Vacuum deposition by pulsed laser radiation: Plasma composition, deposition techniques and thin film properties

VACUUM DEPOSITION BY PULSED

LASER RADIATION: PLASMA I COMPOSITION,

DEPOSITION TECHNIQUES

AND

THIN FILM PROPERTIES

Liang Shi

r

TR diss

LASER RADIATION: PLASMA COMPOSITION,

DEPOSITION TECHNIQUES AND THIN FILM PROPERTIES

PROEFSCHRIFT

Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus,

Prof.drs. P.A. Schenck in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen

op dinsdag 17 oktober 1989 te 14.00 uur

door

Liang Shi natuurkundig ingenieur

geboren te Shanghai, Volksrepubliek China

TR diss

1756

Dit proefschrift is goedgekeurd door de promotor Prof.dr.ir. HJ. Frankena

Het in dit proefschrift beschreven onderzoek is verricht in het kader van een samenwerkingsproject tussen de Vakgroep Optica van de Faculteit der Technische Natuurkunde, Technische Universiteit Delft, en Optische Industrie De Oude Delft B.V. Het is gefinancierd door De Oude Delft met een subsidie van het Ministerie van Economische Zaken.

behorende bij.het proefschrift "Vacuum Deposition by Pulsed Laser Radiation: Plasma Composition, Deposition Techniques and Thin Film Properties" van Liang Shi

1

-De uitspraak van C.Cali et a l \ dat bij een CO2 laser een pulsduur van 0.1-1 ms kort genoeg is om ontleding bij het laser verdampen van chemische verbindingen te voorkomen, is onjuist^, 1. C.Cali et al, Appl.Opt., 15, 1327 (1976)

2. Hoofdstukken 3 en 4 van d i t p r o e f s c h r i f t . 2

-Gezien het feit dat er een groeiende behoefte bestaat aan geschikte depositietechnieken om dunne lagen van steeds uiteenlopende*^ materialen te vervaardigen, is het van groot belang om de PLISD techniek en varianten hiervan, zoals beschreven in dit proefschrift, verder te onderzoeken en ontwikkelen.

3

-Voor het zelfstandig beoefenen van moderne technologie is de

handvaardigheid van de beoefenaar even onmisbaar als zijn denkwerk. 4

-In de huidige z.g. "no-nonsense" atmosfeer moet er juist meer begrip en ruimte komen voor mensen die niet alleen rationeel redeneren, maar vooral ook door hun gezonde intuitie tot uitspraken komen, waarover vóór de realisering moeilijk te argumenteren valt. Ook academische en industrële managers en onderzoekers dienen zich hiervoor open te stellen.

5

-Bij het opstellen van nieuwe onderzoekprojecten zou toepassing van een geschikte voorstellingsmethode de systematiek en de creativiteit van de beoefenaren sterk kunnen verbeteren (een voorbeeld van zo'n methode is een mathematische formulering met matrices of tensoren, waarvan de elementen de methoden, instrumentele mogelijkheden en vraagstellingen weergeven).

6

-Een belangrijke verdienste voor de samenleving van de methodiek die de fysica volgt is het leren denken en redeneren op basis van

fundamentele aspecten, hetgeen ook toe te passen is op niet-fysische processen van materiële en immateriële aard. Dit is een goede reden voor de samenleving om het kennisniveau van de fysica op een hoog peil te houden en om een zo groot mogelijke groep mensen van deze methodiek kennis te laten nemen.

7

-Er valt weinig voordeel te behalen uit zaken waar bijna iedereen grote belangstelling voor toont; "Europa 1992" is daarvan een voorbeeld. Daarom is het onverstandig voor bedrijven binnen de EG om hun activiteiten thans bijna uitsluitend op West-Europa te richten.

8

-In een periode van snelle maatschappelijke en technologische

ontwikkeling zal een duidelijke kloof ontstaan tussen academici van verschillende generaties; het komt een organisatie ten goede niet alleen te trachten die kloof kleiner te maken, maar ook een omgeving te scheppen waarin die groepen als aanvulling op elkaar kunnen worden ingezet.

9

-Hoe meer een systeem gevuld is met informatie, des te moeilijker wordt het om het aan nieuwe situaties aan te passen; daarvoor is een dynamische en flexibele besturing nodig. Dit geldt in gelijke mate voor geheugens van computers, experts in bepaalde vakgebieden en lang

bestaande culturen.

10

-Uitgebreidere kinderopvang, op te zetten in verband met het scheppen van kansen voor vrouwen om buitenshuis te gaan werken, beidt ook de mogelijkheid om peuters in "team-verband" te laten opgroeien. Dit is van groot belang omdat de eigenschap "het kunnen werken in team-verband" in veel vacature beschrijvingen voorkomt.

11

-Een nieuwe generatie van producten of productiemethoden, die in staat is in een zeer korte tijd de vorige generatie te verdringen (bijvoorbeeld CD's), moet in ieder geval aan één voorwaarde voldoen: wat de oude kan, moet de nieuwe ook kunnen.

12

-Bestuurlijke beslissingen kunnen niet altijd uitsluitend op rationele overwegingen genomen worden, daarom kan een menselijke bestuurder niet door een computer met een hoge mate van kunstmatige intelligentie worden vervangen.

alchemist as a physicist "

Prof. S. Tolansky

aan Nai-Ping aan onze ouders

VOORWOORD

Het werk, beschreven in dit proefschrift, geschiedde binnen een samenwerkingsproject tussen de Vakgroep Optica van de faculteit der Technische Natuurkunde van de Technische

Universiteit Delft en B.V. Optische Industrie De Oude Delft. Het experimentele werk is uitgevoerd in het research

laboratorium van De Oude Delft. Op deze plaats wil ik allen bedanken, die in verschillende mate hebben bijgedragen aan de totstandkoming van dit proefschrift. Speciaal zou ik de

volgende personen willen noemen:

Allereerst ben ik dank verschuldigd aan mijn promotor,

Prof.dr.ir. H.J. Frankena, voor zijn intensieve begeleiding en de wijze waarop hij mijn manuscripten evalueerde en

verbeterde. De directieleden van B.V. Optische Industrie De Oude Delft wil ik dank zeggen voor de steun aan dit project en de ruimte en vrijheid die ik kreeg voor dit onderzoek.

Binnen de organisatie van De Oude Delft gaat mijn

erkentelijkheid speciaal uit naar ir. H. Mulder voor vele waardevolle discussies en adviezen gedurende het hele onderzoek, dr.ir. M.J.M. Beerlage voor zijn stimuleren en zijn kritisch toezicht in de beginfase van het project, en dr. H.J. van Elburg voor vele nuttige discussies gedurende latere fasen van het onderzoek. Verder dank ik ir. J. van Spijker en ing. J.J. Houtkamp voor een korte, maar prettige samenwerking.

Van de vakgroep Optica van de TU Delft wil ik speciaal ir. C.J. van der Laan bedanken voor vele vakkundige

discussies en adviezen, en de heer A.Kuntze, die t.b.v. het onderzoek oxyde lagen van hoge kwaliteit leverde, die als referentie hebben gediend.

Geen technisch onderzoek is mogelijk zonder technische

bijstand. In dit opzicht ben ik bovenal dank verschuldigd aan ing. H. Plum, zijn veelzijdige technische ondersteuning was onontbeerlijk. Voorts gaat mijn erkentelijkheid uit naar de heren A. Kaiser, J.M.J. v.d.Lubbe, M.C. Welling,

H.J. Hendriksen, J.J. Kaiser, ir. J. Leeuwenburgh,

P.M. Verbarendse, R.J. Buining, G.E.R.M. Streng en ing. A. de Geus, voor hun technische hulp. Mw. M.Th. van Asperen en Mw. J.F.M. Bergfeld bedank ik voor secretariële hulp.

