An electron exposure system for recording and printing

AN ELECTRON EXPOSURE SYSTEM

FOR RECORDING AND PRINTING

AN ELECTRON EXPOSURE SYSTEM

FOR RECORDING AND PRINTING

Bii^LlOIHtLf;

DER

TECHNISCHE HOGESCHOOL

DELFT

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. ft. VAN NAUTA LEMKE, HOOGLERAAR IN DE AFDELING DER ELEKTROTECHNIEK, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP MAANDAG

27 MAART 1972 TE 16.00 UUR

DOOR

LEENDERT ANTONIUS FONTIJN

natuurkundig ingenieur geboren te Amsterdam

Dit proefschrift is goedgekeurd door de promotor PROF. DR. IR. J. B. LE POOLE

Gaarne betuig ik mijn dank aan de directie van de Technisch Physische Dienst TNO-TH voor de geboden gelegenheid dit werk te verrichten. Voorts bedank Lk een ieder, die aan de werk-zaamheden heeft bijgedragen.

CONTENTS

I INTRODUCTION 7 References 10 II THE ELECTRON EXPOSURE SYSTEM 11

A. Further i n t e r e s t i i ^ points for the electron

exposure system 22 References 23 III POSSIBLE SCREEN ORIENTATIONS AND

CORRESPONDING SCREEN RULINGS 24

References 30 IV A. PHOTORESIST MATERIALS 31

Electron exposure characteristics of r e s i s t s 33 Experimental r e s u l t s obtained with Kalle XTU (or PK 13) 35

B. PHOTOGRAPHIC MATERIALS 42

References IV.A 44 References IV. B 45 V THE ELECTRON PENETRATION AND ENERGY LOSS

INTO ORGANIC MATERIALS AND COPPER 46 General relations in electron penetration 52

References 66 VI CALCULATION OF THE TEMPERATURE DISTRIBUTION

IN A PHOTORESIST LAYER ON A COPPER SUBSTRATE

BY ELECTRON-BEAM EXPOSURE 70

A. Introduction 70 B. Thermal properties 70

C. Calculation of the temperature distribution 72

D. Some examples 79

References 83 APPENDIX A. POSSIBILITIES FOR LIGHT-OPTICAL

EXPOSURE 85

References 91 APPENDIX B. AN ELECTRON EXPOSURE SYSTEM

FOR NEGATIVE PHOTORESIST 92

References 96 APPENDIX C. ELECTRON BEAM ENGRAVD^G 97

References 104 SUMMARY 105 SAMENVATTING 107 STELLINGEN 109

Wie tevreden is over zijn arbeid, heeft reden tot ontevredenheid over zijn tevredenheid.

C h a p t e r I INTRODUCTION

A new method for combining light optics and electron optics has come into being in the graphic industry. These technologies are a l -ready known in the communications industry, such as in television, microbeam machining for computer memories, integrated circuits and specialized weldir^ techniques. Current image data processing in the printing industry is being effected almost entirely with light-optical techniques and by the use of light-sensitive materials. The modest complementary role of an electron optical system may con-sist of the exposure of electron-sensitive materials coated on print-ing surfaces for the transference of image data.

By coupling an electron exposure system to a lightoptical r e a d -out system, such as a t h r e e - or four-colour scanner, far-reaching automation is possible. A l a i ^ e part of current photographic p r o -c e s s e s , su-ch as making intermediate negatives or positives, -can now be dispensed with. The image processing is effected per image element and results in a special screen configuration on the exposed surface. For a suitable speed it is essential that many image e l e -ments are processed per second. With a screen ruling of 84 lines per cm, correspondii^ to a maximum a r e a per image element of 0.120 X 0.120 mm^, most of the experiments have been c a r r i e d out at a rate of 20,000 image elements per second, but a speed up to 50,000 image elements per second i s also possible. The various screens used have a logical structure, with the screen ruling and angulation coupled. For colour printing a choice may be made from a combination of different s c r e e n s in order to minimize the effect of moiré patterns. In principle the electron exposure system i s suitable for letterpress, offset and rotogravure in the printing industry and for data storage on photographic films.

With l e t t e r p r e s s and offset the text need not be screened a s it has to be with rotogravure. For proper tone rendering the images must be screened. In rotogravure the screen s e r v e s to support the doctor blade, which prevents the ink from wiping out of the etched a r e a s during printing. In rotogravure a distinction is possible b e -tween conventional, half-tone and semi half-tone copperplate p r i n t i i ^ , which is illustrated in fig. 1.1.

For the reproduction of a full-tone subject the continuous tones in the original are represented in this way by a large number of image elements or screen dots. The fineness of the screen is largely governed by the number of grey tones required and by the quality of the printing paper. Various thicknesses of ink, according to the size and the depth of the etched cavities, are deposited on the printing surface. The ink soaks into the pores of the paper and, depending upon the quality of the paper, the screen s t r u c t u r e s can be rendered

invisible by a slight sidewise diffusion.

In photomechanical printirg p r o c e s s e s r e s i s t s a r e used which a r e photosensitized and serve a s a protective layer during etching. Such resists a r e pigment paper, rotofilm (Du Pont), rotargo (Agfa Gevaert) and photoresists (Kodak, Kalle). As an etchant ferric chlo-ride is almost exclusively used for copper, as no gaseous products a r e produced during etching.

The best indication of the likely place for an electron exposure system in the printir^ process is obtained by comparison with the cylinder engraving machines in use. With a cylinder ei^raving m a -chine the screen dots a r e chiselled into the copper cylinder surface with a small, rapidly vibrating chisel. A few of the most well-known machines are the Helio-Klischograph made by Hell (Kiel) and the Scanagraver made by Fairchild. Similar equipment has been d e -veloped by Werkspoor (Amsterdam) for textile printing.

The principle of these engraving machines for rotogravure is a s follows. The mounting or read-out cylinder has the same diameter as the cylinder to be engraved and the two cylinders rotate at the s a m e speed. The density signal is obtained by helically scanning the mounting by an optical system (transmission or reflection) and handlii^ the signal in an electronic interface. Depending upon the density signal, the vibrating chisel is brought more or less force-fully down into the cylinder to be engraved. The resulting er^raved dots are semi half-tone in shape. High demands are made on the hardness of the chisel and the effect of wear and tear upon it.

4000-5000 cuts a second a r e possible with the small chisel, which is equal to 0.6 cm^ per second of treated surface. A reasonable handling rate of the cylinder surface is found by using several of these chisel units in parallel. The several problems a r e the b u r r s on the e(%e of a dot, which a r e removed by a second chisel close to the surface and also depends upon the iiardness of the copper layer which has been applied by electroplating, and furthermore upon the prevention of troublesome moiré patterns in colour printing. More-over a photographic interference is necessary owing to a difference in the supply of ink and a smaller volume capacity of the engraved dots a s compared with the etched dots.

In view of the high handling rate of the electron exposure system, coupling to a colour scanner is required for the necessary image data. With the colour scanner three or four part-negatives a r e ob-tained on photographic film for the printing inks cyan, yellow, magen-ta or black by spotwise scannir^ of the original in the three prin-cipal colours, red, blue and green respectively. Other corrective techniques in which, however, the image data are processed as a whole, are photographic masking and retouching. The main tasks of the colour scanner are tone and colour correction, under-colour removal by black printer generation and also compensation of out-of-balance originals and increase of the apparent sharpness of detail by unsharpness masking. Tone correction i s necessary, a s the density

range of a transparent object may be three or more and for an object in reflection it is about one and a half. Colour correction is the c o m pensation for the inadequacy of the printing inks, as each of the p r i n t -ing ink colours cyan, magenta and yellow partly contain the other colours. A few colour scanners a r e the Diascan 3000 and Magnascan 450 made by Crosfield Electronics, the Combi-Chromograph of Hell, the K.S. Paul P.D.I, colour scanner made by Printing Developments International and the Fairchild colour scanner.

