Residue-Free Reactive Ion Etching of Silicon Carbide in ... - CiteSeerX

Residue-Free Reactive Ion Etching of Silicon Carbide in Fluorinated Plasmas II. 6H-SiC P. H. Yih* and A. J. Steckl** Nanoelectronics Laboratory, Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0030 ABSTRACT

We have previously reported the residue-free reactive ion etching (RIE) of 3C-SiC in CHFJO2, SFJO2, CF4/O2, and NF3/O2 mixtures with H2 additive. The minimum He concentration for residue prevention was found to vary with the O2 content of each gas plasma and from gas to gas. In this paper, we report on the reactive ion etching of 6H-SiC in the same gas plasmas. The 6H-SiC etch rate and etched surface morphology under different etching conditions are presented. A similar pattern of minimum H2 concentrations for the residue-free RIE of 6H-SiC is obtained. This indicates that the process involving H 2 additive in the fluorinated plasma could be applied to other SiC polytypes. A residue-free etching surface can be obtained without the H2 additive only in pure CHF3 plasma. Other plasma conditions do need various levels of H2 concentration in the plasma to obtain a clean etched surface. In both 3C- and 6H-SiC the etch rate decreases as the H2 concentration increases. No etching undercut was found for 6H-SiC with Al as etching mask. A graphite sheet covering the powered electrode also produced residue-free RIE but suffers from undesirable side effects. A comparison of minimum H2 levels for residue-free etching of 3C- and 6H-SiC in different plasmas is discussed. SiC is very well known as a semiconductor which has many advantages for operating under challenging conditions requiring high breakdown electric field, high thermal conductivity, chemical and physical stability, and suitably large energy bandgap. I'2 In the last few years, SiC technology has made substantial progress in areas ranging from 3C-SiC grown on Si 3-5 to 3C-SiC, 6 4H-SiC, 7,8 and 6H-SiC 9,10 grown on 6H-SiC. SiC substrates of 3C, 11,12 6H, 13,14and 4H 7,13polytypes have been grown by sublimation with varying levels of success. A comparison of the electrical properties of p-n homojunction diodes of 3C-SiC grown on Si i~ with those of 3C-SiC 16 and 6H-SiC 17 grown on 6H-SiC substrates indicates that a better device performance can be obtained from chemical vapor deposition (CVD) homostrueture growth. No electronic devices have yet been reported on 3C-SiC substrates grown by sublimation. However, 3C-SiC grown on Si can reduce the total cost and also lead to integration with Si technology. In order to operate under high temperature, high speed, and high power conditions, SiC device technology could be changed from 3C- to 6H- and 4H-SiC or other SiC polytypes in the future. 16 The chemical stability of SiC makes device structure patterning difficult. To chemically etch ("wet" etch) SiC usually involves molten salts (such as KOH, NaOH) and high temperatures (600 to 800~ For wet etching at room temperature, a photoassisted process is required. 19 On the other hand, plasma-assisted etching of SiC at room temperature has been quite successful 29 and is now widely utilized. Reactive ion etching (RIE) of 3C-SiC has been reported in a variety of fluorinated plasmas: CBrFJO2, 21 CHFJO2, 21,22 SFJQ, 21,22 CFJO2, 22-26 and NFJO2.22,25 Specific goals for SiC etching are to develop a high etch rate, high selectivity to etch mask material, anisotropie profiles, and a residuefree etch surface. These requirements have been studied for 3C-SiC etching in different fluorinated gases mixed with oxygen plasma. 21'22 Etching selectivity of 3C-SiC to Si greater than unity was achieved in a low fluorine plasma by decreasing the Si etch rate. 21'22For long-term RIE etching (>i0 min) of 3C-SiC, a considerable amount of residue was formed in the etch field for most mixtures of fluorinated gases and oxygen mixture. 22'26This is due to the so-called micromasking effect. 26 To prevent residue formation one can either use an H2 additive 22'26 to change the chemical reaction or utilize a nonmetallic material covering the powered-electrode. 22'25However, the reproducibility of the * Electrochemical Society Student Member. * * Electrochemical Society Active Member.