Tenslotte heeft dr. A. van Veen van IRI (TUD) mijn onderzoek ondersteund door het uitlenen van de ionenbron waarmee de massa-energie analyzer kon worden geijkt.

Het onderzoek werd gedeeltelijk gesubsidieerd door het Ministerie van Economische Zaken.

1 . INTRODUCTION

1.1 Thin films and deposition techniques 1

1.2 Thermal evaporation 2 1.3 Sputtering deposition technique 5

1.4 Laser induced evaporation 5 1.5 Laser induced sputtering 9 1.6 Scope of our investigation

and outline of the thesis 11

2. EFFECTS PRODUCED BY THE ABSORPTION OF PULSED LASER RADIATION ON A SOLID TARGET SURFACE

2.1 Introduction 15 2.2 Heating without phase change 19

2.3 Laser-induced melting and vaporization 21

2.4 Laser-induced electron emission 27 2.5 Laser-induced ion emission 28

2.6 Plasma production 29 2.7 Particle emission created by

non-thermal processes 29

3. INSTRUMENTATION FOR THE STUDY OF THE KINETIC ENERGY DISTRIBUTION AND MASS COMPOSITION OF POSITIVE IONS PRODUCED BY PULSED LASER EVAPORATION OF SOLID MATERIALS (PAPER)

3.1 Introduction 32 3.2 Systems description 35

3.3 System performance 45

4. MASS COMPOSITION AND ION ENERGY DISTRIBUTION IN PLASMAS PRODUCED BY PULSED LASER

EVAPORATION OF SOLID MATERIALS (PAPER)

4.1 Introduction 55 4.2 Experimental setup 59

4.3 Results and discussion 61

5. MECHANICAL AND OPTICAL PROPERTIES OF OXIDE THIN FILMS DEPOSITED BY

PULSED LASER REACTIVE EVAPORATION (PAPER)

5.1 Introduction 67 5.2 Experiments 71 5.3 Results and discussion 79

6. PLISD: A NEW HIGH-VACUUM SPUTTERING

TECHNIQUE FOR THIN FILM DEPOSITION (PAPER)

6.1 Introduction ' 8 9

6.2 Experimental setup '93 6.3 Results and discussion 95

7. CONCLUDING REMARKS 102

SAMENVATTING 106

- L.Shi, H.J.Frankena and H.Mulder, "Instrumentation for the Study of the Kinetic Energy Distribution and Mass

Composition of' Particles Produced by Pulsed Laser

Evaporation of Solid Materials", Rev.Sci.Instrum. 60 (3), 332, (1989)

- L.Shi. H.J.Frankena and H.J.van Elburg, "Mass Composition and Ion Energy Distribution in Plasmas Produced by Pulsed Laser Evaporation of Solid Materials", accepted for

publication by Vacuum

- L.Shi, "PLISD: A New High-Vacuum Sputtering Technique for Thin Film Deposition", accepted for publication by Vacuum

- L.Shi, H.J.Frankena and H.Mulder, "Mechanical and Optical Properties of Oxide Thin Films Deposited by Pulsed Laser Reactive Evaporation", accepted for publication by Vacuum

CHAPTER 1

INTRODUCTION

1.1 THIN FILMS AND DEPOSITION TECHNIQUES

Generally, we refer' to a thin layer of solid material on an underlying supporting substrate (usually another solid material), as a thin film. Its physical thickness ranges from several tenths of a nanometer, which corresponds to a few monolayers of atoms or molecules, to several micrometers. Thin films of various materials on solid substrates are employed to achieve a desired change in surface properties of the substrates either as passive or as active devices to perform certain tasks. Here, we concentrate on thin films for optical purposes. Examples of the first 'category are various thin films on glass substrates, like decorative layers and antireflection coatings. To the second category belong metallic and dielectric mirrors,, multilayer interference (optical) filters and coating's for the absorption of solar radiation. Examples of thin film's' for electrical and opto-electrical applications are thin film resistors and capacitors,

coatings for magnetic recording, photocathodes and layers employed' in integrated opto-electrohic components. Thin films are widely applied and play a.vital role in science and many sectors of present-day technology.

2

The wide variety of thin film applications requires that films of a large number of different materials have to be deposited on various substrates in a productive and reproducible manner. Along with scientific research, several deposition techniques have been developed to produce films with the required properties. With reference to their basic mechanism, most of the thin fil-m deposition techniques can be divided into three categories:

- electro-deposition,

- chemical vapour deposition,

- physical vapour deposition in vacuum.

The choice of a particular 'deposition technique depends on several factors, e.g. the material to be deposited, the nature of the substrate, the desired film properties arid the required thickness. For optical applications, the technology of physical vapour deposition is most widely used to deposit thin films. The two major techniques in this category are thermal evaporation and cathödic sputtering. An extensive tre'atmerit of these techniques can be found in references 1 to 4 listed at the end of this chapter. The present thesis deals with pulsed laser induced evaporation and sputtering for thin film deposition. These techniques are to be ranked in the category of physical vapour deposition. In the following sections we first give a brief outline of the well-established technologies (thermal evaporation and sputtering), after which an introduction to

laser-based deposition techniques will be presented.

1.2 THERMAL EVAPORATION

in Fig.1.1. The deposition process of thin films by thermal evaporation consists of several distinguishable steps:

- Transition of a condensed phase, which usually is solid, into the gaseous state by heating the source material in a resistance-heated vessel or, with an electron-gun;

- Vapour traversing the space between the evaporation source and the substrate at a reduced gas pressure (in the order of 1 0- 4 Pa for non-reactive, or 10~2 Pa in cases of

reactive evaporation)-;

- Condensation of the vapour onto the substrate upon arrival (if required, at an elevated temperature, e.g. 300°C).

According to the way in 'which the source material is removed, thermal evaporation is an equilibrium process involving the thermodynamics of a phase transition; the equilibrium vapour pressure of the source material is an essential parameter. If a compound or an alloy is used as the source material, the evaporation is usually accompanied by dissociation or by fractionated- evaporation. The formation of a thin film on the substrate is a complex process involving condensation, nucleation and growth phenomena. Besides these" purely physical phenomena it may also involve chemical reactions.

Thermal evaporation, especially electron-gun evaporation, is a versatile technique able to produce thin films of different materials such as metals, oxides, hal ides and sulphides. It offers usually high evaporation rates and pure films in which only few foreign gas atoms are included. Reactive evaporation, two-source evaporation and flash evaporation (for resistance-heated evaporators only) are special techniques used to obtain thin films with desired stoichiometry.

k vacuum chamber e beam V f k N\\ | /source ■naterial pump

Fig.1.1 Schematic layout of a vacuum thermal evaporation apparatus Gas inlet valve Sputtered material High voltage Grounded screen Target Glow discharge Substrates

1.3 SPUTTERING DEPOSITION TECHNIQUES

Fig.1.2 shows a simplified set-up of a sputtering system. In contrast to thermal evaporation, sputtering is a nonthermal process. Instead of heating the source material, sputtering produces vapour by bombarding the source material (target) with energetic ions (several keV's). As a result, atoms (or molecules) are ejected from the target surface (Fig.1.3). Subsequently the ejected particles strike the substrate and adhere, gradually forming a thin film. The bombarding ions are usually produced from a glow discharge of a noble gas (mostly Ar), or of a mixture (Ar and O2) in the case of reactive sputtering. The sputtering deposition technique has many variants according to the way in which the glow discharge is created and supported (see also Chapter 6). For practical applications, the most important feature of sputtering deposition is that it usually yields thin films with better mechanical properties than the thermal evaporation, like a higher adhesion strength and a

lower porosity. This is presumably due to the higher kinetic energy of the ejected particles (1-40 eV, compared with <0.5 eV in thermal evaporation) and to reflected primary ions in the sputtering process, leading to enhanced diffusivity and mobility of the condensing particles, as well as to sputtered cleaning of the film during the whole formation process. Compared with thermal evaporation, the main disadvantage of sputtering is the usually lower deposition rate.