Some experience has been gained with an optical read-out system for transmission and reflection according to the Kohier illumination system. Things of practical importance a r e the size of the read-out spot, the colour temperature of the illuminating lamp, the scanning speed and the corresponding band-width and signal-to-noise ratio of the photomultiplier signal, which is lowest for the blue filter. The b e s t r e s u l t s a r e obtained by maskii^ in transmission.

A read-out system consisting of three l a s e r beams at wave-lengths in the main colours red, green and blue might b e interesting. The advantages are the far g r e a t e r depth of focus, an improvement in band-width and signal-to-noise ratio. At present such a system for one colour only is the F e r r a n t i Model 71 Laser Scanning Analyser, designed for paper quality control.

The investigations as to possible application of the electron beam exposure system in rotogravure were conducted for Acigraf Sri. A survey on the possibility of direct electron beam engraving of cop-per and other materials, conducted for the European Rotogravure Association, is summarized here. In view of the simplicity with which the power density of an electron beam can be varied, it is to be expected that for the investigations with direct laser beam en-graving the same limitations in the form of b u r r s around the image dots as obtained with the electron er^raving, are or will be expe-rienced.

The various chapters further refer to the electron exposure c h a r a c t e r i s t i c s of r e s i s t s , electron penetration and temperature effects in a r e s i s t layer. In addition the probable moiré patterns of the selected screens will be reviewed. For the sake of completeness another electron exposure system with a quadrupole i m a g i i ^ system and an electrostatic biprism modulator for a negative r e s i s t will be described, as well a s the possibilities for light optical exposure. References:

G. Andreotti II poligrafico italiano

\ C h a p t e r I I

THE ELECTRON EXPOSURE SYSTEM

V i

With the experimental electron exposure system it is possible to carry out both static and dynamic electron exposure t e s t s . For static exposures a gauze is placed closely above the exposure surface. For dynamic exposures this gauze is omitted and image elemeiits of dif-ferent size and shape are formed which fit into the selected type of screen. The schematic diagram of the set-up is given in fig. II. 1.

MODULATION SIGNAL RESPI SWITCH UNIT BEAM MODULATION UNIT I SCREEN LOGICS POWER SUPPLIES ROTARY INCREMENTAL ENCODER VACUUM SVSTEM ELECTRON GUN CHAMBER tmith Suf+LiÉè

FOR LENSES AND DEFLECTION COILS ELECTRON

EXPOSURE COLUMN

VACUUM WORK CHAMBER

DRMNG-MOTOR

VACUUM SYSTEM

F i g . n . 1

Schematic diagram of the experimental electron exposure system

Test strips of electron sensitive material can be fitted around the exposure cylinder. These strips are made of copper or aluminium coated with a r e s i s t film, offset plates or 35 mm photographic film. A section of the cylinder is provided with a track of calcium tung-state crystals as a fluorescent screen, a sliding contact for the col-lection of the absorbed electrons in the isolated cylinder and a groove for measurii^ the beam current. With a variable drive motor an adjustable speed is obtained. The i n t e r e s t i i ^ data of the exposed area are shown in table II. 1.

Table II. 1 Number of revolutions per minute 100 200 300 400 500 600 700 800 900 Exposed area ( c m V s ) 0.86 1.73 2.60 3.45 4.32 5.20 6.02 6.90 7.80 Surface speed (m/s) 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 Number of image elements per second 6000 12000 18000 24000 30000 36000 42000 48000 54000 The cylinder pitch i s 0.120 mm.

The number of image elements is valid for the basic screenof 84 lines per cm; with an available exposure area of 0.120 x 0.120 mm^ per image element.

A rotary incremental encoder, type ROD 1/45.7 of Heidenhain, is directly fitted to the exposure cylinder siiaft to avoid backlash. With the corresponding electronic interface (type 59.3) 43200 pulses and a zero-pulse a r e obtained per revolution. The angular position of the exposure cylinder in relation to the zero pulse is determined by addir^ these pulses. Figure II. 2 shows the encoder and a part of the exposure cylinder in the opened vacuum chamber.

In the screen logics the pulses and zero pulse are worked up to the positions of the image elements in 6 different s c r e e n s . The various screen combinations will be separately described in chapter III. In the basic screen 12 pulses are available per image element. The zero pulse is used for the c o r r e c t phase-shift between the suc-cessive exposure lines in the selected screen and for the Respi switch unit in order to obtain the Respi dual dot pattern in the high lights, illustrated in fig. II. 3. The screen logics contain the screen generator, beam deflection and Respi switch unit. The entire range of the driving motor is covered electronically by the screen logics, and thus more than 50,000 image elements per second for the basic screen can be handled. Synchronization deals with fluctuations in the rotational speed.

The information for the c o r r e c t image element positions in the 6 screens is simultaneously obtained and submitted as block-shaped

Fig. II. 2

The rotary incremental encoder and a part of the exposure cylinder, shown in the opened vacuum work chamber

^ ! • ! • !

»:•

•

si^4 {»••{

%v..

F i g . n . 3

A screen with the Respi dual dot pattern (Andreotti 1968) 13

The Respi switch unit influences the c u r r e n t passing through the mo-dulation coils for every second image element according to the double dot pattern. A linear relationship between input and output voltages for the modulation coil current is selected, which is adjustable round about the maximum value. The Respi switch unit can be switched on or off at will. Depending upon the s c r e e n selected, an extra amplifi-cation of the modulation coil current is selected in such a way that with a given value of the input modulation signal the percentage of the a r e a exposed is the same for all s c r e e n s . The values of this extra amplification with respect to the basic screen are \r2 for the 1:1 screen, l-vTS for the 1 :2 s c r e e n s and ^"/lO for the 1 : 3 s c r e e n s . With a constant current density distribution in the electron image the electron beam c u r r e n t then only depends upon the modulation coil current at a given exposure speed, as an equal percentage of area is exposed.

The electron exposure column (fig. II. 4) provides the image of a single image element by imagir^ two specially shaped (fish-tail) a p e r t u r e s on the exposure surface. A constant current density d i s -tribution in the electron image is required for an exposure wliich is independent of the shape and size of the image element or the screen selected. A simple construction and good accessibility of components is achieved in the electron optics by the use of magnetic lenses and deflection coils, illustrated in fig. II. 5. A set of current stabilized power supplies for the various lenses and the deflection coils for alignment results in simple operation. The effects of image rotation of the magnetic lenses are easily offset. For correct fitting of the image element into the various s c r e e n orientations, the electron exposure column can be rotated around i t s axis. Not rotated a r e the deflection coils mounted in the column between the final imaging lens and the exposure surface, whose positions are fixed in relation to the exposure cylinder. The maximum angle of rotation is 4 5 ° each way from the column position at the basic screen.

The c o r r e c t angle of column adjustment is determined by the screen selected and is 18°26' for 1 :3 screens, 26^34' for 1:2 screens and 45° for the 1:1 screen, all taken from the zero position at the basic screen.