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latter technique is dependent on the geometry of the etching reactors A material with lower etch rate and sputter yield is required for this purpose. In this paper, w e continue our investigation of SiC plasma-assisted etching by reporting on the RIE of the 6H-SiC polytype in fluorinated plasmas. In particular, w e have focused on obtaining the conditions for residue-free 6H-SiC RIE through the addition of H2 to the plasma. W e follow the approach and model reported previously for residue-free 3C-SiC RIE, 22 referred to hereafter as Part I. As shown in Part I, various levels of H2 flow are needed for residue prevention under different etching conditions. The mechanisms of gas-phase chemistry and ion bombardment effects involved with residue formation and prevention were also discussed in Part I. The 6H-SiC etch rate, surface morphology, and profile anisotropy were obtained for various oxygen dilutions of the fluorinated gases. The 6H-SiC residue-free etching conditions are given for various fluorinated gas/oxygen mixtures and the general trends and mechanisms are discussed. As in the case of 3C-SiC (Part I), the use of H2 additive in the etching process of 6H-SiC involves a trade-off between the etch rate and a residuefree etching surface.

Experimental n-Type (0001) 6H-SiC substrates 13 with doping concentration of 7.2 • 1017 cm 3 were used for most etching experiments reported here. All etching experiments were performed on the Si face. The etching mask, consisting of 3000 A of Al was deposited by sputtering. For comparison, SiC with higher n-type doping concentration and p-type SiC were also etched to investigate the effects of doping concentration and dopant type. As in Part I with 3C-SiC, the reactive ion etching system (Plasma-Therm PKI241) used for the experiments is a parallel plate reactor with AI electrodes. A base pressure of I • i0 5 Torr was achieved in the etched chamber prior to performing the experiments. Four different fluorine compound gases (CHF3, CF~, SF6, and NF3) mixed with 02 were used for the etching experiments. Several etching parameters were kept constant: an RF power of 200 W, an etch time of 30 rain, and 20 sccm total flow rate for the mixture of fluorinated gas and oxygen. H2 additive was introduced to investigate the prevention of residue formation in the etch field. When different levels of hydrogen gas were added to the main gas stream, the working pressure was increased as necessary in 5 mTorr increments from 20 to 40 mTorr in order to accommodate the increase in the total gas flow rate. The 6H-SiC sample

J. Electrochem. Soc., Vol. 142, No. 1, January 1995 9 The Electrochemical Society, Inc.

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J. Electrochem. Soc., Vol. 142, No. t, January 1995 9 The Electrochemical Society, Inc. Table III. Surface morphology of 6H-SiC etched in the condition of Table I (etching mask, AI).

Table I. Minimum H2 flow rate of 3C-SiC/6H-SiC for prevention of residues in several etching ~lases mixed with various levels of oxygen (unit, sccm; etching mask, AI). Gas

0%O2

CHF3 0 "/0 ~ CF4 6/6 SF6 >16/>16 NF3 0 b/>16 H2 flow rate 1 to 5 (sccm) Process pressure 25 (mTorr)

10%O2

50%O2

90%02

2/2 12/>16 >16/>16 >16/>16 6 to 10

2/8 7.5/8 10/>16 >16/>16 11 to 15

2/2 2/2 5/8 10/4 16 to 20

30

35

40

size was k e p t small such t h a t the s a m p l e to electrode ratio was less t h a n 1%. The p u r i t i e s of the gases used in e t c h i n g w e r e 99.6% for 02, 99.99% for CHF3, 99.95% for CF4, 99.75% for SF6, 99.5% for NF3, and 99.999% for Hf. S o m e of the e t c h i n g e x p e r i m e n t s w e r e p e r f o r m e d using a g r a p h i t e sheet (with a p u r i t y of 99.9% and thickness of 0.245 mm) c o v e r i n g the p o w e r e d electrode. A f t e r RIE, t h e A1 m a s k was r e m o v e d b y c h e m i c a l e t c h i n g p r i o r to o t h e r m e a s u r e m e n t s . The e t c h step h e i g h t was m e a s u r e d by surface p r o f i l o m e t e r (DektaM. The e t c h e d surface m o r p h o l o g y was o b s e r v e d by s c a n n i n g electron m i c r o s c o p y (SEM). To o b t a i n the e t c h rate, samples w e r e e t c h e d for a l o n g e r t i m e (30 min) u n d e r conditions of no residue formation and for a much shorter time (5 min) when significant residues were formed. The latter case was required because the thick residue layer formed on the etched surface after the long-term etching prevents accurate step height measured by the surface profilometer.

