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Ion beam assisted film growth by high dose implantation of carbon into a liquid medium
ELSEVIER
Thin
Solid Films 278 ( 1996) 87-95
Ion beam assisted film growth by high dose implantation of carbon into a liquid medium R.R. Manory *, R. Sahagian *, S.N. Bunker, A.J. Armini Implant Sciences Corporation, 107 Audubon Road, 5 Corporate Place, WakeJSeld,MA 01880-1246, USA
Received 15 May 1995;accepted12 October 1995
Abstract
A method of thin film growth by ion implantation into a medium of liquid metal is presented. Thin layers of aluminum and indium deposited on silicon and silicon carbide (6H-type) substrates were liquified and implanted with carbon ions (at 190 keV). The resulting layers were characterized by X-ray diffraction, Auger electron spectroscopy and scanning electron microscopy. The results obtained on silicon substrate show growth of a carbonaceous layer on an intermediate layer of beta Sic when either Al or In were used. On Sic (6H-type) substrates no diamond growth was observed, with either Al or In. The experimental problems faced during this work are also presented and discussed. Keywords: Ion implantation;
Liquid metal; Diamond; Silicon carbide
1. Introduction Recently, a number of studies [l-3] reported diamond film growth by implantation of carbon (C) into a metallic medium to facilitate nucleation. These attempts can be classed into two distinct groups: Rrins and Geigher [ l] have implanted C + ions into a non-carbide-former metal such as copper or nickel, whereas Shimada et al. [ 21 and Yehoda et al. [ 31 have (surprisingly!) used thin films of iron to enhance diffusion of C species generated in an RF plasma towards an unscratched silicon (Si) substrate [2] or a silicon carbide (Sic) substrate [ 31. The latter technique can be defined as s surprising in its approach because it makes use of a strong carbide former, iron, which has been shown to obstruct diamond film formation [ 4 ] when used as a substrate. Shimada et al. [ 21 found that a precursor of the diamond layer growing under the iron was a layer of P-Sic. The process presented here is a new approach to diamond deposition in which a thin film of liquid metal-rather than a solid-is used as the intermediate medium. To our knowledge, prior to this work ion implantation into liquids has never been deliberately attempted, although it had, most probably, occurred accidentally when implant targets melted owing to * Corresponding author. ’ Permanent address: Department of Chemical and Metallurgical neering, Royal Melbourne Institute of Technology, bourne, Vie 3001, Australia.
EngiP.O.B. 2476V, Mel-
0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10040-6090(95)08187-9
overheating. Although unsuccessful for its original purpose of growing single crystal diamond, the experimental technique presented herein could be refined and used in other applications. 2. Principles of the technique Fig. 1 illustrates the concept of this technique. C is implanted into the solvent layer using a modified ion implanSINGLE
IMPLANTED ATOM FLUX
LIQUID SOL&NT
FILM
CRYSTAL SUBSTRATE
‘GROWING SINGLE CRYSTAL THIN FILM
Pig. 1. Schematic diagram of the technique: a thin film of molten metal covers the substrate; a capping layer is applied to prevent sputtering; implanted atoms come to rest within the liquid layer and migrate in two directions.
R.R. Munory et al. /Thin Solid Films 278 (1996) 87-95
88
ter which enables control of high substrate temperatures. As the carbon concentration exceeds its solubility limit, it is expected that atoms precipitate on the substrate and crystallize. The bombardment parameters and the thickness of the solvent layer can be selected such that the distribution peak of the implanted species can be placed close to the solventsubstrate interface to facilitate nucleation and growth without damage to the substrate or the growing crystal by the incoming beam. Some basic requirements from the medium material are: 1. a relatively low melting point; 2. the metallic liquid has to wet the substrate without agglomerating or “balling up” into discrete droplets; 3. the liquid should have low vapor pressure to prevent its evaporation in the vacuum system. A basic requirement regarding the process parameters for the technique is that the C ions must come to rest inside the solvent coating. As schematically shown in Fig. 1, it is expected that the undissolved C atoms will migrate in two directions, towards the liquid-substrate interface and towards the liquid-vacuum interface, with the former becoming the growing diamond layer and the latter forming a graphitic layer which is then sputtered away by the incoming beam. Capping of the liquid layer was done since sputtering of the liquid was expected, the capping layer is also depicted in Fig. 1. Such a cap should not completely cover the liquid film to allow for volume expansion underneath. Expansion of the liquid layer is expected to occur for two reasons: thermal expansion due to heating by the beam, and increase in the volume under the cap as a result of film growth. The purpose of this study was to explore whether the basic concept produces a film. Experiments aimed to test this concept were conducted on silicon and Sic wafers using Ga, Al and In, and the results are presented herein.
