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W. van Heumen, M. Hendriks
To cite this version:
W. van Heumen, M. Hendriks. QLTO, A CLASSICAL PROCESS STILL GOING STRONG. Journal de Physique Colloques, 1989, 50 (C5), pp.C5-595-C5-604. �10.1051/jphyscol:1989570�. �jpa-00229602�
JOURNAL DE PHYSIQUE
Colloque C5, supplBment au n 0 5 , Tome 50, mai 1989
QLTO, A CLASSICAL PROCESS S T I L L G O I N G STRONG W. VAN HEUMEN and M. HENDRIKS
ASM Micro Electronics Technology Centre, PO Box 100, NL-3720 AC Bilthoven, The Netherlands
Resume.
L'article dkcrit un prockdk industriel de formation de Quartz, Oxyde B Basse Temperature (QLTO) dans un r6acteur turbulaire horizontal A parois chaudes, pour le traitement d'une charge de 100 plaquettes de 150 mm de diambtre. I1 pr6sente l'influence des parambtres du procMB et de la conception de la cassette B cage. I1 dkrit une mkthode d'injection du gaz qui permet d'6quilibrer le profil sur le bateau.
La phosphine et le tri-mkthylborate ont kt6 utilids comme sources d'agents dopants. Enfin, l'article prksente les niveaux de particules que le procbd6 permet d'atteindre dans des conditions vari6es.
Abstract.
An industrial Quartz Low Temperature Oxide (QLTO) process for 150 mm wafers and a guaranteed load of 100 wafers with a diameter of 150 mm in a horizontal tube, hot wall reactor is described. The influence of process parameters and caged cassette design is dealt with.. A method of gas injection is described that allows balancing of the profile over the boat. Phosphine (PH,) and Trimethylborate (TMB) were used as source materials for phosphorus and boron doping. Finally, particle levels that can be achieved in this process under various conditions are presented.
1. Introduction.
The oxidation of silane is a widely used method for the deposition of SiOz films [I-91. The advantage of this process over other methods is its low deposition temperature. This allows deposition of the silicon oxide films over Aluminium metallisation. Initially the process was operated at atmospheric pressure [2]. More recently, the oxidation of silane at low pressure was forwarded as a suitable production process [I]. Generally, at low pressure mass transport limitations are not present due to the much higher diffusivity than at atmospheric pressure. As a consequence, the wafer can be placed at close spacing. The oxidation of silane is an exception. The severe reaction between SiH4 and O2 requires caged cassettes surrounding the wafers and distributed gas injection to achieve satisfactory uniformities in deposition rate over each wafer and over the reactor. Recently, special complicated reactors were designed to achieve a better process performance [4,5].
In the present study we show that the performance of a classical horizontal tube low cost production reactor can cope with present day requirements regarding uniformities and particles. An undoped Si02 and phosphorus doped SiO, process for a load size of 100 wafers with a diameter of 150 mm is described. Technical aspects like cassette design and gas injection method will be dealt with. In addition, a BPSG process is described using TMB as the source material for boron doping.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989570
2. Experimental set-up.
The depositions were carried out in a 5 zone hot wall furnace at a 450 O C flat temperature profile.
The inner diameter of the liner in the process tube is 220 mm. Two stainless steel injectors was used for the homogeneous distribution of gases, one for SiH,, pH3 and one for 02, TMB. A schematic set up is given in figure 1. For the evacuation of the system a rotary vane pump with mechanical oil filtration in combination with a rootsblower was used. An especially designed mechanical trap was used to prevent that solid material enters the pump. The pressure in the process tube was conuolled by a baratron pressure sensor in combination with a butterfly valve. The 150 rnrn silicon test wafers
EXH
COWROLLER
Figure 1: Schematic set-up of the used ASM QLTO reactor.
100 200 300 400
TOTAL SiH4 FLOW (SCCM)
Figure 2: The deposition rate as afunction of the total silaneflow at a constant O2 to SiH4 ratio of 1.6. The pressure is shown as a parameter for the individual datapoints. The tempqature was kept constant at 430 "C during all runs.
were loaded in three caged cassettes, which were placed on a quartz carrier. The'loading pattern was proximity back to back with a pitch of alternately 2.38 mm and 9.52 mrn and a total load size of 132 wafers. A baffle was placed in front of the boat to increase the local surface area tbvolume ratio.
The film thickness was measured with a Gaertner ellipsometer and a Nanospec reflectometer. The phosphorus content was determined with Electron Probe Micro Analysis and the boron content was measured with Fourier Transform Infrared Spectrometry. Particles were detected with a Tencor Surfscan 4000 Surface Particle Counter.