Resuffs In Part I, we have reported on the reactive ion etching of 3C-SiC in the following fluorinated gas plasmas: CHF3, CF4, SF~, and NF3 mixed with oxygen. This included the maximum etch rate, the etching selectivity of 3C-SiC to Si and of 3C-SiC to SiOf, and the anisotropic etch ratio of 3C-SiC. In the case of 3C-SiC grown on St, an etch selectivity of 3C-SiC to Si greater than unity is important as an etch stop during device fabrication. The minimum H2 concentrations in each fluorinated plasma for 3C-SiC residue prevention are shown in Table I at different oxygen concentrations. Large H2 flow rates (>16 seem) are needed for residue prevention under plasma conditions of high fluorine density plasma (as measured by the actinometry technique). 28 The minimum H2 concentrations for residue prevention while etching 6H-SiC under the same plasma conditions are also shown in Table I. A maximum H e flow rate of 16 seem was also used, as in Part I. With a few exceptions, for most plasma conditions the minimum H2 flow rate for the 6H polytype was the same or higher than that for the 3C polytype. The plasma pressure as a function of the flow rate is also shown in Table I. The etch rates under the same plasma conditions (witlh and without H 2 additive) are shown in Table II. The presence and absence of residues for 6H-SiC etched under the conditions given in Table I are summarized in Table III.

Table II. Comparison of 6H-SiC etch rate with/without minimum H2 additive for several etching gases mixed with various levels of oxygen (unlt, A/min). Gas

0%O~

10%O2

50%O2

90%O~

CHF3

32 ~ 92/278 301 b/41O 298 b/483

87/152 123 b/311 315 b/451 360 b/568

80/263 139/292 191. "/321 261, b/480

41/186 94/208 130/308 115/303

SF~ NF3

0%02

CHF3

N

CF~

N

SF6 NF3

Y Y

10%O2 N }

Y

50%02

I

Y Y

90%O2

N

N

N

N

Y Y

N N

Y: residues uniformly cover the etched region. N: no residues. The details of e t c h i n g 6 H - S i C in each of the f o u r gases are discussed in t u r n below.

H2 not required. b Isolated spikes present.

CF 4

Gas

No H~ additive. Etch rate for 16 seem H2 additive, residues stillpresent.

Etching in CHF3.--In CHFdO2 plasma, the m a x i m u m etch r a t e of 3C-SiC was r e p o r t e d p r e v i o u s l y in P a r t I to o c c u r at a b o u t C H F j S 0 % O ~ , close to the highest [F] d e n sity. This indicates that a h i g h e r etch r a t e in C H F d Q p l a s m a can be o b t a i n e d by increasing the [F] density. F o r 6H-SiC, no Hz a d d i t i v e is n e e d e d in p u r e CHF~ p l a s m a to o b t a i n a residue-free etched surface. However, the corres p o n d i n g etch r a t e is very low, only 32 ,s as s h o w n in Table II. This is the only p l a s m a c o n d i t i o n w h e r e residuefree etching of the 6H polytype was obtained without H2 additive. In the cases of 10 and 90 % 02, a low level of 2 seem (10%) H2 additive (Table I) is needed for residue prevention, 29 whereas a higher value of 8 seem is needed at CHFJ 50%02. On the other hand, 2 sccm H2 is needed for etching 3C-SiC at all non-zero values of 02 dilution (Table I). The 6H-SiC etch rates with and without H2 additive in the CHFjO2 gas plasma are presented in Table II. A comparison of etch rate and minimum H2 flow rate for residue prevention in 6H-SiC samples with various doping levels and types was performed in the CHFJ50 % 02 plasma: (i) n+-type with 1 x 1019 cm -3 concentration; (it) p-type with doping concentration of 1.28 • i0 I~ cm ~. The etch rate and minimum H2 are shown in Table IV along with the corresponding values obtained for the standard n-type SiC results. No significant effects of the doping concentration and dopant type were found for residue prevention. As usual, the etch rate decreases as H2 is added to the plasma. Interestingly, the n§ SiC has a higher etch rate than the lower doped 6H-SiC for all values of H2 additive. This is consistent with the RIE etching of St, where as the n-type doping increases, it raises the Fermi level and thereby reduces the energy barrier for charge transfer. 3~ Approximately the same level of H2 additive (7 to 8 seem) is needed for SiC residue prevention, independent of doping concentrations and dopant types. This indicates that the H2 additive process is generally applicable for the fabrication of various SiC devices. Examples of n § and p-type 6H-SiC etched in CHFJS0%O= without H~ additive are shown in Fig. la and 2a, respectively. Because of the thick residue layer, a direct etch rate measurement on these surfaces may result in an inaccurate value. The effect of the H2 additive in preventing residue formation is clearly observed in the etching surface morphologies shown in Fig. lb and c for n*-SiC and in Fig. 2b for p-SiC.