1.4 LASER INDUCED EVAPORATION

Another technique for physical vapour deposition results, when the source material is evaporated by means of high

Ion

Gas

Solid

Fig.1.3 Collision of a highly energetic ion with a solid material during ion bombardment yields the

ejection of several atoms in a sputtering process

(from Ref.4) .

power laser radiation. Depending on the parameters describing the laser light, the evaporation process can be either thermal, nonthermal, or a mixture of the two. This technique was first reported by Smith and Turner in 1965^. They used a pulsed ruby laser (both in Q-switched and normal mode) to evaporate a number of elements and compounds in vacuum for film deposition purposes. Fig.1.4 shows a typical

laser evaporation system designed for thin film deposition. It consists of a continuous wave (CW) or pulsed laser and focusing optics, a vacuum chamber with a pumping system and an optical window allowing the laser beam to enter'the chamber. Inside the chamber the laser beam is directed towards a movable boat carrying the source material. A thickness monitor and a substrate holder with a variable temperature controller allow control over the deposition process.

In principle, the deposition technique based upon laser-induced evaporation has a number of distinct features and advantages. These can be summarized as follows:

- It yields high-purity films due to the absence of contamination by the evaporator and to reduction of the outgassing within the chamber. This is because the amount of power delivered to the target is small and localized, causing less thermal radiation and temperature increase. - The absence of high-temperature heating elements allows various gases to be introduced into the chamber at a high pressure without the danger of burning out the heating elements and/or initiating an electrical discharge. This feature is of particular interest for reactive deposition. - The setup is very flexible. Multiple sources can-be evaporated ' simultaneously by splitting and focusing the laser beam onto separate sources or ' by exposing individual

8

targets to the laser beam sequentially.

In the case of pulsed laser evaporation, some additional features are of considerable importance^! :

- The conditions for congruent evaporation are usually automatically met when using a multi-component source. The stoichiometry can be preserved during the evaporation process.

- Pulsed laser evaporation is usually accompanied by the production of highly energetic particles (ions and excited neutrals) which, similar to the case of sputtering, affect the properties of the deposited film positively. The energies of positive ions created during the pulsed laser evaporation range from several eV's up to several hundreds of eV's. In contrast, the energy of the evaporant during thermal evaporation is, according to the thermodynamics of the process (in the order of kT), no more than a few tenths of one eV.

Since this novel technique was conceived, much research work, in this area has been done by investigators using various types of high power lasers6-40. The majority of the

literature on film deposition by means of laser light is reviewed by Cheung and Sankur in a recently published paper (1988)41. The research work in this area has had a sporadic

character, reflected in the fact that this technique does not have a common name. Laser-assisted deposition and annealing (LADA), pulsed laser ablation and laser molecular beam epitaxy are among many other- names used in the

literature for the same technique (see also Section 1.5). In general, the investigation in this area has, like most of the work on thin films, a phenomeno.logical character and is oriented to practical results. Fundamental research4^.43 i n

the field of the interaction between laser radiation and solid materials has provided the necessary insight and a better understanding of the laser evaporation process (see Chapter 2). Due to the complexity of the whole process and the diversity of source materials and lasers, however, a satisfactory model linking the (pulsed) evaporation process and the properties of the deposited films is still not available.

1.5 LASER INDUCED SPUTTERING

To our knowledge, the method to deposit thin films with a sputtering process (see Section 1.3) initiated by a pulsed laser, is entirely new. We call it Pulsed Laser Induced Sputtering Deposition (PLISD). It is described in detail in Chapter 6 and some of its physical background in Chapter 2. Our experimental setup is shown in Fig.1.5. Instead of a glow discharge, a laser-generated plasma originating from the ion-generating target is used as the source of bombarding ions. Through sputtering, this generates the deposition atoms from the sputtering target. This new deposition technique eliminates the use of a working gas as required in normal sputtering deposition processes. Therefore, it can be carried out in an ultra high vacuum environment. It is to be expected that this unique feature and the use of ions of a solid material instead of a noble gas will have important effects on the deposited films. The possible advantages of this novel sputtering technique still have to be fully investigated and exploited.

Our deposition technique based on pulsed laser induced sputtering differs fundamentally from those published in the

o

TEA CO2 LASER

SPUTTERING" POWER SUPPLY +. I CHARGE STORAGE = p CAPACITOR

1

Fig.1.5 Experimental setup for Pulsed Laser Induced Sputtering Deposition

sputtering44< laser-induced sputtering45 and laser-assisted

sputtering4^_ ^s mentioned in the previous section, these

are simply different names for laser-induced evaporation. .

1.6 SCOPE OF OUR INVESTIGATION AND OUTLINE OF THE THESIS

The present investigation deals with pulsed laser evaporation. Its nonthermal character- is expected to have some effects (see Section 1.4) on- the deposited films which are not obtainable by the thermal ,evaporation technique including CW. laser evaporation. A. pulsed TEA CO2 laser is used as the evaporation source. The following two related aspects (Chapter 4) are investigated experimentally:

- The mass composition and the energy distribution of the positive ions emerging from laser generated plasmas for a number of different target materials. -The influence of the

laser parameters on these quantities is evaluated.

- The mechanical and the optical properties ,, of Si.02, AI2Q3 and TiÜ2 films deposited by, pulsed laser evaporation, as well as by thermal evaporation (electron gun). The properties of

films deposited by these two different methods are compared.

To our knowledge, quantitative experimental .results .as presented in this thesis concerning the former aspects have not yet been extensively reported.

Beside these aspects we • report in this thesis a new thin film deposition technique based on the laser-induced sputtering principle. . < • • 1. . '.- ■ . ;• . ■ .

The thesis is divided into seven chapters.-Chapter 1 gives a brief introduction to_ ■ conventional . physical vapour

12

deposition techniques and to deposition techniques based upon laser-induced evaporation and sputtering. The scope of the present investigation is outlined. Chapter 2 is devoted to the fundamental aspects of the laser evaporation process, which are useful for the understanding of the chapters to follow. Chapter 3 describes the measuring system forthe mass composition and the energy distribution of positive

ions generated by laser pulse. The results of these measurements for a number of different target materials are given in Chapter 4. In Chapter 5, the deposition conditions, the film properties and their measuring methods are described for SiC>2, Al2°3 a n d T i 02 films. Comparisons are

made for these films deposited by the laser and thermal evaporation (electron gun).

■Chapter 6 is devoted to the new deposition technique (PLISD), which was conceived in the course of the laser evaporation experiments described in this thesis. Closing remarks concerning practical aspects, outlook and suggestions for the laser-induced evaporation and sputtering techniques are summarized in Chapter 7.