3. Experimental The substrates selected for this series of trials were ( 111) Si and Sic wafers (6H type, supplied by CREE Research Inc., NC). The latent oxide was removed prior to the deposition of the solvent layer in order to ensure good adhesion of the metallic solvent film to the Si substrate, and also to facilitate the expected nucleation of diamond at the metalsilicon interface. A procedure developed by Fork et al. [5] to etch Si and cap it with a temporary hydrogen monolayer Table 1 List of samples and the experimental
conditions
was used. This technique terminates the Si surface with hydrogen, causing its passivation. Hydrogen is removed at a later stage by heating above 500 “C,. and the wafer is then covered with the appropriate metallic medium. The capped substrates can be exposed to air for a short time with no detrimental effects. The procedure was performed in a nitrogen filled positive pressure glove box and the samples were transferred to the process chamber in a nitrogen filled vessel. A number of methods were used for the deposition of the solvent layers: Thermal evaporation of Ga was proven unsuccessful, as evaporated Ga was found to form small discontinuous droplets which did not entirely wet the surface. This effect could not be improved after a number of attempts, and was abandoned as a candidate for the solvent layer. Al, on the other hand, was successfully deposited by ion beam sputtering, with a typical thickness of 1 pm. Indium deposition by evaporation could not produce the desired silvery metallic films because the films were oxidized. Successful indium layers were finally deposited using a highly modified, low power, low temperature RF sputter system at Brown University’s Thin Film Laboratory. In view of the fact that indium is a very soft material, its thickness could not be measured by surface profilometry and the thickness prior to implantation was assessed by electron spectroscopy in one spot to be 350 nm. Calculations using ProfileCodeTM [6] showed that indium would require a capping layer of silica (SiO,), which was deposited using reactive ion sputtering. The In films produced were circular spots about 5 mm in diameter, which were subsequently covered by SiO* spots of 4 mm diameter. (The difference in size between the metal and the cap was intentional, to allow for expansion of the liquid, as indicated). Implantation was performed using a mass analyzed ion beam of C directed at the sample at a 0” incidence angle. Implantation parameters were selected such that the concentration peak of the profile would be at an approximate depth of 213 of the layer thickness, but because of many unknowns these calculations have proved to be only approximate. For the SIC substrates a C 13+ ion beam was used, to enable differentiation between the C from the base material and implanted C in various analytical techniques. The Al coated samples were implanted above the Al melting point at 700 “C. The implanted doses are given in Table 1. All implants were performed at 190 keV. A specially designed tube furnace was used to heat the substrate. A thermocouple mounted under the substrate was used for temperature control. The temper-
for implantation
Sample
Solvent
Substrate
Ion
Dose
Temp (“C)
Tests
3a.b 5a,b 6 7 8
Al Al In In In
(111)Si 6H-SiC (lll)Si (1ll)Si 6H Sic
Cl2 Cl3 Cl2 None Cl3
1.8/3.6X 10” 0.91/1.7x 1018 2.0x lo’*
700 700 500 500 400
XRD, AES Microprobe XRD, AES Control (evaporation XRD
2.0 x 1OL8
test
R.R. Manory et al. /Thin Solid Films 278 (1996) 87-95
ature reported is the actual measured temperature including beam heating. To obtain variation in the dose on the same sample wafer, groups of deposited spots were masked after the initial dose. The implant was continued and unmasked areas of the sample wafer received an additional dose. The techniques used to analyze the samples were X-ray diffraction (XRD) ( using copper radiation), Auger electron spectroscopy ( AES) , and scanning electron microscopy (SEM) and were performed by specialist contractors.
89
4. Results and discussion 4.1. UsingAl as the liquid layer on silicon The first results to be discussed here are those obtained from Al layers on Si. The two parts of sample 3, one receiving a 10~ dose of 1.8~10” C+ cm-‘and one receiving twice that dose were examined by XRD and the results are shown in Fig. 2 and Fig. 3 respectively, with d-spacings shown in
*lo6 J
2.00
1.62 1.26 0.96 0.72 0.50 0.32 0.16 0.06 0.02 10.0 2.00
15.0
20.0
25.0
‘30.0
35.0
40.0
45.0
50.0
05.0
90.0
-
1.621.26 0.960.72’
-
0.50 0.320.16.
+
0.06 0.02
50.0
I 55.0
i!