Table IV. 6H-SiC etch rate in CHFJSO%O2 plasma as a function of H2 flow rate. (Etching mask, AI). H2 flow rate (seem) 0 6 7 8

n-type (7.2 • 1017) 263 147 117 80"

6H-SiC etch rate (/k/rain) n+-type p-type t(1 x 1019) (1.28 • I0 ~) 302 214 151 a

330 144 115 a

Doping concentration, crn -3. Etch rate at minimum H2 flow rate for residue-free etching.


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J. Electrochem. Soc., Vol. 142, No. 1, January 1995 9 The Electrochemical Society, Inc. etch field were found for all conditions of CFJO2 mixture in the absence of H2 additive. In the case of 0 and 90%02, the minimum H 2 flow rates (Table I) were at the same level as for 3C-SiC. Examples of 6H-SiC etched in pure CF 4 for 30 rain, without and with H2 additive, are shown in Fig. 3. The corresponding etch rates (Table If) are 278 and 92 A/ rain, respectively. At 10%O2 in CF 4 near the maximum IF] concentration in the plasma, residues were still observed to cover uniformly the etched 6H-SiC surface for the maximum (16 sccm) H2 additive attempted. For 3C-SiC, the CF4/ 10 % 02 plasma also generated the most residues, however, a residue-free condition-was reached for 12 secm H2 flow rate. The minimum H 2 flow rate decreases as the IF] density decreases as a function of 02 percentage. An example of 6H-SiC etched in CFJ50%Q for 30 rain is shown in Fig. 4a. To produce the residue-free etching surface shown in Fig. 4b, 8 sccm of H2 are required. As shown in Table If, introducing the H2 additive decreases the etch rate from 292 A/rain without H2 to 139 A/min with 8 scem H2. The minimum H2 additive is reduced to only 2 sccm at 90%02, yielding an etch rate of 94 A/min. For a higher etch rate under residue-free etching conditions, a reproducible result was o b t a i n e d at C F J 4 0 % O 2 plus 10 sccm H2 as

Fig. 1. Microphotographs of n+-type 6H-SiC etched in CHFJ50%02 plasma plus (a) O, (b) 6, and (c) 7 sccm H2.

Etching in CF~.--In C F J Q plasma, the m a x i m u m etch r a t e for 3C-SiC was o b t a i n e d in a 20%02 m i x t u r e as rep o r t e d in P a r t I, c o r r e s p o n d i n g to the highest [F] density in the plasma. F o r 6H-SiC, residues u n i f o r m l y covering the

Fig. 2. Microphotographs of p-type 6H-SiC etched in CHFJ50%02 plasma plus (a) 0 and (b) 7 sccm H2.



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surface. Comparing to 3C-SiC in Table I, a higher H2 is needed for 6H-SiC etching to prevent residues, even in the case of 90%02. Examples of 6H-SiC etched in SFd90%O 2 without and with 112 additive are shown in Fig. 7a and b, respectively. The corresponding etch rates (Table If) are 308 and 130 A/min, respectively. The 6H-SiC etch rate decreases as the H 2 additive is introduced in the SF6/O2 gas plasma, as shown in Table II.