REFERENCES

1 L.Holland, Vacuum Deposition of Thin Films, Chapman & Hall Ltd. London 1, 1956

2 L.I.Maissel and R.Glang, edi. Handbook óf Thin Film Technology, McGRAW-HILL book company, 1970

3 R.J.Hill edi. Physical Vapour Deposition, Temescal, Berkeley, 1986

5 H.M.Smith and A.F.Turner, Appl.Opt., 4, 147 (1965). 6 G.Groh, J.Appl.Phys., 39, 5804 (1968)

7 H.Schwarz and H.A.Tourtellotte, J.Vac.Sci.Tech., 6, 373 (1968)

8 G.Hass and J.B.Ramsey, Appl.Opt., 8, 1115 (1969) 9 D.K.Hohnke et al., J.Appl.Phys., 42, 2487 (1971)

10 M.S.Hess and J.F.Milkosky, J.Appl.Phys., 43, 4680 (1972) 11 P.S.P.Wei et al., J.Chem.Phys., 62, 3050 (1975)

12 C.Cali et al., Appl.Opt., 15, 1327 (1976)

13 S.V.Gaponov et al., Sov.Phys.Sol id.State, 19, 1736 (1977)

14 N.I.Pozdnyak and V.S.Mylnikov. Sov.Phys.Tech.Phys., 23, 492 (1978)

15 Y.A.Bykovskii et al., Sov.Phys.Tech.Phys., 23. 578 (1978) 16 T.F.Deutsch et al., Appl.Phys.Lett., 35, 175 (1979)

17 N.I.Pozdnyak and V.S.Mylnikov, Sov.Phys.Tech.Phys., 24, 107 (1979)

18 M.Hanabusa and A.Namiki, Appl.Phys.Lett., 35, 626 (1979) 19 I.E.Morichev and V.P.Savinov, Sov.J.Opt.Tech., 47, 118

(1980)

20 M.Hanabusa et al., Appl.Phys.Lett., 38, 385 (1981) 21 D.Dimitrov et al., J.Mater.Sci.Lett., 1, 334 (1982) 22 O.V.Luksha et al., Inorg.Mater., 18, 231 (1982) 23 S.Fujimori et al., Thin Solid Films, 92, 71 (1982) 24 J.T.Cheung and D.T.Cheung, J.Vac.Sci.Tech.. 21, 182

(1982)

25 J.T.Cheung and T.Magee, J.Vav.Sci.Tech. Al, 1604 (1983) 26 H.Sankur and M.E.Motamedi, IEEE Ultrason. Symp., Atlanta

(nov.1983)

27 H.Sankur and J.T.Cheung, J.Vac.Sci.Tech. Al, 1806 (1983) 28 J.T.Cheung et al., Appl.Phys.Lett., 43, 462 (1983)

29 T.Nakayama, Surf.Sci., 113, 101 (1983)

30 J.J.Dubowski and F.Williams, Appl.Phys.Lett., 44, 339 (1984)

14

31 H.Sankur and W.H.Southwell, Appl.Opt.. 23, 2770 (1984) 32 H.Sankur, Mater.Res.Soc.Symp.Proc., 29, 373 (1984)

33 B.Pashraakov et al.v J.Mater.Res.Sci.Lett., 4, 442 (1985) 34 M.T.Baleva et al., J.Mater.Sci.Lett., 4, 442 (1986)' 35 J.T.Cheung et al . ,■ J .Vac .Sci .Tech. , A4, 2086 (1986) 36 H.Sankur et al.', J.Vac.Sci.Tech., A5, 2869 (1987) 37 D.Dijkkamp et al . , ^Appl .Phys .Lett. 51, '619 (1987-) 38 J.Narajan et al., Appl.Phys.Lett., 51, 1845 (1987)

39 L.Lynds et al., Am.Vac.Soc.,34th 'Nat 1.Symp., Anaheim, CA, 1987

40 S.G.Hansen and Robitaille, Appl.Phys.Lett., 50, 359 (1987)

41 J.T.Cheung and H.Sankur. CRC Cri.Rev.Sol id State & Mater.Sci., 15, 63 (1988)

42 J.F.Ready, "Effects of High-Power Laser Radiation", 'Academic Press, New York 1971

43 J.F.Eloy, "Power Lasers", Ellis Horwood Limited, 1987 44 B/Pashmakov et al., J.Mater.Res.Sci.Lett., 4, 442 (1985) 45 J.Reif et al., Appl.Phys.Lett., 49, 770 (1986)

CHAPTER 2

EFFECTS PRODUCED BY THE ABSORPTION OF PULSED LASER RADIATION AT A SOLID TARGET SURFACE

2.1 INTRODUCTION

The interaction between laser radiation and materials is a field with many aspects and a wide range of applications. In this chapter we briefly consider the physical phenomena that occur when high-power laser radiation interacts with a solid target. This subject has been studied extensively since the advent of the laser, and it has been reviewed by some authors, like Ready1 and, more recently, Eloy^. The

material presented in this chapter is derived primarily from Ready's book. It provides the necessary background for the next chapters in this thesis.

The parameters characterizing the laser radiation and the physical and chemical properties of the target determine which phenomena, predominate their interaction. Table 2.1 gives a summary of the most important phenomena that will occur during the laser-material interaction for various laser power density regions. In view of the scope of this thesis, the emphasis in this chapter will be on the phenomena occurring for laser power densities at the target surface ranging from 1x10^ Wcm-^ up to 1x10^ Wcm-2. This is

the range typically obtainable from a commercially available TEA CO2 laser with a pulse energy of several Joules, a

Table 2.1 Summary of Phenomena in Lasei—Induced Particle Emission (from Ref.1)

Phenomenon Dominant mechanism(s) Typical laser and irradiance (W cm- 1)

Electron emission Electron emission Ion emission Ion pulse shapes High-energy ions Directed expansion Neutral molecule emission Plasma production

High-velocity plasma expansion High electron densities Highly ionized species X-ray emission Charge collection Pressure pulses on target Neutron production Plasma heating and expansion High plasma temperatures Electron emission from

dielectrics

Thermionic emission ." Multiphoton effect Thermionic emission Displacement current effects Expansion kinetics Expansion kinetics Thermal desorption

Vaporization and thermal ionization Heating by absorption of laser light Thermal ionization of blowoff Thermal ionization of blowoff Deexcitation processes in hot plasma Expansion kinetics of plasma Recoil of blowoff material Thermonuclear fusion

Inverse Bremsstrahlung and gas dynamics Absorption of light in plasma

Thermionic emission

Normal pulse ruby: 10s—10" G-switched ruby: 10' Nd-glass or GaAs: 10 6-switched ruby: 10' G-switched ruby: lO'-lO» G-switched ruby: to 10" G-switched ruby: 10'-10' G-switched ruby: 10'

G-switched ruby and Nd-glass: I0'-I0" O-switched ruby and Nd-glass: ÏO'-IO" G-switched ruby: ÏO'-IO1»

G-switched ruby and Nd-glass: to 10"

G-switched ruby and Nd-glass and picosecond Nd-glass: 10"-1013

G-switched ruby: 10"-10" G-switched ruby: ÏO'-IO"

Q-switched or picosecond Nd-glass: IC'-IO" analysis

O-switched and picosecond Nd-glass and ruby 10*-10" Normal pulse ruby: 10s; Q-switched ruby: 10"

temporal pulse length in the order of a few hundreds of nanoseconds and a beam spot size between several mm2 and one

cm2. As a consequence of reflection, the laser irradiance

absorbed by the target surface will be less than that of the incident beam. If we limit our considerations to targets opaque for 10.6 yon wavelength (CO2 laser), the energy

balance of the interaction can be expressed by

Ea - Eo - Er,

where Ea is the laser (photon) energy absorbed in the material, Eo the laser energy incident on the material, and Er the laser energy reflected from the target surface.

In any discussion of the energy balance, it is important to be aware of two effects associated with the interaction between a high-power laser beam and a target:

- the presence of the vaporized target material and the plasma generated by the laser pulse; especially their absorption and reflection of the laser pulse energy,

- the temporal change of the reflectivity of a metallic target for the laser pulse, resulting from the modulation of the free carrier density and the phase change of the target surface.

In general, the interaction between a high-power laser pulse and a target is a complex process and involves many different physical phenomena. In the sections 2.2-2.7, we consider the most important of them separately. If not stated otherwise, the presented numerical results are valid for metals.