All the lenses a r e mechanically precentred. Adjustable a r e the electron gun, aperture holders and the angular position of several deflection coils. For reproducible exposure results, high demands a r e made upon the stability of mechanical adjustments of the elec-tron-optical system and upon long-term drift of the electrical sup-plies.

-F i g . n . 4 -Fig. II. 5

The four sections of the electron exposure column will now be described in g r e a t e r detail, viz:

1. the illumination system; 2. the modulation system; 3. the imaging system; 4. the viewing optics.

1. The illumination system gives an adjustable current density d i s -tribution on the first aperture in the modulation system.

The various p a r t s of the illumination system a r e : a. an electron gun (fig. II. 5-lA);

b. a condenser lens with deflection coils for alignment (5-1 B); c. a beam-limiting aperture ( 5 - l C ) ;

d. a beam-switching deflection coil (5-1 D); e. a deflection coil (5-1E);

f . a manually operated vacuum lock (5-1 F).

The ranges of the gun conditions a r e an adjustable high voltage value up to 30 kV, an emission c u r r e n t of 0.2-0.5 mA, a Wehnelt voltage of 300-500 V and an electrostatic field strength of 8-lOkV/mm between Wehnelt and anode.

The filament current is adjusted for auto-bias operation. The majority of experiments a r e c a r r i e d out at 20kV. When the operating voltage is varied an adjustment of the currents throi^h the deflection coils controlled from the screen logics is necessary in order to keep the excitation p a r a m e t e r K = (ni)^/V constant.

The condenser lens focusses the c r o s s - o v e r of the electron gun in the vicinity of the first aperture of the modulation system. This chiefly determines the current density distribution in the plane of this aperture. From current density measurements on an electron spot size of 0.150 x 0.150 mm^ at the exposure plane position the values at the edges are more than 80-90 ^ of the maximum value at its centre for under- or over-focussing of the condenser lens of 2 ^ and 4 % respectively. With the same focussing condition the maxi-mum value of the current density increases with high tension.

With a condenser lens aperture an adjustment of the current density value at a given distribution is possible. This aperture is, however, only used as a test facility for large differences in electron sensitivity of the material to be exposed, such a s between r e -sists and photographic films.

A fine position control of the illuminating electron beam is en-sured by deflection coils, situated inside the bore of the condenser lens. The coils of the five magnetic lenses in the column are identical and their contribution to astigmatism is minimized by a r o t a

-screen logies a r e r e s e t for the next image element while the electron beam is intercepted. The block-wave signal of the beam switch is ob-tained from a saw-tooth signal and two voltage comparators in the beam deflection unit. The saw-tooth signal is coupled to the rotary encoder by a frequency-to-voltage converter. In view of the high ex-posure speed provisions a r e made to avoid eddy c u r r e n t losses counteracting the current through the deflection coils.

For this reason the deflection coils, mounted on an insulator, are placed around a glass tube at the vacuum side and are enveloped by a cylinder of ferrite to close the magnetic field. The relative current variation for a step function is given by

^ = 1 - exp ( - ^ t)

The switch on effect is therefore limited to within 1 ^is by values of the self-induction L of less than 1 ^xE, the resistance R of about lOOn and the capacity C of less than 100 pF.

For adjustment of the lenses in the modulation and imaging s y s -tem a deflection coil for varying the incident angle is used.

2. The modulation system consists of a. 2 aperture holders (fig.II. 5-2A); b . 2 identical lenses in between (2B);

c. the modulation deflection coil (2C) with a double deflection system (3B) in the imaging part connected in s e r i e s .

If the f i r s t focus of the first lens is in the plane of the first a p e r -ture, the second focus of the second lens will be in the plane of the second a p e r t u r e because of the symmetry of the system. This is i l -lustrated schematically in f ^ . I I . 6 for a thin lens approximation. In order to avoid angular deflections by an off-axis position of the beam a telescopic system is preferred, which means that a = b = c = d. The modulation deflection coil is situated between the two lenses and its centre of deflection coincides with the first focal plane of the second lens. The direction of its deflection can be adjusted during operation by rotation around the column axis. Since the positions of the aperture planes and the two lenses a r e fixed, the resultir^ image rotation when imaging is the same for a given direction of the lens current. This means that no further readjustment of the modulation coil position is required at different high-tension values. The image rotations of the two lenses are counter and equal in value.

The imaging properties of the modulation system a r e considered in the plane of the second aperture, as this plane is further imaged by the lenses in the imaging system at the exposure surface.

The imaging properties of the modulation system for the thin lens approximation a r e in general represented by:

VI

f f [ f 1

rn

1 y —- — — — Y-o ] '~--T

FIRST APERTURE FIRST LENS CENTRE PLANE SECOND LENS SECOND OF

MODULATION DEFLECTION COIL

APERTURE PLANE

DOUBLE DEFLECTION COILS IN SERIES WITH MODULATION DeaETTOR COIL

}B

Fig. n . 6

Principle of the modulation s y s t e m

yDo

yoo

The deflection angle of the modulation is y.

The influence of defocussing under the conditions a = b = c = d=f, fi = f2=f + Af and 7= 0 is represented by

So the size of the first aperture in the second aperture plane at de-focussing is

y, = 2 A f ( l + f ) y i - ( l -2{ff)Yo

The position of the exit pupil in the illumination system is at the prin-cipal plane of the condenser lens and, in the case of a beam-limiting aperture, at this condenser aperture plane. The semiangular a p e r -t u r e angle a is defined by -the radius of -the elec-tron beam a-t -the con-denser principal plane or the radius of the beam-limiting aperture and the respective distances to the c r o s s - o v e r image around the first aperture plane in the modulation system. The range of the beam angle VQ at the first aperture plane with a distance p to the exit pupil plane is given at an off-axis position yg as

Z o - a < y ; 5 ^ Z o + a p ^ "^ ^ p

The shift of the main ray in the second aperture plane at defocussing from its imaging position when focussing is given by

Ay. = 2 y o ( ^ + ( f ) ^ l 4 ) )

P r* The diameter of the blur around this point is 4 f a . -7- (1 + - p ) The influence of an axial shift of the modulation deflection coil, r e -lated to the position with its centre of deflection half-way between the two lenses, i s represented in the thin lens approximation for a = d = fj^ = f2=f and b + c = 2f by:

y{ = -Y'o + n-j)y Yi = - y o + i y

This means that a relative axial shift 1 - j of the centre of deflection with respect to the position half-way between the two lenses results in the plane of the second aperture when imaging as a beam tilt over (1 - T)'y, which is proportional to the modulation deflection angle y, and the desired beam shift of fy.

Spherical aberration and astigmatism do not figure significantly in the modulation system. The blurring of the image at the second aperture plane due to spherical aberration of the two lenses i s l e s s than 0.25 ^m at an illumination angle up to a = 10"^ rad.

The shape of the apertures determines the shape of the image element. For a square image element the fish-tail apertures a r e similar to the selected a r e a diffraction apertures with an angle of

90° in an electron microscope. The first and second aperture form the two opposite angular points in the image element and the change in size occurs along the diagonal. This is c o r r e c t for proper m e -chanical centring of the apertures and of the direction of deflection of the modulation coil. At the minimum spot size^with no deflection by the modulation coil the angular points or the centre of the r e -maining figure is positioned on the column axis. This position i s determined by current reversion on imaging the mechanical p r e c e n -tred magnifying lens (fig.H. 5-3A) of the imaging system. In o r d e r to obtain the centre of the image element on the column axis for the final imaging lens a shift of the image element is prevented by a double deflection system in s e r i e s with the modulation coil. This is also illustrated in fig. II. 6.