Etching in NF3.--In NF3/O2 plasma, the m a x i m u m 3CSiC etch rate was r e p o r t e d in P a r t I at 10%O2, w h i l e the highest [F] density was found in p u r e NF3 plasma. For 6 H - S i C RIE w i t h o u t H2 additive, residues w e r e f o u n d unif o r m l y covering the etch field in all NFJO2 mixtures. An example of residues after etching in pure NF3 plasma is shown in Fig. 8a. Due to the high etch rate, the residues were formed with a spike (or delta) shape with high vertical-to-horizontal aspect ratio. In Part I, 3C-SiC etching in pure NF 3 plasma was observed to produce only occasional spikes in the etch field, p-Type 6H-SiC with doping concentration of 1.28 • i0 ~8 cm -3 and n+-type with doping concentration of i • 1019 cm -3 were also etched in pure NF3 plasma, resul}ing in residues with similar density and shape. An example of n+-type 6H-SiC etched in pure NF3 is shown in Fig. 8b. No dopant type or concentration effect on

Fig. 3. Microphotographs of 6H-SiC etched in CF4 plasma plus (a) 0 and (b) 6 sccm H2.

s h o w n in Fig. 5. In this case, an etch rate of 158 A/rain was obtained.

Etching in SF~.--In S F J O 2 plasma, the m a x i m u m etch r a t e of 3C-SiC was r e p o r t e d in P a r t I at S F J 2 0 % O 2 , close to the highest [F] density. F o r 6H-SiC, the residues w e r e f o u n d u n i f o r m l y c o v e r i n g the etch field in all conditions of S F J Q m i x t u r e p l a s m a w i t h o u t H2 additive. The residues w e r e observed to h a v e a spike-like shape. Residues w e r e f o u n d u n i f o r m l y covering the etched surface in the conditions of 0 and 10%O2 plasmas. This is consistent w i t h the corres p o n d i n g results for 3C-SiC r e p o r t e d in P a r t I. S E M observ a t i o n of these samples r e v e a l e d t h a t a layer of residues is f o r m e d in the etch field in u n d e r 5 rain etch time. This i n t e r f e r e d w i t h the a c c u r a t e m e a s u r e m e n t of the etch rate. To bypass this problem, the etch rates s h o w n in Table II for these t w o conditions w e r e m e a s u r e d w i t h a g r a p h i t e sheet c o v e r i n g the electrode to p r e v e n t residues. In the case of 50%02, the residues were also found still uniformly covering the etched region for the 16 scem H2 flow rate. The 6H-SiC surface etched in SFJ50%O~ without and with H2 additive are shown in Fig. 6a and b, respectively. Both conditions exhibit dense residues covering the entire etched

Fig. 4. Microphotographs of 6H-SiC etched in CF450%O2 plasma plus (a) 0 and (b) 8 sccm H~.


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J. Electrochem. Soc., Vol. 142, No. 1, January 1995 9 The Electrochemical Society, Inc. (Table I). This also results in a reduction in the etch rate from -300 to 115/~/min. The minimum flow rate of H2 is significantly lower than that required for 3C-SiC (Table I). The minimum H2 flow rate for residue-free 6H-SiC RIE with NFJ80 %02 flow rate increases significantly, from 4 to i0 sccm. This indicates a large amount of H2 is needed as the 02 percentage decreases. As shown in Table I, we need in excess of 16 secm H2 in the conditions of 0, i0, and 50%02. In the cases of 0 and 10%O2, residues interfered with accurate etch rate measurements. To be able to accurately measure the etch rate in these two conditions (as shown in Table If), we once again utilized a graphitecovered electrode to prevent residue formation. The etch rate decreases as the H2 additive is introduced in the NFJQ

gas plasma for all 02% levels. The NFJO2 plasma also produced the highest 6H-SiC etch rate of all the gas plasmas investigated.