18

500 2 0 nsec

10 _{-,,. . ' 0 \}

10-OEPTH (cm)

Fig.2.la Calculated temperature rise as a function of depth, with time as a parameter, caused by

absorption of a Q-switched laser pulse in copper (from Ref.1) ABSORBE0 FLUX DENSITY (WATTS/CM2) 1.5 xlO7 I.OxlO7 0.5 xlO7 /^^^ - ' . ° l0 20 30 ;._ TIME (NANOSECONDS)

Fig.2.lb Laser pulse shape used for calculation (from

2.2 HEATING WITHOUT PHASE CHANGE

In this section we consider the temperature rise, generated by the absorption of laser energy, below the

level that causes melting or vaporization of the target surface. For absorbing metallic surfaces, the mechanism for the temperature rise is, that electrons absorb photons and collide with lattice phonons and other electrons. These are the same collision processes which govern the transfer of heat. The energy absorbed by an electron will be distributed and passed on to the lattice at a time scale in the order of 10~^3 s Therefore, for a laser pulse lasting longer than

picoseconds, the energy transport occurs so rapidly (as compared to the time scale of the pulse) that the optical energy can be regarded as being turned into heat

instantaneously at the point where the light was absorbed. The above mentioned arguments justify the use of the

temperature as an entity and the usual equations for heat flow.

In order to simplify the mathematical treatment, we assume that the thermal properties of the absorbing material are independent of the temperature and that the energy lost from the surface through réradiation or convection is negligible. With these assumptions, the differential equation for heat flow in a semi-infinite slab of material with a boundary at the plane z=0 on which laser radiation is incident is1

VT(x,y,z,t)-(l/k)U(x,y,z,t)/H A(x,y, z, t)/K,

where T is the absolute temperature as a function of position (x,y,z) and time (t), k is the thermal diffusivity, K the thermal conductivity, and A the heat production per unit volume per unit time as a function of position and

20 10 5 o 10' 10" 10 W/CM SURFACE VAPORIZATION BEGINS SURFACE VAPORIZATION BEGINS 10 W/CM SURFACE VAPORIZATION BEGINS 10"' 10'5 10 10 TIME (SECONDS) 10 10

F i g . 2 . 2 Melted depth in yellow brass as a function of time for various laser flux densities (from Ref.1)

Table 2.2 Time to reach Vaporization Temperature, tv (from Ref.1) Metal Bi Cd Pb Zn Mg Sn Ni Fe Al Mo Cu W 10'

Laser flux density (W cm-*) 10s 2.460 msec 24.6 ;tsec 8.970 11.770 12.770 89.7 117.7 127.7 245.1 599.8 1.842 msec 1.855 2.666 5.557 8.26 10.46 10» 0.246 /isec 0.897 1.177 1.277 2.451 5.998 18.415 18.550 26.66 55.57 82.60 104.61 10' 0.060 fisec 0.184 0.186 0.267 0.556 0.826 1.046

time. The initial condition may be T(x,y,z,0)=To, where To equals the temperature of the surroundings at t=0. The boundary conditions are that the temperature approaches zero as z approaches infinity and that there is no heat flux across the surface zm0.

The problem will be simplified further, without any sacrifice of practical significance, if we regard the problem as one-dimensional, i.e. by assuming that the transverse dimensions of the laser beam are large compared to the depth to which heat is conducted during the passage of the laser pulse. Ready has given calculated results for a few cases in which some additional assumptions are made. Fig.2.1 shows results for one case with the assumptions that the absorption coefficient of the material is large, and that the reflected laser energy is not a temporal function of the incident laser power density. Recent studies^ show, however, that the reflectivities of materials vary considerably during the laser pulse as a consequence of the time dependence of the free carrier density near the surface (within the skin depth).

The calculations based upon the model given above have been evaluated experimentally using various techniques. The results (e.q. see Figs.2.7 and 2.8 and Ref.1) show that at least qualitative agreement exists between the calculations and the experiments.

2.3. LASER-INDUCED MELTING AND VAPORIZATION

We now turn to a higher regime of absorbed flux densities and consider melting and vaporization of surfaces absorbing laser radiation. The situation of melting without

22 • ■ y ■ ' ■ Bi Ni — Fe — lol Cd Zn Pb A l / w / lo) i Ë ' 0 . -■ . • -Ib) 2 nA B iA \ Mg S Pb-v \ \kL r Fe \ V w ^M„ wN i Ib) 150 . - 2 0 0 250 500 ENERGY OENSITY ( j W ) '6000 8000 10000 12,000 14,000 16,000 ENERGY. OENSITY (J/cm2)

Fig.2.3 Calculated depth of hole vaporized by a 700 us duration of absorbed energy per unit area for various metals: (a) low energy per unit area;

vaporization can only be reached within a fairly narrow range of laser parameters. For calculation of a simple estimate for the melting depth, one may use the method mentioned in the Section 2.2 to calculate the maximum depth at which the melting temperature is exceeded. It is assumed that no difference occurs in the thermal properties of the melted and the solid material, which is a reasonable assumption for metals. Fig.2.2 shows the calculated results for yellow, brass. It indicates that vaporization will predominate melting for the laser parameters in our range of

interest (e.g. 1 us pulse duration and an absorbed laser power density above 10^ W c m- 2) . The melting depth will be in

the order of tens of a micrometer. In contrast to this," longer pulses of lower " power densities may cause much melting before surface vaporization takes place.

The existence of target material in the melted state may be the main cause of spitting, which occurs during laser evaporation of some materials. Then, material is removed in the liquid state rather than being vaporized. The moiten material is ejected by pressure due to surface vaporization of the material and, sometimes, by thermomechanical effects

(rapid thermal expansion).

For typical parameters, the surface of the material is raised to the vaporization temperature (boiling point) in a time tv given by1

tv = (TT/4) (Kf«c/F2) (Tv-To)2,

where K, c, f , F, Tv and To are the thermal conductivity,

heat capacity per unit niass, density, absorbed laser power density, the vaporization and initial temperatures, respectively. In Table 2.2, some values of tv are shown for

Fig.2.4 Experimental depth vaporized for various metals irradiated by a 700 \is laser pulse (from Ref .1)

O.lr IE O < 0.01 0.0001 2 0 0 0 4000 6000 8000 10.000 12.000 14.000 16.000 ENERGY OENSITY U/cmz)

0.001

1000 2 0 0 0 3 0 0 0 4 0 0 0 'ENERGY DENSITY ( J / c m2)

5000

Fig.2.5 Depth vaporized in carbon by a 700 ^is duration laser pulse as a function of absorbed energy per unit area. The solid line is a calculated curve.

•,smooth graphite; o ,rough graphite;

a number of metals and absorbed laser power densities.

Supposing that the material is exposed to a constant high-power laser pulse lasting much longer than tv, the rate of material removal will approach a steady state rate given by

vss=F/p[L+c(Tv-To)],

where L is the specific heat of vaporization per unit mass.

Experimental verification of the above mentioned relations has been carried out for various kinds of carbon and for some of the metals mentioned in Table 2.2 (Figs.2.3 to 2.5). It appears that the agreement between calculated and experimental results is much better for carbon than for the metals. This originates from the fact that a number of effects for which the theory does not account are virtually absent1 in the case of carbon. These effects include the

material removal by spitting and the loss of laser energy caused by reflection.

The melting and vaporization discussed above are considered to _ be ordinary thermal processes. This is justified when the laser pulses driving the process last long enough, i.e. at least about one millisecond. The power densities absorbed by the target material are in the regime below 10^ Wcm-2. For shorter laser pulses (ns) with higher

power densities (>107 W c m- 2) , the interaction between the

blown-off material and the laser pulse becomes important and will influence the interaction at the target surface considerably (e.g. by plasma formation).