3. With the imaging system the image element as obtained with the modulation system is imaged on the exposure cylinder. This system consists of:

a. a magnifying lens (fig. II. 5-3A);

b. the double deflection modulation coils (5-3B);

c. a final lens with deflection coils for alignment (5-3C);

d. a set of x, y deflection coils controlled by the screen logics (5-3D). The magnification lens has two functions, firstly to define the column axis in the modulation system, secondly to s e r v e as an aid to proper imaging. The magnification is about 15 times that of the final lens for exposure. The direction of deflection of the double deflection system is determined by the aperture position in the modulation s y s -tem and can be adjusted during operation. The x, y and y* deflection coils, situated between the final imaging lens and the exposure cylin-der, are controlled by the beam deflection unit for proper positioning of the image elements in the selected screen and to avoid blurring by the movement of the cylinder during exposure. For the fitting-in of the image element in the selected screen the column and consequently the image element obtained can be rotated to the appropriate s c r e e n angle. The rotation of the image in the final lens is also compensated in this way at the basic screen position.

The x, y and y* deflection coils a r e not rotated by their position being related to the exposure cylinder and they have the following functions:

1. The X deflection coils yield a displacement in the cylinder axis direction. With a cylinder pitch of 0.120 mm the displacements are:

for the basic ( 1 : =) and 45° (1 :1) screens 0 mm; for the 1 :2 screens 0 and 0.060 mm;

By this dynamic compensation, blurring of the exposed image ele-ments as a r e s u l t of cylinder movement is avoided. The slope of t h e saw-tooth c u r r e n t is made proportional to the cylinder speed by a frequency-to-voltage converter.

3. An extra possibility for displacements in the y direction is pos-sible with the y* coils.

T h e separation and b o r e of the final lens a r e 20 mm. With a focal length of 50 mm and a beam aperture angle of less than 2.10"^ rad the contribution by spherical aberration in the final image is less than 0.1 nm. The final lens demagnifies about twice. For a varia-tion in the axial posivaria-tion of the exposure plane of ± 0.5 mm the b l u r at the edges is l e s s than 2.5 ^m. This unsharpness is of the o r d e r of the r e s i s t thickness. For exposure only the resulting current density change is important. The tolerance in the diame-t e r of prindiame-ting cylinders in rodiame-togravure is 0.01 - 0.02 mm.

For test adjustment procedures the signals to the deflection coils can be switched to dummy r e s i s t o r s . In the present set-up an extra electron-optical magnification system for intermediate checks on the performance of the deflection coils is not possible and the p r e s e t values a r e obtained in a separate test unit. Such a system may for instance consist of two identical magnetic lenses with r e -v e r s e d image rotation and a magnification of 25 to 50 times. Such a magnification p a r t is certainly desirable in the exposure system. For calibration a gauze of known dimensions may be used at the exposure plane. The alignment coils in the final lens may serve for slight displacements.

4. T h e optical system (fig. II. 5-4) magnifies the electron image at t h e exposure plane about 50 times. On the holder for the objective lens (f = 25 mm) and the two m i r r o r s an aluminium box is fitted for measurement of the back-scattered electrons. In order to avoid interference with the electron beam the optical axis has an inclined position on the exposure surface. The numerical aper-t u r e is limiaper-ted so as aper-to obaper-tain a large focal depaper-th (n.a. = 0.04) wiaper-th a resulting optical resolution for blue light of about 5 fjm.

A. Further interesting points for the electron exposure system a r e : 1. The investigation of the electron beam current density distribution

in order to find the optimum gun condition at a selected high-tension value for various types of filaments. This may lead to improvement of the maximum value and pertirtent distribution of the current density in the i m ^ e element.

2. The use of an electron back-scatter detector for quality control during the exposure process. The detector may be an aluminium or carbon ring around the image point of which the c u r r e n t ab-sorbed by the back-scattered electrons i s used. This back-scatter signal can be plotted against the quadrated modulation current s i g -nal for an almost linear relationship.

3. The aim of the electron exposure system i s to effect coupling with a read-out system. This may be a mechanical or electronic coup-ling with a colour scanner system. The electronic coupcoup-ling with a buffer memory offers a solution of the greatest flexibility.

An adapter for conversion of the density information of the colour scanner to the input voltage of the modulation current has to be added.

4. The application of cylinders with different dimensions of length and diameter is required. The number of pulses per revolution obtained from a rotary incremental encoder and interface system has to be made such that it is proportional to the circumference of the cylinder. Further modifications in the screen logics are: a. Improvement of the exposure time of each image element,

which can be determined by a fixed number of p u l s e s .

b . A continuous screened image without an interference between the b ^ i n n i n g and end of the screen i s possible. In the present apparatus a r e s e t line occurs at the position of the zero pulse in the rotary encoder at which the electronics a r e properly r e s e t according to the screen selected. The occurrence of the r e s e t line in an i m ^ e is unwanted. T h i s effect can be com-pletely overcome under the following conditions:

1. The number of pulses per revolution divided by the number of pulses per image element must b e divisible b y :

2 for the basic and 45° (1 ; 1) s c r e e n s 5 for the 1:2 screens (2^ + 1)

10 for the 1: 3 screens (3^ + 1) 17 for the 1:4 screens (4^ + 1)

Here use is made of the repeating patterns in the various screens and a situation for closed exposure lines around the cylinder circumference is created by the sweep deflection. 5. The principle of the electron beam exposure column can also be

applied to digital recording on discs or cylinders to which a photographic material has been applied. The modulation system can then be replaced by an aperture holder with an enlarged a p e r ture shape of the image element desired, as only one size t h e r e -of is wanted. The reduction -of this shape can be effected by a 2-lens system, the image rotation of which can be compensated. To obtain an image element of 5x 5 |jmf which can be read out by a light optical system, the enlarged aperture shape can be 125 ^m with an electron optical reduction by a two lens system of 25 times. In respect of the electron beam system for digital record-ing as described by Loeffler (1967) an improvement in the handl-ing speed for the electron exposure process is possible by 10 to 100 times. The reason for this lies in the exposure of the image element as a whole and in the higher current density value herewith, as lens e r r o r s play a relatively smaller role.

References:

K. H. Loeffler, IEEE catalog nr. F-79 pp. 344-362, 1967

(IEEE 9th Ann. Symp. Electron, Ion and Laser Beam Technology Berkeley 1967, San Francisco P r e s s Inc. Calif.

Patent application numbers for ' A device for recording with electron beams" Netherlands 6807439 USA 827,510 Canada 052,646 Great Britain 26.845/69 Japan 4318/1969 Eastern Germany AP57d/140055 and 79664 Western Germany P19 26849.6

C h a p t e r I I I

POSSIBLE SCREEN ORIENTATIONS AND CORRESPONDING SCREEN RULINGS

A moiré pattern, which is a normal interference phenomenon, is likely to result when two or m o r e periodic structures a r e superim-posed. In the printing process this may occur in colour prints. A de-tailed description of moiré patterns, screen periods and screen angles is given by Yule (1967), Tollenaar (1968) and Sweerman (1971). In the normal four-colour reproduction the following screen orienta-tions have been adopted for quaternary dot screens: yellow 0°; cyan 15°; black 45° and magenta 75°.

