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EPITAXIAL SILICON GROWTH IN A REDUCED PRESSURE AND TEMPERATURE CVD REACTOR
J. Regolini, D. Bensahel, J. Mercier, E. Scheid
To cite this version:
J. Regolini, D. Bensahel, J. Mercier, E. Scheid. EPITAXIAL SILICON GROWTH IN A REDUCED
PRESSURE AND TEMPERATURE CVD REACTOR. Journal de Physique Colloques, 1989, 50 (C5),
pp.C5-519-C5-527. �10.1051/jphyscol:1989561�. �jpa-00229592�
JOURNAL DE PHYSIQUE
C o l l o q u e C5, s u p p l 6 m e n t au n"5, T o m e 5 0 , m a i 1 9 8 9
EPITAXIAL SILICON GROWTH IN A REDUCED PRESSURE AND TEMPERATURE CVD REACTOR
J.L. REGOLINI, D. BENSAHEL, J. MERCIER* a n d E. SCHEID'"
C e n t r e N a t i o n a l d ' E t u d e s d e s T 6 1 6 c o m r n u n i c a t i o n s , B P . 9 8 , F - 3 8 2 4 3 M e y l a n Fedex, F r a n c e
L E P E S / C N R S , avenue d e s M a r t y r s , F - 3 8 0 4 2 G r e n o b l e C e d e x , F r a n c e
* " L P C S / C N R S , avenue d e s M a r t y r s , F - 3 8 0 3 1 G r e n o b l e C e d e x , F r a n c e
RESUME
En procede thermique rapide dans un systeme sous pression totale de quelques torrs, I'epitaxie selective du silicium est realisee B des temperatures aussi basses que 650°C. Les sources gazeuses du silicium sont soit le silane soit le dichlorosilane (DCS) diluees dans I'hydrogene etlou I'helium.
Les aspects cinetiques de la croissnce du siljcjum dans Ies deux systemes DCS/H2 et SiH4/HCI/H2 sont presentes et discutes. Le taux de croissance avec DCS est fonction lineaire de la pression partielle d'hydrogene alors que pour SiH4/HCI, une variation quadratique traduit les resultats experimentaux.
La selectivite ainsi obtenue est expliquee selon un modele de decomposition des gaz reactifs et n'est pas affectee par la nature du gaz porteur, alors que le taux de croissance I'est. Nos resultats montrent I'existence de conditions experimentales pour lesquelles le taux de croissance est presque independant de la temperature.
Ce type de CVD, a pression et temperature reduites, apporte une contribution positive aux problemes poses par la VLSl qu'exige des jonctions abruptes des couches epitaxiques selectives et un procede basse temperature.
ABSTRACT
In a Rapid Thermal Processing system working at a total pressure of a few torr we have obtained Selective Epitaxial Growth of Si at temperatures as low as 650°C. The studied gas systems are based on the decomposition of either silane or dichlorosilane (DCS) diluted in hydrogen and/or helium.
The kinetic aspects of the two systems: DCS/Hzand SiH4/HCI/H2 are presented and discussed. The growth rate for DCS is found to be a linear function of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989561
C5-520 JOURNAL DE PHYSIQUE
the hydrogen partial pressure. Nevertheless, for SiHdHCI a quadratic approach fits the experimental results better.
The obtained selectivity is explained b y the reacting gases decomposition model and is not affected by the nature of the carrier gas as the growth rate is. These results show the existence of experimental conditions under which the growth rate is nearly temperature independent.
A new contribution to the VLSI challenge is provided by this type o f CVD at reduced pressure and temperature for which abrupt junctions, selective epitaxial layers and low temperature processes are required.
INTRODUCTION
Selective silicon epitaxy is a promising technique for very large scale integration (VLSI) where device isolation is in the range of the submicron scale geometry. In addition abrupt transitions in dopant concentration cannot be achieved using conventional high temperature (>lOOO°C) CVD. As a result a reduction in the epitaxial temperature has a significant effect on high performance integrated circuits as shown for an isolated npn bipolar transistor by Srinivasan and Meyerson
i l l . The n+ buried layer profile is very sensitive to the epitaxial deposition
temperature. Moreover, recent results have shown the good performance of a Si/Si-Ge heterojunction bipolar transistor made by low temperature CVD 121. In this case, the thermal exposure should be minimized since these layers are metastable. Thus, excessive thermal exposure will cause the films to relax and form misfit dislocations which may degrade the device performance.