26 LASER TRANSPARENT WINDOW BEAM SPLITTER LENS OSCILLOSCOPE COLLECTOR => ATTENUATOR

A

y

f—0

x

n

2.6 Typical experimental arrangement for observation of laser-produced electron emission (from Ref.1)

\A 10.000 - i 200 4 0 0 600 800 1000 1200 1400 1600 TIME U s » g . 2 . 7 C a l c u l a t e d and o b s e r v e d e l e c t r o n e m i s s i o n s i g n a l from Ta t a r g e t a t 1700K f o r i n d i c a t e d l a s e r p u l s e shape (from R e f . 1 )

2.4 LASER-INDUCED ELECTRON EMISSION

Electrons can be emitted under conditions where no or little damage to the surface occurs. The mechanism for this phenomenon can be either a thermionic or a photoemission effect, depending on the laser wavelength. For most target materials, the electron emission produced by a laser pulse in the visible or at IR wavelengths is of thermionic origin, described by Richardson's equation:

j = AT2exp(-$/kT),

where j is the current density,9 the work function of the target surface, T and k the temperature and Boltzmann's constant respectively, and A is a constant equal to

60.2 Acm-2deg-2 for many metals.

A typical experimental arrangement for investigating electron emission is illustrated in Fig.2.6. Fig.2.7 shows the calculated and observed electron emission produced by the indicated laser pulse from a pre-heated Ta target (1700 K ) . The good agreement shown in Fig.2.7 indicates that Richardson's equation is obeyed and that the emission occurs as a result of a surface temperature increase which can be calculated from classical heat transfer theory. Good agreement has also been found for short laser pulses (see Fig.2.8). Fig.2.8 shows the laser irradiance as a function of time and the time dependence of a thoriated tungsten target calculated from the experimental electron emission data, using Richardson's equation under the assumption that all the emission is purely thermionic.

The peak current density produced by even a modest laser pulse can be very high. Current densities as high as

28

10° Acm-^ were obtained from tantalum using a 0.3 J laser

pulse focused to a 0.1 mm diameter spot size. The energies of the electrons produced by a short laser pulse (Q-switched laser) were measured by a retarding potential method. These observations indicated that the electron emission peaks at two distinct energies, one in the order of 2 eV and the other at approximately 14.5 eV.

2.5 LASER INDUCED ION EMISSION

The range of laser power densities for laser induced ion emission without clear plasma formation is confined to 104

-106 Wcrn-2. According to the thermionic theory, a metallic

sample heated to a temperature in the order of 2000 K or higher will emit positive ions of the materials present as impurities at the surface and also of the parent metal itself. Like electron emission, the ion current emitted from a pure metal surface can be estimated from the Richardson-Smith equation

i+ = ApT2exp(-4>p/kT) .

where Ap is a constant and 4>p is the positive ion work

function, the values of which were experimentally obtained for Al and Cu (5.3 and 7.25 eV, respectively).

For experimental studies of the ion emission, a circuitry similar to that shown in Fig.2.6 was used (with the polarity of the collecting voltage reversed). Using the laser pulses mentioned in the previous section, ion current densities in the order of 10-3 Acm-2 were observed for a Ta surface and

2.6 PLASMA PRODUCTION

When the laser power density approaches 108 Wcm-2, plasma

formation will start to take place. It is produced by vaporization of some target material and subsequent absorption of laser energy. The difference between the phenomena described in this section and in Sections 2.3 and 2.4 arises because different ranges of laser power density and particle density are being considered. In front of the solid surface, the laser absorption takes place in two zones (see Fig.2.9): in the absorbing vapour (2) and in the plasma (3), where more ionised particles are created. The higher (>108 Wcm-2) irradiance will produce denser, hence

more absorbing blown-off material, leading to a stronger plasma formation. The absorption is caused by an inverse Bremsstrahlung process, which involves absorption of a photon by a free electron.

Laser generated plasmas have been investigated by many techniques, including optical interferometric measurements, optical spectroscopy, mass spectroscopy and charge collection measurements. In Chapters 3 and 4 we describe the techniques and the instrumentation for measuring the mass composition and the energy distribution of positive ions emerging from a laser generated plasmas for a number of different targets, including dielectric materials.

2.7 PARTICLE EMISSION CREATED BY NON-THERMAL PROCESSES

Besides thermal processes, such as thermal evaporation and thermionic emission of electrons and ions, there are also non-thermal processes causing particle emission from a target during the interaction between the material and the

1500 1000 500 TEMPERATURE: / " EXPERIMENTAL ~ 7 THEORETICAL^ / i / / 1 / \ / 1 / \ / * / \ / / / \ 1 11 > I I I I I I I I I I I I I I I I I I 1 1 1 11 1 II 1 h / 1' / by IS i i ■ v ^ \ " \ \ S LASER POWER V ' DENSITY i 0 20 40 60 80 100 TIME AFTER START OF LASER PULSE (NANOSECONDS)

Surface temperature of thoriated W as -a time, determined experimentally from electron emission data and also calculated using

indicated laser pulse shape

function of

the (from Ref.1)

Distance (2)

9 Diagram of the laser-solid material interaction and the phases that occur (from Ref.2)

laser radiation. For example, surface absorption of laser radiation can cause a sudden change in the electron density which prevents the penetration of the laser energy into the material. This results in a sudden appearance of high charge gradients, leading to a breaking of the chemical and physical bonds of the target material on the surface. Particle emission is also possible due to desorption stimulated by a shock wave, caused by pressure pulses due to either the recoil of the evaporating material or just surface heating. These effects are considered to be responsible to the spitting problem encountered in pulsed

laser evaporation (Chapter 7 ) .

REFERENCE

1 F.Ready, "Effects of High-Power Laser Radiation", Academic, New York, 1971

2 J.-F.Eloy, "Power Lasers" Ellis Horwood Limited, Chichester, 1987

32

CHAPTER 3

INSTRUMENTATION FOR THE STUDY OF THE KINETIC ENERGY DISTRIBUTION AND MASS COMPOSITION OF POSITIVE IONS PRODUCED BY PULSED LASER EVAPORATION OF SOLID MATERIALS

3.1. INTRODUCTION

The energy distribution and mass composition of particles which emerge in the pulsed laser evaporation of solid materials have been studied by many investigators^""^0. Until

now, most investigations in this field use Q-switched ruby or Nd-Yag lasers, yielding a very high laser power density on the target surface (>109 Wcm-^). The use of pulsed TEA

C02 lasers giving a medium power density (from 106 Wcm-^ up

10° Wcm~2), is increasing in fields like laser mass spectrometry^1 and laser-induced evaporation for thin film

deposition^' 13. Hence, it becomes relevant to produce experimental data on the energy distribution and mass composition of particles emitted by solid targets using this type of lasers. In thin film deposition, it is well-known that the properties of the films depend strongly on the energy and mass composition of the impinging particles from which the thin film is formed1^. Pulsed laser

evaporation with a power density exceeding lO'7 Wcm~2 usually

involves plasma production, i.e., the material removed from the target consists of neutral and charged particles, viz., electrons andions. Furthermore, the evaporation is often accompanied by dissociation when a molecular compound is used as target.

In most published investigations1-111, ions are produced in

short bursts generated by radiation from Q-switched solid state lasers with a pulse duration in the order of 10~° s. There, Time-of-Flight spectrometers are most frequently used to measure either the energy distribution of ions with one particular mass (single species plasma), or the mass constitution of almost monoenergetic ions obtained by accelerating them to an energy level much higher than the energy spread of the ions. Neutral particles can be measured as ions by first ionizing them with an electron beam (impact electrons).

