Pattern Dependency of Pure-Boron-Layer Chemical-Vapor Depositions
V. Mohammadi, W.B. de Boer, T.L.M. Scholtes, L.K. Nanver Delft Institute of Microsystems and Nanoelectronics (Dimes),
Delft University of Technology,
Feldmannweg 17, 2628 CT Delft, The Netherlands, Phone: +31 (0)15 27 86294, Fax: +31 (0)15 27 87369,
E-mail: v.mohammadi@tudelft.nl
The chemical-vapor deposition of pure boron has in the last years been very successfully applied for fabricating extremely shallow, less than 10-nm deep, silicon p+n junction diodes for a number of leading-edge device applications. This so-called PureB technology has provided particularly impressive performance of photodiode detectors for low penetration-depth beams for which 2-nm-thick PureB-layers are reliably implemented as the front-entrance window [1]. Ideal low-leakage diode characteristics are achieved for deposition temperatures from 400ºC - 700ºC, which together with the fact that the deposition is conformal and highly selective to Si, also makes PureB technology an attractive candidate for creating junctions on silicon nanowires and advanced CMOS transistors including source/drain in p-type FinFETs [2]. In the latter applications, sub-3-nm thick layers are required to avoid excess series resistance through the high-resistivity boron layer. Therefore, a very good control of the layer thickness is crucial, in which respect the uniformity over the wafer and the pattern dependence are the important factors.
In this paper, deposition parameters have been optimized for 700ºC depositions with respect to uniformity over the wafer with and without patterned oxide coverage. A series of 100-mm Si (100) wafers patterned with various oxide window sizes and different oxide coverage ratios (OCR, ratio of the oxide mask area to the entire surface area) were used to investigate pattern dependency and loading effects. The dependency of the PureB deposition rate on window size and also the influence of the OCR of the patterned wafer on the deposition rate are shown in Fig. 1. The higher the OCR the higher the deposition rate, and as the feature size of the Si opening decreases the deposition rate is increased.
The influence of the pressure was investigated at both atmospheric pressure (ATM) and 60 torr. Different diborane partial pressures and main gas flows where applied as specified in the Fig. 2. The results show a reduction of the pattern dependency with decreasing diborane partial pressure as well as main gas flow. Furthermore, no significant improvement in the pattern dependency was observed when decreasing the total pressure from ATM to 60 torr. In Fig. 3 results are shown for wafers where 14 oxide windows to the silicon, each 1x1 cm2 in size, are positioned so that the surrounding oxide area is different in each case. This gives different loading effects that become very small when the diborane partial pressure and main gas flow are low.
The deposition rate will also be influenced by the proximity of neighboring windows. On the basis of experiments such as the one illustrated in Fig. 4, where the boron thickness can be related to the distance along the surface that the boron can travel before being adsorbed, it has been possible to estimate the diffusion length of boron on both the Si and the oxide surface. It was concluded that the depletion volume and the diffusion length of diborane should be around 1-1.5 cm.
With this work it has been demonstrated that it is possible to control pattern dependency and loading effects to such a degree that uniform 2-nm-thick PureB-layers can
be deposited uniformly with only a few angstrom thickness variations. The long diffusion length of the boron on the oxide and Si surfaces means that the loading effect in micron sized windows will not lead to thicker layers than seen in the larger windows.
0 10 20 30 40 50 60 70 80 90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.54 0.56 0.58 0.6 0.62 0.64 0.66 100 150 200 250 300 350 400 450 OCR (%) De p. R a te (nm/min ) De p. R a te (nm/min ) Si opening area (mm2)
Figure 1. The deposition rate of the PureB-layer as a function of (a) oxide coverage ratio (dashed line) and (b) the area of the individual oxide windows to Si with an OCR of around 80% in all cases (solid line).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 40 50 60 70 80 90 100 110 De p. Ra te ( nm/min )
Diborane partial pressure (%)
100 200 300 400 bare
Figure 2. The deposition rate of the PureB at atmospheric pressure as a function of diborane partial pressure and for different Si opening windows as well as a bare Si wafer. The same trend was also found for other main gas flows. 0 2 4 6 8 10 12 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 De p. Ra te (nm /min ) Die number ATM_P1F1 ATM_P2F1 ATM_P3F1 ATM_P1F2 ATM_P1F3
Figure 3. The deposition rate of PureB plotted against the die number for a
wafer patterned with 14 1x1 cm2 dies in the manner shown in the wafer
layout of the inset. Here P1 (F1) is the maximum diborane partial pressure
(main gas flow) and P2 (F2) and P3 (F3) are 75% and 50% of the maximum
values, respectively.
Figure 4. The deposition rate of the PureB in a row of 1x1 cm2 test dies
adjacent to a 3x1 cm2 die that is designed as (a) one big window to Si (b)
an area covered with oxide, and (c) 3 dies of the same type as the test dies. The deposition rate in the big window to Si is included for comparison. [1] A. Šakic, et. al., IEDM 2010, pp.712-713.
[2] L.K. Nanver, et. al., RTP 2010, pp. 136-139.
Si opening size (µm) (a) (b) Si reference (a) (b) (c) Abstract #862, 221st ECS Meeting, © 2012 The Electrochemical Society