The l a i ^ e overlapping spectral absorption areas of black, cyan and magenta half-tones a r e separated by angles of 30°, which is the maximum regular division possible over the available angle of 90° arising from the fourfold symmetry of the screens. A second order moiré pattern resulting from the cyan and magenta combination with the black one may sometimes be troublesome. It is impossible to find a combination not producing moiré under any circumstances. The visibility of the moiré depends upon the following factors: 1. The screen ruling of the moiré. For a screen ruling finer than 20

lines per cm moiré is no longer observed as a disturbii^ pattern, though the sharpness of definition is affected. A very large or in-finite moiré cannot be observed either, but this has the disadvan-tage of being extremely sensitive to shifts over very small angles of rotation and of showing parallel displacements which may cause colour shifts all over the page.

2. Large steps in the density profile normal to the lines of import-ance in the moiré can be avoided by a certain selection of the shape and definition of the dots, and also by the unsharpness of the dots, as described by Tollenaar. Therefore, the basic lines of the screens give moiré patterns that a r e more clearly visible than any other row of dots.

3. The printing a r e a of the interferir^ s c r e e n s . A combination of two 50^ printing a r e a s gives the best visible moiré. The contrast of the moiré depends upon the gradient of the printing area as related to the density function.

4. In multi-colour printing, the spectral absorption distribution of the inks for dotfS printed next to or over each other. The better the

In order to obtain a simple screen logic system with the possi-bility of simultaneous registration of images with different screen angles, quite a different screen pattern concept was taken as a s t a r t -ing point. The directions of r ^ i s t r a t i o n of the screen in question a r e shown in fig. III. 1.

V

\ \ \ \ \ \ \ \ \ \ \ 1:00 1:3 1:2 F ^ . i n . 1

The direction of registration of the various s c r e e n s

By rotating the square image element through the c o r r e c t angul-ar value it becomes incorporated in the appropriate screen. The image elements beyond the registration line a r e obtained by a shift of the electron beam per image element. In this way the rotated s c r e e n s are r e g i s t e r e d according to diagonal directions and the square dots that do not lie on the diagonals a r e filled up by lateral displacements of the electron beam. This results in a slight lack of sharpness due to the difference in position of reading and recording.

The screens show a change in lineature according to the formula X — i—, in which x is a scale factor and n and p a r e positive in-t e g e r s . The uniin-t of lengin-th has in-to be referred in-to in-the axial displace-ment after one revolution or pitch of the recording drum, which has the s a m e value for all recording drums for the various s c r e e n

pos-sibilities.

The screen angle is given by a = arctan— .

Table HI. 1 shows the various screen possibilities, to which others may be added. In the practical application only the first four s c r e e n types have been selected so as not to make the realization of

Table HI. 1

The various screen possibilities, which can be further extended Screen l:oo 1 : 1 1:2 1:3 1 :4 2 : 3 2 : 5 Angle a 0° = arctan 0 45° = arctan 1 26°34' = arctan 1 18°26' = arctan | 14°02' = arctan 1 33°41' = arctan 1 21°48' = arctan 1 Change in lineature ^^l = 1 't \'~2 = 1.42 ^ = 1.12

-T

-

'•»»

-^f . 1,08

the screen logics unnecessarily complicated. Thanks to the asym-metry a distinction is possible in the 1 :2 and 1:3 s c r e e n s to the left and to the right. In this way six different screens a r e obtained, which a r e represented in fig. III. 2.

The calculation of the direction and size of the m o i r é for two line screens with n^ and n^ number of lines per cm and an ai^le j3 between the lines, is given by a simple geometrical equation as fol-lows:

n = n , / l + ( § ] - 2 ( ^ ) c o s ^

and

- sin /3 tan y =

where n = screen ruling of the moiré

y = the angle between the line s c r e e n with ruling n^ and the

• • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • • • • • • • • • a • • • • • • • • • • • • • • • • • • I •• V V V » v v * • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • a V V • • • 4> • • 4 • • • • « • • * « 4 4 • 4 * 4 • • • • • • •• # • V V V • 4 • • • • • • • • • * « V V V « V 4> 4 • • • • •

•! vvvvvvvvvvvwv

•• •vvvvvvvvvwvv

_{v v w w v v w w w}

• v w v w w w w •

v v v v w v v w w w

• v v v w v v v v w w

v v v w w v v v w w

•vvvvvv • • • • • • •

v w v v w w w • • •

• v v v v v w v w v • •

iVïi • • • • • * • » • • • • • • •••• " • • • • • ; ' • ' * * • • * " • * " • • • * • . • • » • • . * • . • • , ,'•.'», r».'».'».'»: •* •* •* »» • l : c o 1:1 1:2L 1:2R 1:3L 1:3R Fig. i n . 2

The 6 different s c r e e n s selected for the electron exposure system, which are each on the same scale with reference to the basic (1 : co) screen !<>• c ' 4 > -o ,-^ • W ^ M _^TT»| j.-v,'-^^p '••^ •-J-v '•-"il • • / < -'.„••,'.'.\,-;l'.^-/-VV, f • • . ' r • % '

^i

0-'

s^

1:2R 1:3R 1 :«> 1 :2L 1:2R Fig. i n . 4

An illustration of the moiré pattern for the screen combinations as indicated. The s c r e e n s a r e on the same scale as depicted in fig. III. 2

Table HI. 2

The values given for the moiré patterns a r e for a basic screen ( 1 : oo) of 85 lines per cm above the infinity line and the factor below it. Not considered a r e the coarsening and the corresponding rotation of the moiré pattern caused by the Respi effect

1 : CO 1:1 1:2L 1 :2R 1:3L 1 :3R 1 :4L 1:4R 1 :oo inf.

K^

K^

ï^^

1 ^ 1 0

iVio

iV^^

iV^^

1 : 1 60 inf.

l-rio

jl^lO

¥^

|.re

3-4-'"^^

H-'"»*

1:2L 38 27 inf. 0.566 0.13

i^^

0.208 0.65 1:2R 38 27 48 inf.

K.

0.13 0.65 0.208 1:3L 27 38 11.1 60 inf. 0.60 1:3R 27 38 60 11.1 51 inf. 1:4L 20.6 43.5 17.5 55 inf. 0.47 1 :4R 20.6 43.5 55 17.5 40 inf.

n

n2

n1

Fig. i n . 3

Illustration of the geometrical equation

In table III. 2 the screen rulings of the moiré patterns for the two s c r e e n combinations are indicated. The Respi system, in which a s c r e e n coarsening of \f2 and a screen rotation through 45° occurs, has not been considered. With the above equations it is possible to obtain an idea of the m o i r é pattern to be expected. An illustration is given in fig. III. 4.

However, when a number of colour dot screens a r e superimposed the problem becomes so complicated that calculations alone no longer give a sufficiently reliable indication of the degree of disturbance all the possible m o i r e s may be expected to cause. The best practical approach, therefore, is to make the required screens and sharp trial prints so as to b e able to determine the combination with the least possible occurrence of moiré.

These experiments were c a r r i e d out at the Research Institute for the Printing and Allied Industries TNO (I.G.T.) by Sweerman and du Pont (1971). The prints were made via an electrographical p r o c e s s . In the Gevaproof prints a sharp print is obtained for clear visibility of the moiré patterns that occur.

The followir^ combinations a r e recommended as beir^ the most favourable:

magenta 1: oo or 1 : oo also possible is 1:1

cyan 1:2R 1: 3R 1:2R black 1:2L 1: 3L 1:2L yellow 1:3L 1:2L 1 : 3L

References J. A.C. Yule

D. Tollenaar

A. J. W. Sweerman W. M. du Pont

Principles of Colour Reproduction 1967 J. Wiley and Sons

I.G.T. Publication no. 30, 1968 Moiré in Screen Printing

VIII Int. Conf. of Printing Research Institutes 1965 Helsinki.