6
i .i L
zI 8 .i
60.0
I *
;
65.0
70.0
75.0
a ?
t *
1 60.0
Fig. 2. XRD spectrum for sample 3a. (Low dose implantation; Al on Si). Preparation conditions as per Table 1.
X104 1.00 0.01-
a
0.64’ 0.49-
I
0.360.25
2
-
c: :
0.160.0§-
10.0 1.00 0.61. 0.640.490.36 0.25 0.16-
15.0
20.0
25.0
30.0
65.0
70.0
35.0
40.0
75.0
60.0
45.0
50.0
-
k ;
0.090.040.01
-7 -4 50.0
65.0
60.0
Fig. 3. XRD spectrum for sample 3b. (High dose implantation;
Al on Si). Preparation
65.0 conditions
90.0 as per Table 1.
R.R. Manmy et al. /Thin Solid Films 278 (19%) 87-95
90
100.0
10.0
(d= 1.263 A) can also be distinguished. A peak which appears as a small “hump” at 35” in Fig, 2 becomes the second highest in Fig. 3 and is clearly attributed to SiC( 111). The relative intensity (Z/XI) of this peak has increased markedly because of the combined effect of the film growing on the surface and increasing its relative intensity while in the same time reducing the relative intensity of the Si substrate being buried underneath. Fig. 4 shows some relevant comparison spectra from the diffraction data file [7 1. Comparison of the spectra in Fig. 3 with those in Fig. 4 shows no indication of graphite formation. (Not included in Fig. 4 is data for aluminum carbide, which was also not observed in this case). The P-Sic layer appears to have grown well, and a very strong peak is observed for ( 11I), the highest intensity peak of this structure (d = 2.520 A). Other peaks are not observed, most probably indicating that this growth is preferentially (111) oriented. This is the expected result on a ( 111) Si substrate. The peak observed at 75” can be attributed to p-SiC(222) (d= 1.258 A> (which would be consistent with the ( 111) preferred orientation), or diamond (220) (d = 1,261 A), or both. Surface analysis was also performed on sample 3 to determine its composition with depth, and the AES results are shown in Fig. 5. As also shown by XRD, no Al is observed on the surface, and its concentration reaches only a modest 20 at.% deep below the sample surface, suggesting that it was either lost during the implantation or that it was indeed completely covered by the grown layer. These results indicate that film growth has occurred, and that the sample is covered with a layer consisting primarily of carbon mixed with Sic. The Si curve is especially revealing: it shows a nearly constant distribution of about 8 at.% until the substrate Si is encountered.
DIAMHMIO-JC 6- 67s
I 20.0
40.0 20.0
30.0
40.0
100.0 80.0 60.0 40.0 20.0
70.0
60.0
00.0
90.0
Al
I
60.0 50.0
20.0
50.0
40.0
30.0
ioo.0
10.0
C
I
1 42::: 60.0
60.0
I,
50.0
.
! 50.0
AL”&“M 4- 797
.I:.
70.0
.
20.0
90.0
27-i2 I, m.0
40.0
._,,I;,
60.0
100 .o BO.0 SO.0 40.0
,I, 00.0
100.0
PSiC 29-5329
I
20.0
I io.0
‘IiE/ 10.0
( 120.0
20.0
30.0
. .j 20.0
40.0
, 30.0
I 50.0
60.0
,j, ~ [ ,. 40.0
50.0
60.0
70 .o
. 70.0
. 60.0
90.0
J yyy$ BO.0
90.0
Fig. 4. Comparison of powder XRD spectra of diamond, aluminum, silicon, j3-Sic and graphite (from Ref. 171).
zlngstroms. It should be noted that the peak intensities in these figures differ significantly as the sensitivity has been increased for Fig. 3 to reveal peaks of low intensity. The spectrum in Fig. 2 basically shows the presence of Al and Si. Two additional low intensity peaks, at about 35” and 75” were identified by the computer. The relative intensities (Z/XI) of these two peaks increase significantly when the dose is doubled, as shown in Fig. 4. The maximum peak intensity in the latter figure is much lower than in the former. The lower overall intensity of the Si peak in this figure is attributed to the presence of a film on top of the wafer. The previously sharp ( 111) Si peak has also broadened, indicating damage to the single crystal. A broad peak at 75” AL/S1 121-ALlS.DEP
94x3
..“‘....’
,,,,,,,,,,,__,,.,).......,.,.........,.....
2000
40 4000 SPUTTERING
“.‘..
60 6000 m
Fig. 5. AES depth profile for sample 3b.
. ...,.