The Time-of-Flight technique, due to its nature, usually does not provide information of both mass composition and energy distributions of the particles when a multi-species plasma is involved (e.g., if a compound or an alloy is used as the target). Furthermore, its usefulness is very limited when the time duration of the ion bursts approaches or exceeds the travel time of the ions in the field-free travel tube used in the Time-of-Flight experiment. Our goal, however, is to measure both the mass composition and the energy distribution of particles, evaporated from a compound or an alloy by means of a pulsed TEA C02 laser. Its

radiation pulses will last from 10 to 1000 times longer than those from the Q-switched solid state lasers15'16.

Therefore, it is neccesary to consider other measuring techniques.

Recently, experimental results from a study of kinetic energy distributions and mass compositions of laser desorbed ions of organic materials were published by Van der Peijl11.

He used a pulsed TEA CO2 laser and a magnetic sector for mass filtering, followed by an electrostatic hemispherical condenser for energy analysis. Although good mass as well as

TEA CO2 LASER •c-DIFFERENTIAL PIMP

nn

good energy resolution are obtained by this configuration, it only allowed a very limited energy range (up to 15 eV) and required auxiliary electron optical devices resulting in a relatively complicated measuring system.

This chapter describes the equipment designed to measure both the two aforementioned quantities using a TEA C02 laser

in an alternative way. Some experimental results are presented. The equipment and the results obtained under conditions described in this chapter have not yet been extensively described in the available literature. This paper reports the first stage of an investigation which aims at establishing relations between the laser parameters

(pulse energy and duration, power density), the energy distribution and mass composition of the evaporated particles, and the optical and mechanical properties of the produced thin films.

3.2. SYSTEMS DESCRIPTION

The system is shown schematically in Fig.3.1. The main part of the measuring instrument is a modified cylindrical mirror analyzer (CMA) for energy analysis, in serie with a quadrupole mass spectrometer (QMS), see Fig.3.3. Both are produced by VG Gas Analysis Ltd. Compared with earlier published measuring instruments for mass-energy analysis of particles generated by laser pulses1-13-, as well as with

configurations consisting of a normal CMA as is amply described in the literature1^-^^,the new analyzer has the

following advantages:

-Broad mass (up to 300 amu, enough to cover most inorganic materials) and energy (up to 1 keV) ranges that are covered

36 ^jaJjfi^gM :£s§§*l UJSESSmKJJ J i W h E—M ■« v • ' B &~^Bi'^^H _{Rï^MIMi» ^^^^H} ■ ^ ^ ^ ^ | T

h i *

■ V » " » *

with a resolution which is satisfactory for our purpose (see Sections.3.1 and 3.2).

-Using an adequate data-acquisition system, the analyzer is suitable for measurements of pulsed particle beams with different pulse durations as well as of continuous particle beams. This makes it useful to other applications such as SIMS studies of non-conducting materials23.

-The analyzer is compact and, due to the modified CMA, able to measure parallel beams (distance target-analyzer 45 cm; radius of the opening of the analyzer 4 mm) such that it can be easily placed within a normal coating chamber as is indicated in Fig.3.1. Using this configuration, the particles measured by the analyzer will virtually have the same properties as the particles impinging the substrate that is placed some distance away from the target.

The possibility of measuring parallel beams and a direct coupling between the CMA and a QMS are two special features of the CMA. These distinguish the CMA from a normal CMA as

is utilized in Auger electron spectrometers17. The latter

accepts divergent ion beams (an angle of incidence into the CMA of 42° to achieve second-order focusing) emitted from a point source (sample) placed in front of the CMA, and its exit beam is not directly suitable for coupling into a QMS.

In the following subsections the different parts of the system will be described in detail.

3.2.1 THE VACUUM SYSTEM

The main vacuum chamber is a 60-cm-high, 51-cm-diam stainless-steel bell jar (Balzers, BA 500) and is pumped by a 360 1/s turbo-molecular pump (Leybold-Heraeus). The CMA

38

8,'y ill M.'4 é è i.'y i.'i i.'4 iJiTi.'n Ü.'D 2.2 I.'I 2,'t i F l g TIME p s F i g . 3 . 2 L a s e r p u l s e s h a p e u s i n g d i f f e r e n t g a s c o m p o s i t i o n s a / 82% He; 2% CO; 8% N2; 8% C 02 ( s t a n d a r d c o m p o s i t i o n ) b / 58% He; 2% CO; 5% N2; 35% C02 c / 58% He; 2% CO; 30% N2; 10% C 02 channeltron 6 e l e c t r o n m u l t i p l i e r Quadrupole mass 5 spectrometer ion o p t i c s c y l i n d r i c a l m i r r o r analyzer ion optics ionization source

1 d

Fig.3.3 Layout of the M-E analyzer based on a

modified cylinderical mirror analyzer (CMA) and a quadrupole mass spectrometer (QMS)

and the QMS are mounted within a stainless steel tube which is differentially pumped by another 170 1/s turbo-molecular pump (Balzers) to keep the vacuum pressure inside the tube below 10- 5 Pa while the pressure within the main chamber is

about 10~3 Pa.

3.2.2 The LASER AND ITS BEAM SHAPING OPTICS

The laser used for evaporation is a pulsed multimode TEA CO2 laser (Laser Applications Ltd, UK, model L300, wavelength 10.6 urn). Using the prescribed laser gas composition (82% He, 8% C02, 8% N2, 2% CO), the laser is

able to generate pulses whose typical temporal shapes are characterized by a peak with a rising front of less than 50 ns and a FWHM of 150 ns, followed by a tail lasting for about 1 us (see Fig.3.2). The temporal shape of the laser pulses can be varied by using other laser gas compositions-1-5 >16 as is indicated in Fig. 3. 2. The maximum

pulse energy is 2.5 J at a maximum repetition rate of 10 Hz. The laser pulse energy can be varied over more than one decade by using various beam absorbers consisting of thin polyethylene foils of different thicknesses29. The laser

beam (cross section at the laser exit 3x4 cm with a beam divergence of about 10 mrad) is focused by means of an air-spaced doublet consisting of germanium lenses, both antireflection coated with ZnS. The focusing system has a focusing length of 50 cm. Behind the focusing system, the laser beam traverses the vacuum chamber through an 8-mm thick germanium window. This window's outer surface is anti-reflection coated with ZnS while its inner surface is coated with diamond-like carbon which can be kept clean more easily. The focused laser beam is directed towards the target by a molybdenum mirror. The size of the laser spot on

40

Uk

'i

»«* • »

Photograph showing the mass-energy analyzer on the top of the vacuum chamber

the target surface can be varied by adjusting the distance between the focusing lenses and the target. The minimum spot size is 1.5x1.5 mm which corresponds to a maximum laser power density (averaged over the whole pulse duration) of about 10° Wcm~2. The target is placed on xy-translation stages which are moved (driven by two in-vacuum stepper motors equipped with a water cooler), in order to avoid crater formation by repeating irradiation of laser pulses at the same location of the target.

3.2.3 THE MASS-ENERGY ANALYZER

Figure.3.3 shows schematically the layout of the M-E analyzer. It consists of an electron-impact open ion source, a modified cylindrical mirror analyzer (CMA), a quadrupole mass spectrometer (QMS) and a charge detector. The main difference between the modified CMA and a conventional

C M A 1 "-2 2 can be described as follows. The conventional CMA

requires a point ion source whose beam is analyzed and focused by the CMA to a point at the exit of the CMA, where the ions are detected. The modified CMA is designed for a parallel incoming ion beam, which first is transformed by a focusing lens and a deflector (inverted hemispherical type) into a virtual point source, and then analyzed with a conventional-type CMA. Then the CMA collimates the ions into a parallel beam again at the entrance of the QMS, using the inverted optics as at the entrance of the CMA. This construction makes it possible to measure parallel particle beams with the CMA and to couple the CMA to a normal QMS.