A considerable amount of research has been done on low temperature epitaxy. Meyerson et al. I31 have demonstrated that device quality material can be obtained by CVD at temperatures as low as 750°C in an ultrahigh vacuum environment. A plasma assisted system is also used by Donahue and Reif /4/
achieving good quality material, and Vescan et al. 151 using a ultrahigh vacuum reactor down to 760°C. Device quality epitaxial layers can be grown in a commercially available reactor by Borland and Drowley 161 in a pressure range of 10 to 100 torr and at a temperature of about 850°C.
We previouly presented /7,8/ a selective epitaxial growth study of silicon on patterned wafers, in which we showed the full selectivity obtained using DCS in H2 without any HCI addition. We also compared the DCSlH2 system with the SiH4/H2 and the SiH4/HCI/H2 systems and concluded that selectivity is also a strong function of the total pressure of the system. No addition of HCI is needed when using
DCS in H2 in the pressure range investigated in our study.
The aim of the present paper is to show how selectivity does not depend on the nature of the carrier gas while the growth rate does. In addition the growth rate behaviour, when H2 or He are used as a carrier gas, will be studied for the three systems: DCS, SiH4 and SiH4lHCI.
EXPERIMENTAL
Our experimental set-up consists of a rapid thermal CVD system using a bank of tungsten lamps as a source of radiant heat. The system was pioneered by J. F.
Gibbons 191 and the process was named: "Limited Reaction Processing". The actual reactor is a horizontal cold-wall air cooled silica chamber where the base pressure is about 2 mtorr obtained simply by a mechanical oil pump with a molecular sieve oil trap. The substrate temperature, rather than the gas flow, is used as a switch to turn the CVD reaction on and off. The substrate surface is only hot while the reaction is occurring. The sample temperature rises at about 200°C/sec and cools down to about 200°C in 20 sec.
After loading the sample the system is purged using H2 which is also generally used as a carrier gas. Before deposition, the sample is subjected to an "in situ" cleaning step which depends on the sample surface history. For example, the "as received" wafers should be cleaned using a high temperature cycle of a few seconds (lOOO°C, 30 sec) under H21HCI to etch the native oxide from the silicon surface. If only low temperature cycles are allowed, a chemical pre-clean procedure before loading is imperative. In this case, an RCA chemical cleaning / l o / will precede a low temperature "in situ" process (800°C, 1 min), which can include deep UV radiation I 1 1 1 .
In general, all the studied layers were specular and shiny. Some remaining defects could be found since no special care was taken with the laboratory environment and/or sample handling. Patterned in SOn, (100) 4" wafers were used in this study. The growth rate (G) of the epi-layers were measured by a profilometer and the wafer temperature was measured by a thin thermocouple attached to a test wafer under the same experimental conditions. Reactive and carrier gases were all electronic grade and no special purification was used, their flow being established by flowmeters prior to the temperature cycle. The reactive and carrier gas partial pressures indicated in different plots were all measured using a pressure induced capacitance manometer. As our system works under a reduced dynamic pressure, the partial pressure of each gas also depends on the pumping system impedance as well as on the total pressure and flow ratios. Thus, when the partial pressure of a gas is used as a variable, this variable represents the measured value rather than the calculated one as is possible in a static system. The extreme simplicity of the reactor, its negligeable thermal and chemical inertness, together with the easy control of gas pressure and rapid thermal cycle, should be noted. The versatility of the system has been demonstrated by growing low' temperature silicon dioxide on compound semiconductors like InP 1121 and the selective deposition of silicides without silicon consumption 11 31.