PEAK
SCALING FACTOR
ALL SISIC csc OS12
li-.--. l---1”““““”
R. R. Manory et al. /Thin Solid Films 278 (1996) 87-95
The picture revealed by both surface analysis and XRD data suggests the following sequence of events: liquid Al has dissolved the Si substrate underneath (the Al-Si phase diagram predicts about 20 at% Si at 700 “C [ 81) , and C atoms from the incoming beam have preferentially reacted with solid Si to form cubic Sic. There is definitely a layer of a carbonaceous material on top of this sample which is clearly shown by the AES data, and we believe that this layer has originally grown as diamond. As indicated, this experiment was intended to cause diamond growth inside the Al layer, apparently this has not happened. Instead, Si from the substrate was “floated” on the liquid and formed Sic on which the carbonaceous film grew. The Al layer has been buried under the film, exposing the latter directly to the beam. Thus, assuming that diamond had grown, the beam subsequently partially amorphized the layer. Diamond’s structure is known to be highly sensitive to ion bombardment [9] and if this model is correct the XRD data in Fig. 3 reflects its damaged structure: The broad peak at 75” suggests that a layer of small diamond crystallites growing with a (220) preferred orientation has apparently formed on the (222) planes of P-Sic, for which the d-spacing is very close. (This actually means that the habit planes of this growth is (11 l), as the (110) plane in diamond does not have a positive reflection in XRD.) This sequence of events makes sense particularly in view of recently published data [2] in which nucleation of diamond on Si was observed on particles consisting of Sic on the bottom and diamond on top. This model is also supported by the fact that no graphite appears in the XRD data. AES reveals that the top layers of this sample consist of C and a small amount of Si. Had this layer been SIC only, Auger analysis would have shown a Si content of about 50 at%. Had the Sic been completely buried under the C film, Si would not have been detected so close to the top surface. The low intensity broad peak observed in the XRD spectrum at 75” can most probably be interpreted as an indication of a diamond layer crystallizing on the (222) planes of the cubic Sic, since the closest lattice match between P-Sic and diamond is on these planes. The fact that the peaks in Fig. 3 in general are not sharp can be directly attributed to a combination of ion beam damage to the crystal structures of Sic, Si and diamond. It should be noted that the relatively high temperature at which the process was conducted, 700 “C, makes it very unlikely that the carbonaceous layer has grown originally as amorphous on a cubic SIC base. Moreover, because SIC has grown first, and diamond has been shown to grow preferentially on Sic under conditions of fast diffusion [ 2,3] even at temperatures as low as 350 “C, we believe that diamond was originally formed but was then destroyed. Despite the fact that a good diamond film was not grown, the results obtained were encouraging in the sense that a nongraphitic film was shown to grow under these conditions. The bombardment of the growing film by the incoming beam was unexpected because the molten metal layer was expected to cover the film at all times. The disappearance of the aluminum layer can be attributed to two reasons: sputter-
Fig. 6. SEM micrograph of a large aluminum carbide crystal obser ved in sample 4b (Al on Sic-6H). Preparation conditions as per Table 1.
ing by the beam and lack of coverage by the film. It should be noted that initial calculations by ProfileCodeTM [6] did not predict loss of Al due to evaporation or sputtering and thus a cap was not used. It is interesting to compare the effects observed on the Al-on-Si sample in which SIC has grown during the process, with those observed when a similar Al layer was used with a SIC (6H) substrate. The results differed significantly and will be discussed in the next section. 4.2. Using Al as a liquid medium on Sic Examination of the Al-on-Sic sample by optical microscopy showed large crystals protruding out of the surface. An electron micrograph of a typical crystal is shown in Fig. 6. The composition of these crystals was determined using electron probe microanalysis (EPMA) . They were shown to contain Al and C, with a small amount of Si, which most likely originated from the substrate. It was concluded that, in this case, formation of aluminum carbide crystals was observed. The difference between these results and those of the Alon-Si sample discussed before is striking and it stems from the fact that the SIC grown on Si was cubic P-Sic, whereas in sample 4 the substrate was hexagonal Sic. No interaction occurred between the liquid aluminum and its substrate, and the result was significant growth of aluminum carbide crystals, which did not occur on the Si substrate. Surprisingly, no carbonaceous layer was observed in this case, probably indicating that all the available carbon was consumed in the growth of these crystals while the remainder may have been sputtered away. In previous studies [ 2,3] iron, a well known carbide former, was used as an intermediate layer for enhancing diamond nucleation. The fact that Al forms a carbide indicated that it
R. R. Manory et al. /Thin Solid Films 278 (1996) 87-95
92
il/:;J[. , 1,._, )L , ;., X103
20.0 ioo.0.‘ 60.0: 60.0: 40.0: 20.0:
40.0
60.0
20.0 100.0: 60.0: 60.0: 40.0: 20.0: v 20.0
40.0
60.0
ioo.0
120.0 &Ggc
I I
I. 60.0
60.0
100.0
120.0 6H.SiC 29-1128
I
I
40.0
60.0
60.0
100.0
i20.0
100.0 60.0
33RSlC
60.0
22-1316
40.0 20.0 20.0
II 40.0
IllIll 60.0
II I
II 60.0
100.0
II I I 120.0
Fig. 7. XRD spectrum for sample 6 (In on Si) and comparison with possible spectra. Preparation conditions as per Table 1.