Dot shape and contrast of moiré patterns 1971 - Private communications

The patent application numbers of the " F r a m e combination" are: Netherlands U. S. A. Canada Great Britain Japan Eastern Germany Western Germany 6909437 44732 85221 29894/70 53440/70 AP 15b/l 50078 and 84801 P 20 30375.7

C h a p t e r IV

A. PHOTORESIST MATERIALS

The term photoresist r e f e r s to the protective layer formed by the change in solubility of a suitable material sensitive to light so a s to protect the underlying surface from chemical attack. The photor e s i s t layephotor must exhibit sufficient adhephotorence to the substphotorate to photor e -tain the geometrical boundaries of the desired pattern after exposure and development. Photoresists consist in general of polymers which a r e photosensitive themselves or a r e capable of reacting with the photolysis products of added compounds. For the exposed areas the solubility is increased for positive r e s i s t s and decreased for negative r e s i s t s . A developer is selected so as to enhance the d i s c r i m i -nation between the exposed and unexposed p a r t s . Apart from the sensitivity to actinic light, practically all photoresists are also sensitive to electron exposure. The presence of sensitizers in a photoresist may produce a remarkable increase in its sensitivity. The various reaction mechanisms during exposure a r e based on the transference of energy between the incident photons or electrons and the sensitizer and polymer molecules.

We can avail ourselves of the literature in the field of radiation chemistry. As pointed out by Charlesby (1960) the concepts of the molecular weight distribution and the distribution of the cross-links a r e of fundamental importance for the properties of polymer systems. The chemical s t r u c t u r e of the polymer acts more or less as a p a r a -meter. By radiation theory the G value is introduced, which gives the number of chemical changes of a given kind produced per 100 eV absorbed energy. In this definition no assumptions a r e made as to the various reaction mechanisms. Moreover the gelling dose or threshold flux at the gelling point is introduced at which an insoluble polymer network begins first to form. The remaining soluble frac-tion is termed the sol. In this manner the solubility of the exposed p a r t s of the r e s i s t at development is dependent upon exposure. This may also cause a change in the r e s i s t thickness after development, as only the insoluble fraction is left behind on the substrate. Some-times swelling of the cross-linked polymers occurs, which depends upon the density of c r o s s - l i n k s and the type of solvent used. In radia-tion chemistry the polymer degradaradia-tion, for instance, of polymethyl-methacrylate due to prc^ressive reduction in the average molecular weight is known and also the opposite reaction the polymerization of polystyrene by cross-linking.

In nearly all literature references the electron exposure c h a r a c -teristics of r e s i s t s relate to filmthicknesses of less than 1 fxm. In most cases the r e s i s t s have then been applied to a glass substrate with a thin metal coating. The quantity of absorbed energy per volume unit in the r e s i s t layer is the proper guide to the exposure

Table IV. 1

Electron exposure characteristics from index 1: Broyde 1969,1970 index 2: Matta 1967 index 3: Hatzakis 1969 Electron energy keV 5 10 15 20 15 15 15 10 20 30 14 Exposed material Kodak Thin Film Resist high resolution silver halide emulsion Kodak Photo Resist Shipley AZ 1350 Polymethyl-methacrylate Shipley AZ 1350 + 2^ benzotriazole Kodak Thin Film Resist Shipley AZ 1350 Shipley AZ 1350 Initial r e s i s t thickness jim 0.60 0.60 0.60 0.60 0.36 0.60 1.20 0.36 0.30 Minimum charge density MC/cm^ 1.5 3.5 8.0 16.0 10-3 8 a 10 6 8 2 a 3 120 80 40 10 30 80 50 Index 1 2 3

condition. As a practical indication the incident energy density in eV/cm^ for photons and the charge density in C/cm^ for electrons a r e used.

For photon exposure the incident energy density is determined by instruments suitable for measuring radiant energy. These in-struments can be calibrated by some photochemical reaction of known quantum yield. The quantum yield is the ratio of the number of molecules that have reacted to the number of quanta absorbed. In this way the number of quanta with known quantum energy is ob-tained. The absorbed photon energy in the r e s i s t layer is deter-mined from the total incident energy and the reflection coefficient of the r e s i s t coated sample.

For electron exposure the energy absorbed in the r e s i s t layer is determined by the incident current density, the electron energy and the depth of penetration. Here the contribution of the back-scattered electrons from the substrate has to be included. For a given r e s i s t thickness the energy absorbed in the r e s i s t layer de-c r e a s e s as the inde-cident elede-ctron energy inde-creases. This is due to the fact that the depth of penetration is m o r e than proportional to the accelerating voltage. Depending upon the depth of electron pe-netration for thin layers a further decrease occurs in the absorbed energy, which follows from the initial p r o g r e s s of the electron energy dissipation with the penetration depth curve.

Electron exposure c h a r a c t e r i s t i c s of r e s i s t s .

Resist c o a t i i ^ s of the negative Kodak Thin Film Resist (KTFR) and the positive Shipley Azoplate 1350 were investigated by Matta (1967). For several film thicknesses of KTFR the exposure required for complete polymerization is 10-5 C/cm^, with an incident elec-tron energy of 15 to 20 keV, at which voltage the r e s i s t appears to b e least sensitive. The required charge density in the range 10-5 to

10-4 c / c m ^ i n c r e a s e s with an increasing incident electron energy of 5 to 30 keV for an AZ 1350 layer of 0.36 jum.

Broyde (1969) on the other hand shows, as partly represented in table IV. 1, that for a KTFR film with a thickness of 0.60 urn the m i nimum charge density for full exposure also increases with i n c r e a s -ing electron energy. The sensitivity of KTFR increases as the film thickness i n c r e a s e s , which can only partly be explained by the de-pendence of the electron energy loss upon the penetration depth. The sensitivity of KPR (Kodak Photo Resist), KTFR and Shipley AZ 1350 for a r e s i s t layer thickness of 0.60 lum is virtually the same at 15 keV. It is also shown by Broyde (1969,1970) that the amount of

energy t r a n s f e r r e d per unit of volume at the threshold charge density is a constant for a given r e s i s t . This indicates that the threshold is due to the gel point of a negative r e s i s t or the sol point of a positive r e s i s t . Compared with UV light exposure, electron exposure is more efficient in solubilizing AZ 1350, as the incident energy density

(eV/cm^) is less and exactly the opposite applies to KTFR. Overexposure by electrons causes a c r o s s - l i n k i i ^ reaction, which renders AZ 1350 undevelopable. Overexposure of AZ 1350 with UV light of wavelengths g r e a t e r than 310 nm does not result in an insoluble product, but with a wavelength of 254 nm insolubility occurs as a result of cross-linkir^ of the phenol-formaldehyde polymers. The cross-linking reaction is also likely to occur with electron exposure, which is in agreement with our exposure results of Alnovol 320K and 429K, basic materials for photoresists.

The solubilization reactions involved in the response of AZ 1350 to electron exposure a r e likely to b e the s a m e as for light exposure. The quinone diazide groups a r e converted into carboxylic acids (Kosar 1965, Levine 1969), which render AZ 1350 alkali soluble. Certain additives increase the sensitivity of photoresists. The electron sensitivity of AZ 1350 is increased about three times by the addition of small amounts of the benzotriazole, indazole or imidazole solutions (Broyde 1970).