JOURNAL DE PHYSIQUE
RESULTS AND DISCUSSION
I ) . - Influence on G of deposition temperature
Using
H2
as a carrier gas at 2 slm with a constant total pressure of 2 torr, we obtained the growth rate G as a function of temperature, for a given reacting gas flow rate as shown in Figure 1 in an Arrhenius type plot.7 8 9 1 0 1 1
l o 4 / T I ° K ]
Figure 1
.-
The growth rate of silane and DCS in an Arrhenius plot Two regions can be easily distinguished:-
a high T region (T>850°C) where the crystal growth is limited by the supply of reactant via the gas phase diffusion and-
a low T region (Tc850°C) where the G values are controlled by the sample surface reactions which are activated.For silane we measured an apparent activation energy Ea of 38 kcallmol (below 750°C), curve (a), which is close to the dissociation energy of the silylene molecule:
and compares quite well with the values obtained from the literature 114,151.
On the other hand, the measured Ea value for the DCS decomposition is about 59 kcallmol which is slightly higher than previously published values (between 40 and 50 kcallmol 116,171). If a certain reaction is rate-limiting, our result implies that this reaction is different from the one generally assumed:
In the high temperature region, however, no doubt exists in assuming a gas diffusion limitation. Indeed, the G values are very weakly temperature dependent
i.e. in proportion to the molecular diffusion coefficients ( To.9 ) 171. In addition, the G values for silane and DCS are almost equal. This is simply because the diffusion coefficients of both gases are in the reciprocal ratio of the flow rates (40 and 80 sccm respectively).
2).- Influence on G of the reactive aas injection
The growth rate for both systems exhibits a reaction order A defined by the equation:
where p~ stands for the partial pressure of SiH4 or DCS, K is a constant at a given T, and A is close to one at 900°C. At 800°C and below, this A value is well below one which is a clue to a Langmuir-Hinshelwood type adsorption mechanism 1181. For the SiH4 case, we have already mentioned that the SiH2 reaction at the surface must be involved. For the DCS case it seems that SiCI2 should not be involved due to Ea.
3).- Influence on G of the hvdroaen partial pressure
In order to maintain the total pressure constant, we used H2 and He as a mixed carrier gas. In Figure 2 we compare the two systems, SiH4IH2 and DCSIH2, by plotting the G values as a function of the H2 partial pressure. The DCS curve at 750°C is fairly constant.
Figure 2.- The growth rate of silane and DCS as a function of hydrogen partial pressure for two different temperatures.
JOURNAL DE PHYSIQUE
At 900°C the DCS shows an increase in G when increasing the H2 partial pressure pH2. Just the opposite is measured for the SiH4 system where a slight decrease is only observed at these two temperatures. This general behaviour can be explained as follows: for DCS, at high T, the increase in growth rate with increasing pH2 should be attributed to the reactive gas diffusion coefficient which is higher in H 2 than in He (0.39 cm21s and 0.35 cm21s respectively as calculated according to 1191). At low T this growth rate is fairly constant which correlates perfectly with the proposed DCS decomposition, where no H2 is involvedl81. In the case of SiH4, at high and low T, there is a growth rate decrease when increasing pH2 because the silylene decomposition produces hydrogen, (according to [I]). Thus, the presence of H2 as a carrier gas will depress this reaction giving lower G values.
To explain all the DCS results in the activated regime we propose a surface-reaction scheme based on:
SiH2CI2 -> SiHCl
+
HCI [41for which lshitani et al. 1201 have given the dissociation energy of 61 kcallmol, close to our value of 59 kcallmol. Also P. Ho et al. 1211 mentioned the existence of SiHCl as a product at atmospheric and reduced total pressures. This scheme does not involve H2, thus qualitatively fitting our observations. As HCI is also a product , it explains a very interesting feature of our process, namely selective epitaxy.