3. Process parameters and cassette design.
First of all, the influence of the process parameters and the caged cassette design was investigated.
The deposition rate increases linearly with SiH4 flow, see figure 2. The reaction appears to be input rate limited or limited by mass transport type I [lo], with a nearly 100% conversion efficiency of SiH4 [ l l ] . Interestingly, for our horizontal tube reactor the straight line does not go through the origin, whereas for other types of reactors a strictly proportional relationship was observed [5,71.
Because the process is input rate limited, the influence of the process pressure on the deposition rate is insignificant, see figure 2. The uniformity over each wafer (point to point uniformity) on the other hand, is substantially improved by a reduction in pressure, see figure 3. This can be attributed to the improved diffusivity at lower pressure. Because of the severe reaction between SiH4 and OZ, a proper cassette design is essential to achieve a satisfactory point to point uniformity. In an open boat, the deposition rate at the wafer edge would be much to high. A caged cassette is required to eliminate this effect. The optimum clearance between the wafer edge and inner caged wall appeared to be about 0.5 times the wafer spacing [4]. In that case a more or less constant surface area to volume ratio (AIV) is seen by the whole wafer surface. The model to calculate this optimum clearance considers the SIPOS process. Heterogeneous reactions account for the observed effects [12]. We suggest that in the case of QLTO these effects can be attributed to a homogeneous gas phase reaction. In this case, also a constant value of A/V is required for a uniform film thickness.
This reaction is thought to be identical to that forwarded for the oxidation of silane at atmospheric pressure E13, 141 resulting in the formation of siloxane (H2SiO):
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PRESSURE (IN mTorr)
Figure 3: The point to point uniformity as a function of the process pressure. Other process parameters identical to those ofjigure 2 .
The oxidation of siloxane in a subsequent step is thought to be a surface controlled reaction:
The fact that an excessively high deposition rate is observed on the surface that borders a large volume, e.g. the front surface of the first cage, is in support of homogeneous gas phase reactions.
The deposition rate linearly increases with the wafer spacing for the spacings commonly used in this type of reactors, as show in figure 4. A similar dependency was observed by Learn [4].
Extrapolation to zero deposition rate yields a wafer spacing of about 2.5 mm. This agrees with the
Figure 4: The deposition rate as a function of the wafer spacing. The pressure was 275 mTorr, the SiH4flow was 300 sccm and the O2 to SiH4 ratio 1.6. The'deposition rate was measured in the wafer centre.
3 0 1 ' ~ 0.8 ~ 6 "1 .O ' ~ ~1.2 " ~ "1.4 " ' ~1.6 3 " ' ~ ' ~
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Figure 5: The deposition rate and the point to point uniformity as afunction of the slit width. The pressure was 200 mTorr, the SiH4flow was 200 sccm and the O2 to SiH4 ratio 1.6.
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Figure 6: The layer thickness as a function of the position on the IS0 mm wafer scanned along three lines through the centre of the wafer. The thickness was scanned: 1 - verticallyfrom the top to the bottom, 2- at the slit nearest to the silane injector ending at the farthest point from the silane injector and 3- at a position intermediate between two slits.
observation that at the rear of the wafers, which were positioned proximity back to back (spacing 2.38 mm) no deposition was visible in the centre of the wafers.
In deviation with our findings, Learn observed that extrapolation towards zero deposition rate yielded a zero wafer spacing. This can be explained by the fact that he used a lower process pressure (100 mTorr) and smaller wafer size (100 mrn). The influence of the slit width is presented in figure 5. For a large slit width the deposition rate increases because the gases more readily enter the cassettes. Simultaneously, the point to point uniformity degrades for large slits due to a relatively strong local increase of the deposition rate at the position of the slits. For a small slit width, depletion effects result in a low deposition rate in the centre of the wafer and a large thickness variation from the edge towards the centre. The distribution of the slits around the cassette was optimized until a good uniformity over the wafer was achieved. The film thickness was measured along three lines though the centre of the wafer and is represented in figure 6. In the centre of the wafer a uniform deposition rate is observed that with only one exception, falls off near the wafer edge. The vertical scan (1) shows the effect of a wafer support notch at the bottom and of the wafer flat at the top. Around the wafer support notch, depletion occurs and the deposition rate decreases locally. Therefore, the fall-off in deposition rate occurs further away from the edge. Near the wafer flat a high deposition rate is obsered due to the large clearance between wafer flat and cassette wall. At the slit nearest to the silane injector (scan 2, right part) the deposition rate rapidly increases and reaches a maximum 20 mm from the wafer edge. For higher pressures or larger slit widths, this effect is more pronounced and can become a limiting factor for the uniformity over the wafer. The third scan is taken at an orientation intermediate between two slits in the base. At this orientation the deposition rate increases somewhat slowly as a function of the distance from the wafer edge because the slits in the base are relatively far separated. Figure 6 shows the worst spots on the wafer but it also shows that the centre of the wafer is very uniform. A wafer map measured over the total wafer surface (excluding 6 mm from the edge) would show a 1 sigma uniformity well below 1%.