The energy of the ions when entering the CMA, E0, is first

transformed into the CMA pass energy ( Ep a s s) , by applying a

42

the CMA ( Vc m a) . The scan of the CMA across the energy

spectrum is realized by scanning Vc m a. The deflection of the

ions between the two cylindrical electrodes of the CMA is determined by the radially symmetric field defined by the sizes of the electrodes and their potential difference Vd.

In this case, the value of Vd is related to the pass energy

Ep a s s, which is adjustable from 0 to 25 eV, according t o^

Vd=3xlO-3Epass. (1)

The full width at half maximum peak height (FWHM) of the CMA is determined b y "

FWHM =0.1 Ep a s s. (2)

The CMA works in constant pass energy mode which makes it particularly useful for quantitative work because a constant energy window is obtained, over the whole energy range

(0-1 keV).

The relation between the initial energy of the ions E0,

the pass energy Ep a s s, and the retarding/accelerating

potential, Vc m a, is expressed by

Eo = V s s + eVc m a. (3)

Positive ions with an energy E0 exceeding Ep a s s, are

retarded in the CMA by the application of a positive value of Vc m a, while ions with an energy E0 lower than Ep a s s are

accelerated in the CMA by a negative Vc m a.

Early experiments performed using Q-switched solid state lasers24, indicated that at a laser power density of

the order of 1 keV. Such surprisingly high energies were possible due to plasma formation. Since the laser power density in our experiments (108 Won-2) is considerably lower

than the mentioned value, the upper limit of the CMA scan energy is set to be 1 keV.

In order to obtain a well-resolved mass/energy spectrum the energy of the" ions within the QMS must be limited to several eV's (Eiq; up to 10 eV). This is achieved by the

pole bias facility which floats the quadrupole rods' potential Vp^ relative to earth potential according to

e Vpb " Eo - Ei q (4)

In our case, Ejg can be varied from 0 to 25 eV, which allows a trade-off between the mass resolution and the transmission of the QMS for individual measurements.

The QMS (model SXP300) comprises a multifilter25 with a

mass range of 0-300 amu.

A combined Faraday collector/electron multiplier device is. used as the detector. The multiplier is a wide-bore channeltron electron multiplier (Galileo USA), mounted off-axis. The potential difference applied across the multiplier can be varied maximally.to 3 kV. Normally, the channeltron multiplier is used for the detection as the ions are

accelerated towards the multiplier by the deflection field, determined by Vpj-, (-20 V - to 1 kV, depending on the scan energy Eo) and the negative potential applied to the input of the multiplier (max. 3 kV). At the maximal channeltron voltage (3 kV) the multiplier has an amplification factor of about 10^, and its detection efficiency for ions is nearly independent of the scan energy E0. 'This is because the

44

detection efficiency curve of the multiplier shows a plateau for an ion input energy (determined by the accelerating/deflection field between the QMS and the input of the channeltron) above 3 kV (see Ref.24). The Faraday plate detector can be "switched on" by turning the multiplier voltage to zero.

Both the CMA controller and the QMS controller are connected to a microcomputer (IBM PC) which also triggers the laser and performs the data acquisition (see Sec.3.2.4).

3.2.4 COMPUTER CONTROL AND DATA ACQUISITION

The stepwise scan of the M-E analyzer, which is synchronized to the firing of the laser, is controlled by the microcomputer. It allows a mass scan from 1 to 300 amu, or any fraction of this range at a particular (fixed) energy level between 0 and 1 keV, and an energy scan from 0 and 1 keV, or any fraction of this range, at any particular (integer) mass number between 1 and 300 amu. A complete spectrum, either mass or energy, requires (independent of the-scan range) 600 steps and the same number of laser pulses. The triggering circuit of the laser is coupled by an opto-coupler to the computer interface which is able to trigger the laser at a repetition rate up to 10 Hz. The output signal of the channeltron electron multiplier is a series of electric current pulses. Those are fed into an charge integrator (EG&G Ortec) whose maximum output voltage corresponding to an input pulse is proportional to the total charge of that pulse, and therefore to the number of ions detected by the multiplier. The hold time at the output of the integrator is long enough to ensure that the top amplitude of the output voltage pulse can be sampled and

recorded by the computer, but much shorter than the time interval between two successive pulses, which is at least 0.1 s. The output of the integrator is enhanced by a voltage amplifier whose amplification is adjustable from lOx to lOOOx. An analog to digital converter links the amplifier to the computer interface. The time interval between the trigger pulse to the laser and the A/D conversion can be adjusted from 0 to 594 us, in steps of 6 us. This facility ensures a proper sampling of the output by the computer (see also Sec. 3.2).

The mass and energy spectra obtained by this stepwise scanning method are stored in the computer and can be retrieved from it. The computer also provides the possibility to perform data processing and to make corrections.

3.3. SYSTEM PERFORMANCE

3.3.1 PROPERTIES OF THE M-E ANALYZER

The sensitivity of the M—E analyzer versus ion energy for direct ion measurements (i.e., without the use of the internal ion source) can, in the continuous operation mode, be defined as the output current of the channeltron multiplier divided by the ion current at the entrance of the analyzer. It was determined with the aid of an auxiliary ion source as developed by Kornelsen2^. This particular source

was chosen because of its small energy spread (FWHM=0.6 eV) and beam divergence (half-angle 0.4° at 1 keV). Under typical operating conditions, the output ion current versus

ion energy was measured using a Faraday collector, and is displayed in Fig.3.4.

46 < 20 5L 10 H cd « os a o SB

o

Fig;3.4 . Output ion current vs. ion energy of the ion source used for the evaluation of the M-E analyzer

10 -f

io' ioJ

energy eV

Fig.3.5 Sensitivity of the analyzer vs. the scan energy • measured at E_{;urea ax. ^ p a s s}riac!,,=25 eV with the modification _{=^3 e v wxtii-nie muuuii,cn.iuii}

X measured a t Ep a s s= 1 5 eV w i t h t h e m o d i f i c a t i o n

asured at Epa s s =25 eV without the modification

In the continuous operation mode, the output of the channeltron multiplier was directly connected to the. amplifier. The sensitivities of the analyzer at 25 eV and 15 eV pass energies are shown in Fig.3.5. Owing to the simple input configuration of the CMA, the sensitivity at 25 eV pass energy decreases roughly linearly with an

increasing scan energy. It must be pointed out that these sensitivity curves have been obtained after a modification of the analyzer: a voltage which is linearly proportional to Vc m a has been applied to the central element of the ion lens

in front of the CMA, in stead of the fixed voltage (optimized for low energies) that was provided by VG Ltd. The ratio between the voltage applied to the element and Vc m a is experimentally determined such that the overall

sensitivity, especially at high energies (>200 eV), is optimal. The sensitivity at Epass=25 eV measured without the

modification, which is indicated in Fig.3.5 as well, was roughly inversely proportional to the second power of the scan energy, and less than one tenth of the sensitivity measured after the modification, at the same pass energy (except in the region around the pass energy, see Fig.3.5). Thus, the modification reduces the sensitivity variation significantly, and improves the overall sensitivity of the analyzer. These positive effects suggest also that the sensitivity can be further improved by using more sophisticated ion optics in front of the CMA. The present

input optics are not able to ensure a constant- focusing, over the entire energy scan range, as is required by the CMA for a constant functioning. For this purpose, we need a lens system which is able to add and subtract various energies to and from the energies of the input ions without changing its focusing properties^?. At the present stage, the variation in the sensitivity can only be partly corrected (limited by the finite signal to noise ratio) by data processing.

48

i l 10.0 20,0 30.0

ENERGV e«

Fig.3.6 Energy spectrum of Ar+ ions generated by the

internal ion source with 15 eV extraction energy

10.01

Fig.3.7 Mass' spectrum of laser-generated ions from the AI2O3 target at 25 eV ion energy