Discussion The minimum H2 flow rate for 6H-SiC residue-free etching is shown in Table I along with the corresponding values for 3C-SiC. With a few notable exceptions, the genera] trend was found to be the same for both SiC polytypes,

Fig. 5. Microphotograph of 6H-SiC etched in CFJ40%O2 plasma plus 10 sccm H2. residue formation was found in pure NF3 RIE. In the NFJ 90%02 plasma, residues appear in the etch field with a much reduced density, as shown in Fig. 9a. It is interesting to note that given the size and shape of the spike-like residues they could be an appropriate vehicle for SiC quantum structures. As shown in Fig. 9b residue-free etching was obtained at NFJg0%o2 with the addition of 4 sccm H2

Fig. 6. Microphotographs of 6H-SiC etched in SFJ50%O2 plasma plus (a) 0 and (b) 16 sccm H2.

Fig. 7. Microphotographs of 6H-SiC etched in SFJ90%O2 plasma plus (a) 0 and (b) 8 sccm H2.



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higher plasma etch rate results in a residue structure with a higher aspect ratio. The 6H-SiC etch rate is generally lower than that the previously reported for 3C-SiC in Part I. The reason for this difference is very complex and may include a difference in the defect density and doping concentration, different stacking sequence of 3C- and 6HSiC, etc. Usually, when etching SiC with a higher defect density 23 and/or doping concentration, one can obtain a higher etch rate because of reduction in the energy barrier for charge transfer. 3~ It is important to compare the results obtained with the H2 additive to other techniques for obtaining residue-free etching. In Part I, we reported that the use of graphite and kapton sheets covering the sample-bearing electrode resulted in residue-free 3C-SiC etching in CF4/O 2 and SFJO2 plasmas. For comparison, we performed RIE of 6H-SiC in CFJ20%O2 and SFJ20%O2 plasmas with a graphite sheet covering the powered electrode for 30 rain, yielding the corresponding etch rates of 212 and 440 A/rain, respectively. The residue-free etching surface morphology is shown in Fig. lla and b, respectively. While the use of a graphite sheet covering the powered electrode in the RIE system was reported 22'25 in the SiC dry etching, the side effects are still unclear at this point. The effectiveness of a

Fig. 8. Microphotographs of 6H-SiC etched in pure NF3 plasma for 30 min (a) n and (b) n+-type.

namely, that the m i n i m u m H2 flow rate for residue-free etching is related to the fluorine concentration. This relationship is deduced from the following observations: (i) gases with a higher fluorine content (or higher F/C ratio) require a higher m i n i m u m H2 flow rate; (it)the dependence of the m i n i m u m H2 flow rate on 02 dilution in any one gas mirrors that of the fluorine concentration (i.e.,first increasing up to a certain O~~ as more fluorine is generated and then decreasing as the dilution effects~~ take over). Therefore, gases which are [F]-rich, such as SF6 and NF3, require greater values of H2 to provide a residue-free envi~ ronment. O n the other hand, the [F]-rich plasmas produce a significantlyhigher etch rate with or without the H2 additive (Table II).The one case which is a glaring exception to these trends involves RIE of 3C-SiC in pure NF3, where only a few isolated spikes are observed. Generally, in N F J 02 and SFdO2 3~plasmas, the residue shape resembled that of a spike. O n the other hand, the residue was formed as a round column in C H F J Q 28 and C F J O 2 25,54plasmas. Examples of residue shape produced by etching in C H F J 5 0 % 0 5 and NF2 plasmas for 30 rain are shown in Fig. 10a and b, respectively. The samples of Fig. 10a and b were etched in the same conditions as those in Fig. la and Fig. 8b, respectively. This indicates that for 6H-SiC a

Fig. 9. Microphotographs of 6H-SiC etched in NFJ90%O2 plasma plus (a) 0 and (b) 4 sccm H2.