Figure 7: Schematical drawing of the double injection system.
4. Distributed gas injection.
To achieve a good uniformity along the boat (wafer to wafer uniformity), two double gas injectors were used. Essential parameters are length of the injection zone, number of holes, hole diameter and the position of the injection zone with respect to the boat: An inner injector allows process gases to be fed from both sides into the outer injector, see figure 7. This gives the possibility of adjusting the front-back balance. Three methods were evaluated to adjust this balance. First of all both the O2 and SiH4 flow can be balanced. In figure 8 it is shown that this method has only a very minor effect.
The changed gas distribution results in a different axial gas velocity pattern along the length of the tube. Apparently, the effect of a local increased injection rate is compensated by the resulting increased axial gas velocity. In the second method, the total gas flow and flow distribution are kept constant along the reactor. The leveling is done by partial substitution of SiH4 with N2 at the side of the injector where the deposition rate is too high. The oxygen distribution is kept constant and a minimum O2 to SiH4 ratio of 1.6 is guaranteed. As shown in figure 9, the wafer to wafer uniformity rapidly changes by this method. The third method is the reduction of the SiH4 at one side and keeping the oxygen flow constant. This method gives similar effects as the second method but is less sensitive see figure 10. This is due to the face that compensation also occurs in this third method but to a lesser extent than in method 1. The overall profile over the boat is not optimally flat in figure 10. This can be attributed to the injector design. A better optimized injector gave a very satisfactory wafer to wafer uniformity of f 3.6% to f 4.3%, as shown in figure 11. Furthermore the reproducibility is demonstrated. For three consecutive runs, the combined run to run and wafer to wafer uniformity is 5.2%.
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20 40 60 80 100 120
WAFER POSITION
Figure 8: The deposition rate as afunction of the wafer position with theflow distribution between front and back inlet as a parameter
- - - - - - - NO SIH, SUBSTITUTION
AT REAR SIDE
WAFER POSITION
Figure 9: The deposition rate as a function of the wafer position, showing the leveling by partial NZ substitution of SiH, at the back inlet.
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Figure 1 1 : The deposition rate as a function of the wafer position in the boat
5 . PSGIBPSG deposition.
Phosphorus incorporation.
The addition of pH3 results in the incorporation of phosphorus oxide into the film. Over a wide range a linear relationship is found between the molefraction of pH3 in the total hydride flow and the phosphorus content, as shown in figure 12. A similar relationship is observed by Learn [7]
although the slope of the line slightly differs. This is probably related to the totally different reactor set up. The double injector system allows the pH3 flow to be balanced independently from the SiH, flow which gives excellent control of the phosphorus level over the boat. Wafer to wafer uniformities in phosphorus level o f f 0.2 w/o or below can readily be achieved.
Figure 12: The phosphorus content as a function of the molefraction pH3 in the total hydrideflow.
The pressure was 280 mTorr, the SiH,flow was 240 sccm and the O2 to SiH, ratio 1.6.
20 40 60 80 100 120
TMB FLOW (SCCM)
Figure 13: The boron content as a function of the total TMBflow. The pressure was 280.mTorr, the SiH,flow was 240 sccm and the O2 to SiH4 ratio I .6.
Boron incorporation.
Commonly used source materials for boron doping are BC13 [5] and B2H6 [8]. The disadvantage of BCl, is that chlorine is incorporated into the film. This can result in blistering of the film for boron concentrations in excess of 2 w/o. B2H6 is relatively unstable at temperatures in excess of 300 OC.
Therefore, depletion effects are difficult to avoid in a horizontal tube reactor. In this study trimethylborate was used as a source material.The dissociation of this compound can be controlled over a wide temperature range [16, 171. A linear increase in boron content was observed with increasing TMB flow, see figure 13. The dissociation of TMB resulting in the formation of B203 on one hand and the oxidation of SiH4 and pH3 on the other, are independent processes. The slight increase in deposition rate and slight decrease in phosphorus content with increasing TMB flow in BPSG layers can completely be attributed to the additional deposition of B203. When the wafers were exposed to environmental air, the formation of boric acid was visible at the back of the wafer.