The electron exposure conditions of polymethyl methacrylate a r e given by Haller et al (1968) and Hatzakis (1969). Polymethyl methacrylate is a positive electron r e s i s t and is insensitive to visible light. The electron sensitivity is comparable to the other positive r e s i s t s . Within the c o r r e c t charge density range the expo-sure results in a random scission of the molecule chains, which effectively reduces the average molecular weight of the polymer. Development is based on the change in solubility in the exposed a r e a s due to the molecular weight reduction. The developer is a mixture of a solvent and a non-solvent of the original polymer. The exposure range for a 100^ exposed a r e a successfully devel-oped at 7.5 and 10 keV incident electron energy i s 5.10-5-5.10-^ C/cm^. At 14 keV the exposure range is 10"'*-10"^ C/cm^. Hatzakis (1969) considers AZ 1350 unsuitable for reliable micro-circuit fabrication owing to the narrow range of exposure around 5.10-5 C/cm^ at 14 keV. Overexposure of polymethyl methacrylate causes the reactions leading to cross-linking to dominate, and the r e s i s t in the overexposed a r e a s is not removed by the developer.

Thornley and Sun (1967) describe the negative luminescent electron-sensitive r e s i s t plastifluor for microbeam writing in a closed loop control system. This r e s i s t was applied as a 0.8 fjm thick coating on oxidized silicon wafers. At an incident electron energy of 14 and 20 keV complete development and etching is pos-sible with a charge density of more than l O ' ^ C/cm^ due to cross-linkii^ of the polyvinyl compounds. They show that scission is a

In the negative Kodak Photo Resist (KPR), the sensitive compo-nent is polyvinyl cinnamate or a closely related compound. The hardening is due to the cross-linking reaction of the C=C double bond of the cinnamate group (Kosar 1965). Other negative photoresists such as Kodak Metal Etch Resist (KMER) and Kodak Thin Film Resist (KTFR) consist of xylene solutions of polyisoprenes sensitized by b i -functional azides (Hunter 1969, Bulloff 1970). The organic azido com-pounds dissociate into nitrogen and a free radical, characterized by a monovalent nitrogen atom. Cross-linking may be involved in the consequent insolubilization of the exposed polymer (Delzenne 1970).

Exposure c h a r a c t e r i s t i c s of negative electron silicone r e s i s t s TSE200 and SH410 are described by Atoda and Kanaya (1969). These r e s i s t s are used as a recording material in the electron microscope.

Hirai et al (1971) describe highly sensitive negative epoxy elec-tron r e s i s t s , which a r e 50-200 times more sensitive than KTFR.

The application of r e s i s t s in microbeam recording a r e described, inter aUa, by HaHer et al (1968), Kanaya et al (1969), Miyauchi et al (1969), Maekawa et al (1969), Tarui et al (1969) and Samaroo et al (1970).

Experimental r e s u l t s obtained with Kalle XTU (or PK 13)

The positive photoresist Kalle XTU (now under tradename Kalle PK 13) and the appropriate alkali developer EP 11 a r e used in our experiments. As i s known, the majority of diazonium compounds have been synthetized by chemists of Kalle (Kosar 1965). Diazonium salts in the diazotype process are unsaturated aromatic compounds bearing a diazo group attached to a carbon atom of an aromatic nucleus, the other valency being satisfied by an acid iOn. On expo-s u r e the diazonium expo-saltexpo-s a r e decompoexpo-sed owing to the formation of a nitrogen molecule, an acid and a phenolic compound.

According to our information the difference between the r e s i s t s Kalle XTU and Shipley AZ 1350 consists only in the solvent used. The solvent in the Kalle XTU has a lower boiling point, which facili-tates coating.

In order to ensure adherence of the r e s i s t layer the test strips a r e degreased, slightly etched and dried. The r e s i s t layer is applied by means of dip-coating. Other ways of doing this a r e spray- and

roller coating. The required homogeneous layer thickness is about 2 /im. It can be measured with the aid of the focal depth of an optical microscope, with an optical interferometer or with a Taylor Hobson profilemeter. The layer thickness is determined by the concentra-tion and viscosity of the r e s i s t soluconcentra-tion, the solvent and the coating speed. The strip obtained is dried for half an hour at a temperature of 60° C to eliminate the organic solvent. This also results in better adherence of the r e s i s t layer, which is necessary for the develop-ment and etching p r o c e s s e s after exposure. No loss in electron sen-sitivity of the r e s i s t occurs as compared with a strip that has not

been dried at 60°C. It is recommended to expose the r e s i s t within a few days after coating. The loss in sensitivity of the r e s i s t when stored at room temperature and in darkness may a r i s e from oxyda-tionby the air. Storage in an inert atmosphere (e.g. nitrogen) might d e c r e a s e this loss in sensitivity, but these effects have not been in-vestigated.

Investigation with an infrared grating spectrophotometer is a useful guide to the nature of the photochemically active structures in the photoresist. The infrared spectrum obtained from unexposed XTU yields the same results as reported by Levine (1969) for unexposed AZ 1350. Both for UV light and electron exposure of XTU r e -sist the same curve is obtained as for UV light exposure of AZ 1350. The absorption of the doublet of the diazoketone liiikage with a wave number of 2100 cm"-^ d e c r e a s e s . Moreover a slight reduction in absorption with a wave number of 1600 c m - 1 has been noticed.

The exposure experiments for determination of the XTU elec-tron sensitivity are carried out for low as well as for high current densities at the exposed a r e a s . A guide to c o r r e c t exposure is the possibility of etching the exposed a r e a s on the copper test s t r i p s after development. At a low current density there is no increase of temperature of the r e s i s t layer during exposure, but there may be a considerable r i s e in temperature of the resist layer during

exposure at a high current density.

a. Low current density exposure results of Kalle XTU at 20 keV. The current density at the exposed a r e a s ranges from 5.10-'^ to 2.10"" A/cm^. The charge density is varied by the exposure time from a few seconds up to 5 minutes. A gauze of 25 lines per cm with 50^ open a r e a placed l e s s than 1 cm above the r e s i s t layer is projected upon the r e s i s t by a homogeneous current density electron beam of 7 mm diameter. The exposure window is 4.10-5-4.10-4 C/cm^ at 20 keV of an XTU r e s i s t layer of about 2 fim thickness on a clean copper substrate. The development time is 5 to 10 minutes in E P l l developer. Etching is effected with F e C l j , 42° Beaume for 1 minute or less. After develop-ment the exposure results can be viewed from a charge density of 5.10-6 C/cm^ up. At a higher exposure of 5.10-4 C/cm^ the r e s i s t is overexposed and r e a c t s negatively, except for the transition area to the unexposed part where there is always an area with a c o r r e c t exposure.

Various charge density results a r e illustrated in fig. IV. 1. For an effective penetration range of 7 ^m at 20 keV the charge density r a i ^ e of 4.10-5-4.10-4 C/cm^ corresponds to the energy

400 ;iC/cm^ 500 /iC/cm^ Fig.rV. 1

Low current density exposure r e s u l t s of Kalle XTU 20 keV, 2 fim r e s i s t thickness,

Respective exposure times: 15;23;50;150;225;300 s. Gauze of 25 lines p e r cm.

Shown exposed a r e a between b a r s 0.3 x 0.3 mm^.