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J. Electrochem. Soc., Vol. 142, No. 1, January 1995 9 The Electrochemical Society, Inc. etched 6H-SiC in CFJ20%O2 plasma with an indium tin oxide (ITO) mask. As expected, this indicates that the RIE etching reactor (electrodes) dominates the contamination micromasking effect. Therefore, to prevent residues in RIE mode dry etching, one can either modify the plasma chemistry by introducing H2 (as reported here and Part I) or use other contamination-free plasma (such as electron cyclotron resonance plasma 33 or transformer coupled p!asma 34) sources. The information reported in this paper will hopefully give current and future users of fluorinated gases for SiC RIE a series of conditions for obtaining residue-free etching. The specific choice appropriate for a given situation is a rather complex function of several parameters: work environment (e.g., research/development vs. production), prior experience with a given fluorinated gas, the relative importance of high etch rate and cost of gas, etc. For example, if high throughput is a paramount consideration, then conditions of high etch rate are necessary. From our experiments, we have found that the CFJQ/H2 mixture in the 5:5:4 ratio provides the highest residue-lree etch rate (-140 A/min).

Fig. 10. Microphotographs of 6H-SiC residues resulting from plasma etching in (a) CHF3/50%O2 and (b) NF3.

graphite sheet on the powered electrode for obtaining residue-free etching is dependent on the reactor geometry. 27 In a capacitively coupled parallel plate reactor, some contamination may also come from the upper electrode. For example, in our parallel plate RIE system, the experiments with graphite sheet covering the powered electrode (bottom electrode) result in a polymer film being deposited on the grounded electrode (upper electrode). The polymer film eventually cracked as the etching experiments continued, resulting in yellow flakes dropping from the upper electrode to the bottom electrode. Furthermore, under these etching conditions, contamination of the pumping equipment of the RIE system is readily apparent. Mass-loading effect 3~ of the graphite sheet in fluorinated gas is also inevitable. All of these aspects of etching with a graphite cover would be a great concern for SiC device processing in actual production. Since both the powered electrode and the thin film mask used during SiC etching are made of aluminum, they can both contribute to the residue formation process. To determine the dominant contribution, we have performed 6H-SiC etching experiments using indium tin oxide (ITO) as etching mask in CFJO2 plasma. As shown in Fig. 12, residues were found uniformly covering the etch field as we

Fig. 11. Microphotographsof 6H-SiC plasma-etchedwith graphite sheet covering the powered-electredein (a) CFJ20~O2 and (b) SFJ 20%02.



2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12.

Fig. 12. Microphotograph of 6H-SiC etched in CF4/20%02 plasma with indium tin oxide (ITO) as etching mask.

13. 14.

Summaryand Conclusions Residue-free reactive ion etching of 6H-SiC in CHFa/O2, SF6/O2, C F J Q , and NFJO2 plasmas with H2 additive was investigated. A similar trend for both 3C- and 6H-SiC is that the etch rate decreases as the H2 concentration increases in the plasma. Generally, to prevent residues in the 6H-SiC etch field, a higher Ha concentration is needed than for the corresponding 3C-SiC case. Within the range utilized, no doping concentration and dopant-type effects on the etched surface morphology were found. We have shown that residue-free reactive ion etching of 3C- (in Part I) and 6H-SiC can be obtained with the Ha additive process. This indicates that this process could probably be applied to other SiC polytypes. No etching undercut was found in any of the conditions we used. A1 is suitable as the etching mask for long-term etching. A residue-free 6H-SiC etching surface in the absence of H2 additive can be obtained only in pure CHF3 plasma. Other plasma conditions need various levels of H2 concentration in the plasma. We conclude that the process involving H2 additive in a fluorinated oxygen mixture plasma can be widely utilized for the fabrication of different SiC devices. The process of reactive ion etching with H2 additive has been shown to provide reliable residue-free surfaces in dozens of runs in our laboratory. Furthermore, the process has been reliably reproduced in other laboratories.

27. 28.

Acknowledgment

29.

We acknowledge the partial support of this work by the NASA Space Engineering Research Center at the University of Cincinnati. We also gratefully acknowledge the 6H-SiC material provided by Westinghouse Science and Technology Center and Wright-Patterson Air Force Base. Manuscript submitted March 14, 1994; revised m a n u script received Aug. 9, 1994. REFERENCES 1. J. A. Powell and L. G. Matus, in Amorphous and Crystalline Silicon Carbide, Springer Proceeding in

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

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