On the other hand, the front of the wafer remained featureless and bright. The explanation of this phenomenon is suggested to be as follows: the dissociation of TMB into B203 is a surface controlled process. Therefore, the B203 deposition rate can be expected to decrease less pronounced with decreasing wafer spacing than the oxidation of silane and phosphine. As a consequence, at the rear of the wafer (spacing 2.38 mm) the deposited film is much more boron rich and hygroscopic than at the front of the wafer (spacing 9.52 mm). Abandoning the proximity back to back wafer loading pattern and using a regular, equally spaced pattern will eliminate this problem. By means of the double injector, an excellent wafer to wafer uniformity in boron content can be achieved of
+
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6 . Particles.
In the past we observed, that the low local surface area to volume ratio in front of the boat resulted in excessive gas phase reactions and thick, hazy depositions. As a consequence, particle counts were high in the front of the boat and strongly decreased towards the pump side. Therefore a properly designed baffle was used to increase the local surface area to volume ratio and the hazy depositions were completely eliminated.
SCAVERING CROSS-SECTION )= 0.1 2 MICRON )= 0.3 MICRON )
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RUN NUMBER
Figure 14: The particle density as a function of the run number. The particles have a scattering cross- section 2 0.12 p2, which can be compared to particles of 0.3 pm if measured on bare silicon (all particle data were measured on 1000
A
layers).Particle counts below 0.6 #/cm* were consistently achieved for 0.3 micron particles as shown in figure 14. In two cases one wafer was excluded that exhibited a particle count exceeding 10 times the average value. These high values are attributed to the manual wafer handling. Dunng the sequence of runs both undoped oxide and PSG was deposited. In addition the SiH4 balance and process pressure were varied. None of these variations appeared to have a significant influence on the particle level. Furthermore, the distance between boat an baffle was varied. Although for large distances hazy deposits on the outside of the first cassette were observed again, this did not result in an increase in particle level. This is most likely a consequence of the effective shielding of the wafers by the cage. Nevertheless, it is expected that continued processing without a baffle will result in high particle counts after a few microns of deposition. In run 154, after a total cumulative deposition of 9.3 micron, the particle level was measured again. A density of 0.19 #/cm2 was measured. Apparently in the investigated period no systematic increase in particle level had occurred due to the cumulation of deposits on the boat.
7. Conclusions.
It is shown that uniformities over each wafer of lr3 % and over the boat of &4 % can consistently be achieved for a load of 100 150 mm wafers. The balance over the boat can be adjusted effectively by a double injector for SiH,. Dopant uniformities for P and B are
+
0.2 w/o and f 0.1 w/o respectively using PH3 and TMB. Particle levels under a variety of process conditions did not exceed 0.6 #/cm (0.3 p. particle size)Acknowledgement.
The authors thank J. van Putten for technical assistance. The Electron Probe Micro Analysis was performed by dr. ir. W. Sloof of the Delft Technical University.
References.
[l] R.S. Rosler, Solid State Technology, 3 (4) 63 (1977)
[2] W. Kern, W.A. Kurylo and C.J. Tino, RCA review 46, 117 (1985) [31 E.A. Taft, Journal of the Electrochemical Society 126,1728 (1979) [4] A.J. Learn, Journal of the Electrochemical Society
m.
390 (1985) [5] J.C. Mitchiner, 1. Mahawil. Solid State Technology 110, August 1987[61 B. ~orowitz, R.H. Wilson and T.B. Gorczyca, Solid State Technology 97, October 1987 171 A.J. Learn, Journal of the EIectrochemical Society
m,
405 (1985)181 A.J. Learn and B. Baerg, Thin Solid F i s 103 (1985)
[9] W. Kern, R.S. Rosler, Journal of Vacuum Science and Technology U 1082 (1977)
[IO] D.W. Shaw in C.H.L. Goodman (ed), Crystal Growth, Theory and Techniques, vol. 1, Plenum, New York 1974 [Ill P.J. Tobin, J.B. Price and L.M. Campbell, Journal of the Electrochemical Society 127.2222 (1980)
[121 M.L. Hitchman, Vacuum 377 (1984)
[13] J. Graham, High temperatures, High pressures 6,577 (1974)
[14] A. Stock, The Hydndes of Boron and Silicon. Cornell University Press, Ithaca, New York, 1933.
[15] T. Foster, G. Hoeye and J. Goldman, Journal of the Electrochemical Society