Electron exposure of Alnovol 320 K and 429 K confirms the nega-tive reaction due to cross-linking.

b . High current density exposure r e s u l t s of Kalle XTU at 20 keV. At the exposed areas the current density ranges from 0.5 to 1 A/cm^. The charge density is varied by the number of revolu-tions per minute of the exposure cylinder from 50 to 900. The exposure time for the image elements can thus be selected from 20 to 300 ^ s . The exposure window is 10-5-4.10-5 C/cm^ at the same conditions as described under a and with a beam current of 30 /iA.

The absorbed energy density corresponds to 0.2-0.7 10^2 eV/cm^ Increasing the beam c u r r e n t to 100 ^A r e s u l t s in a slight de-d r e a s e in the charge de-density range to 3.10-5 c / c m ^ .

Owii^ to the temperature r i s e in the r e s i s t during exposure an improvement occurs in electron sensitivity at high current density

•PBBHB

f ' v J ^

iA lilg|É|-j|| M

'" ÉÊWM '

43 M C3 O O O

Normal exposure at 20 j i C / c m ^

beam current 25 /lA

Excessive exposure at 40 /iC/cm^

beam current 50 /iA

Overexposure at 80 ;jC/cm^, beam current 100 nA

exposure. Clear melting phenomena at the exposed image elements, sometimes becoming visible, also indicate a considerable temper-ature r i s e of the r e s i s t during exposure.

Some experimental results a r e illustrated in fig. IV. 2. The cylin-der surface speed is 2.1 m / s . The electron exposure of the photo-r e s i s t may be followed by the visible photo-red light emission due to tron luminescence and by the detection of the back-scattered elec-trons.

Also in the high current density experiments, as a result of the cross-linking of the Alnovol the thin films can be seen after development. The exposure range is determined by these films easily p a r t -ing on development or dur-ing etch-ing. At a ratio of the beam- and emission current between 0.3 and 1 a deterioration occurs in the values of the current density in the image element a r e a s . This affects the exposed image dot size as a function of modulation. Also of in-t e r e s in-t is in-the concenin-train-tion of in-the developer and in-the developmenin-t in-time. It needs no further explanation that it is desirable to perfect and standardize the processes of applying the r e s i s t layer, development and etching so as to achieve well reproducible r e s u l t s . F u r t h e r m o r e the properties of other r e s i s t s should be tested as to their being applicable to electron exposure.

A few experiments c a r r i e d out at 15 keV accelerating voltage yield the same results as at 20 keV. At 10 keV the results a r e less favourable.

From a number of simple tests it has been established that X-rays do not cause any noticeable effect on exposure of Kalle XTU. The experimental conditions are obtained with an X-ray diffraction tube at 20 and 40 keV accelerating voltage and 10 mA emission c u r -rent. The tube was provided with a copper target and a beryllium window. The photoresist-coated copper test s t r i p s a r e exposed by the X-rays for 5 to 10 minutes at a distance of 2 to 3 cm from the X-ray window. Development for 10 minutes with E P l l developer afterwards gives no visible results.

A summary of the six different screens, without and with the Respi effect and the reset line, is given in fig. IV. 3. The exposure conditions are 20 keV, beam current about 30 jiA, 18000 image elements per second at a surface speed of 2.1 m / s , a r e s i s t thick-ness of 2 (im, developed in E P l l developper for 10 minutes and slightly etched afterwards. The exposure density corresponds to 2-2.5 10"5 C/cm^. The magnification can be found by comparison with the basic s c r e e n of 85 lines per cm.

• • W a l l • • ' • " • • • • •|p||^pf=jpr^-'.':j:^i|'ir'i|'- • • • • • • • • i B « * Um • • • • • • • B a n • • • • • • • • * • • • • • É • ' • § • • « ' • • ' • • • • • • • • • • • a • . • . • • • < • • • l a a a a a a a a a a a . a a a a a a a a a a a a . • • • « • • • • • • ^ • « a a a a a a a a a a • « « a a a a a a a a . a • ' . . . « . . . . «aa-a a a a a a a a a ttMmm a a a a a a a a . * . . . . • • a a a a a a > a a B • a a a a a a a a a a a . . . . • . • . • . • • • • • • • • • » • • • a a a a a a a a a a a a . • . . " . tPir9'P=^hf'*-m^M-m-M» ^a_a. a a a e a a a a a a • . . « . • . « . • • • • • • • > • • • • • • • a a B B a B e a d o a a , ( . . . • . « . • . • • • • • • • • • • • • B a a o B a a a a a a a , ; , , , . , . . . . . • « • • • • « • • • • j i « - a o B » B - o # O B a a . . , . . . , . • , , . . . / • . > • > " • • • • • • ' • • s e a • a.B a B a a a a . . . . , , , , . , . . . • . • . «I. a a s B B Btt B era B o .'^ , ' , , , ; B*o e B B a B B B B-ao ' * " * . ' . ' . , .B • a a e a ip o B B a 0 * ~ * * . -• ' -• -• -• -• -• -• -• - -• -• -• I • • • • • • • ! • t^^t • > • # a a^ • • • 1 • •' • • a

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B. PHOTOGRAPHIC MATERIALS

The electron sensitivity of photographic materials is many times greater than that of photoresist. Depending upon the emulsion and photographic process used, the sensitivity or spepd in a wide range lies at about 10-10 c / c m ^ for a density D = 1, where this value also depends upon the incident electron energy (Burge et al 1968).

The photographic emulsions a r e normally between 10 and 30 /im thick. The emulsion is a suspension of tiny crystals of silverhalide in gelatin. The mean size of these crystals varies from 0.1 pm or even less to about 1 ^m, depending upon the emulsion selected. The required energy for full exposure of such a crystal according to Valentine (1966) is about 30 eV in the c a s e of light (many photons) and about 500 eV (single-hit process) for electrons. In view of the incident electron energy and application of the Poisson distribution function in the single electron hit process it follows that the density of the processed emulsion is directly proportional to the exposure (the current density) in the useful density range up to a value of about 25^ of the saturation density. The contrast of an emulsion depends then only upon the density. When in that case exposed to the same density, all emulsions will have exactly the same contrast.

The photographic graininess is a statistical phenomenon and is several orders of magnitude larger than the size of the grains of silver that form the image, it being caused by imperfections in the photographic p r o c e s s . The effect of electron diffusion in the photo-graphic emulsion is negligible as compared with graininess.

Kodak panatomic X photographic film is used as it enables the experiments and processing of the film to be carried out in red light. Filament light of the electron gun does not influence the experimen-tal results. The six different screen combinations are depicted in fig. IV. 4. The experimental conditions a r e high voltage of 20 keV, a surface speed of 2.1 m / s at 300 revolutions per minute of the exposure cylinder, a normal setting of all adjustments of the lens c u r -rents which is used for the exposure of photoresists but with a con-denser lens aperture of 20 ^lm (in fig. II. 5-lC). The modulation i s a saw-tooth voltage signal.

The possibility to use silver halide films is useful for testing of the electron optics and s c r e e n logics, the modulation signal from a read-out system or colour scanner and for moiré pattern investigation.

s

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•1 •1 •1 •1 •1 •1 •1 •1 J Fig. IV

Electron exposure of a photographic film with a sweep modulation signal. The 6 different s c r e e n s have the same magnification as the basic screen of 85 lines p e r cm. 20 keV, 2.1 m / s , 10-^ C/cm^. From top to bottom the screens a r e 1:», 1:1, 1:2L, 1:2R, 1:3L, 1:3R.