Selective epitaxial growth is obtained throughout the whole studied temperature range (1100°C
-
650°C) when using DCS diluted in Hp and no epi-layer thickness limitation has been observed. The selectivity can be easily explained by the silicon nuclei reduction on the SiO2 mask through the HCI by product. Increasing the HCI concentration will produce a rapid linear decrease in growthresults eleiminate any explanations based on: SiCI2
+
H2 -> Si+
2HCIl o 4
1000 900 800 700 ["C ]74 kcallmol
.-
Selectiv.E
withoutY HCI
Selectiv.
with 2 sccm HCI
10'
7 8 9 1 0 1 1
10 / T
[OK]rate These
Figure 3.- Growth rate as a function of the reciprocal temperature for the SiH4/HCI/H2 system showing the region where selectivity is obtained only with SiH4.
When using silane, as reacting gas, selectivity is also obtained at temperatures above 950°C, and the addition of HCI is not needed, as shown in Figure 3
On the other hand, below that temperature, just a few percent of the SiH4 concentration of HCI is needed to obtain full selectivity. In this case 2 sccm of HCI in 20 sccrn of SiH4 diluted in 2 slm of H2. Accordingly the activation energy Ea is affected: from 38 kcal/mol for the SiH4/H2 system it goes to 74 kcal/mol for the SiH4/HCI/H2 system and selectivity is obtained up to 650°C.
In order to study the chemical reactions in the SiH4/HCI/H2/He system, the growth rate variation was measured as a function of pH2 and G was plotted versus the square of this pressure as shown in Figure 4.
0 1 2 3 4
(pH2 [torr])
Figure 4.- The growth rate for the two systems DCS and SiH4/HCI diluted in a gas mixture of H2 and He are plotted as a function of the H2 partial pressure.
If we propose a chemical reaction similar to that proposed for the DCS/H2 system, the following equations hold:
SiH4
+
HCI -> SiHCl+
2H2 [51 SiHCI+
-> (SiHCI)* [GIwhich means that the growth rate should depend on the second power of the hydrogen partial pressure and the limiting reaction will come from:
(SiHCI)* -> Si*
+
HCI [71where "*" means adsorbed species or a vacant site. Curve (a) from Fig. 4 shows this quadratic behaviour for H2 pressures above 1 torr. Curve (b) is the same qualitative
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result already shown in Fig. 2 where G is fairly constant above 1 torr of Hp. Curve (c) corresponds to the same system as (a) at 900°C, which follows a similar beh-aviour to DCS at 900°C shown in Fig. 2.
What is striking in the present picture is the important decrease in growth rate at low hydrogen partial pressure, i.e., when there is more He than H2 as a carrier gas (the total pressure being constant) and in the presence of HCI. This behaviour is nearly temperature independent. Curve (a), at 750°C, is in the activated regime and curve (c), at 900°C, in the diffusional one. In all cases the G values decrease dramatically under 100% He as a carrier gas, whatever the system, i.e., DCSIHe or SiH4lHCIlHe. The growth rate'is more affected at 900°C than at 750°C and full selectivity is preserved in the two systems. To our knowledge the growth rate, under 100% He as a carrier gas, is lower than under 100% Hp because of a depletion of reactant in the gas phase. The presence of He enhances the homogeneous decomposition of reacting gases and the products are carried away from the reactor.
A clear advantage of this system, SiH4/HCI/H2/He, is in the region where the volumetric ratio for HpIHe is about one: the G values are almost the same for 750 and for 900°C (around 600 Atmin which is fairly high). This implies that the addition of He is able to remove the pathological sensitivity towards T of the growth rate in the SiH4IHCIlHp system.
CONCLUSION
Full selectivity has been obtained with only DCS diluted
in
Hp. However, in the silane system, a small percentage of HCI in silane is sufficient to obtain selectivity up to 650°C. This is the main characteristic of our system working in the torr pressure regime. The etching of silicon by the presence of HCI is enhanced by this reduced pressure regime.The rapid thermal process applied to Si epitaxial growth has proven its capability of growing selectively good epitaxial layers with a high degree of control and under a wide spectrum of experimental conditions.
The fact that growth rates can be controllably varied from 10 to 1000 Almin makes this procedure a very good candidate for V and ULSl applications.
ACKNOWLEDGEMENTS
The authors would like to thank Mrs. M. Timmins for reading of the manuscript and R. Carre for technical assistance.
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