would prevent formation of diamond. It seems that the lack of diamond growth in this case was due to the incompatibility between the hexagonal substrate and the diamond structure. Some graphite may have formed, which could have been sputtered away by the beam, since it was not observed in the analysis. The data therefore suggest that the presence of cubic Sic at the interface is crucial for any possible diamond formation in this technique. In the next series of experiments In was used. Better results were expected because of its lower melting point, it is not a carbide former and it does not interact with Si. The results, which showed marked similarities, particularly with the Alon-Si case, are summarized in the next section. 4.3. Using indium as the intermediate layer For all In samples an SiO, cap, nominally 1250-1500 A thick was utilized. The XRD analysis performed on sample 6 (In on Si) is shown in Fig. 7. Metallic In is not observed in this spectrum indicating that it has either been completely covered by a growing film, or lost by evaporation or sputtering. In order to better show the smaller peaks the Si peak has been truncated in this figure. The most readily identifiable peaks are attributable to the Si substrate and to Sic. The formation of the latter phase indicates that despite the presence of an intermediate medium (and a capping layer) the beam has penetrated into the substrate. This may be due to the thinness of the In layer (which could not be measured, as mentioned) or to the fact that the indium layer was lost during the process despite the capping. There are a number of overlapping peaks between the various possible SIC structures, as indicated by the comparison spectra in Fig. 7 (from [7] ) : two sharp peaks at around 35” and 60” are common to a number of such structures. Despite this however, we can state with certainty that the phase formed here is the (cubic) j3 phase, because non-cubic phases
exhibit many additional peaks, none of which are observed, and also because the cubic form is commonly obtained by implanting Si single crystal with carbon [ lo]. The XRD data again suggests diamond formation. The large sharp peak at 95” has not been identified, but at its base it is very broad and it seems to include the (3 11) diamond peak at 9 1.5”. As well, the small peak at 75” can be treated in the same manner as in the previous discussion of the Al on Si case. This sample was also analyzed by AES, and the result is shown in Fig. 8. In was not detected, confirming the XRD finding that it was no longer on the substrate. The C deposit has a variable and small amount of Si, indicating that the SIC is again mixed in a C matrix. The SiOZ cap was still present on the surface, but the In has nevertheless disappeared. It should be noted that the capping layer was applied on top of the In film and deliberately allowed for expansion of the metal to the side to prevent cracking of the cap. Nevertheless, such cracks were observed to have formed during the implantation and they have apparently enabled significant evaporation of the metal. A possible explanation for the formation of these cracks is that during film growth under the cap, the liquid metal layer is squeezed out to make room for the growing film. Lateral expansion was apparently not sufficient for this purpose. As encountered in the Al-on-Si case, the overall analysis from AES and XRD data indicates the presence of a carbonaceous layer which was not well crystallized. This lack of crystallinity is again due to two possible causes: either the film grows as amorphous, or it grows as diamond and is then amorphized by the beam. Because of the nature of the process however, we are inclined towards the latter interpretation. Unlike the case with Al however, where SIC has formed from Si dissolved in liquid Al and has covered the Al layer, in this case Sic has formed directly in the substrate, basically showing that the In layer has not served its original purpose of being a medium for film growth.
R.R. Manory et al. /Thin Solid Films 278 (1996) 87-95
93
IN/S1 121-INiS.DEP
SMx3 PEAK
loo1
SCALING FACTOR l-
I--.-, 1---
0
3000
2000
1000
SPUTTERING
4000
Tw
Fig. 8. AES depth profile for sample 6 ( In-on-Si).
Fig. 9 shows the XRD data for the In-on-Sic sample. Peaks identified in this spectrum include the ( 101) and (202) peaks of indium (most probably indicating preferred orientation of the In layer, a commonly encountered feature in sputtered films), as well as the ( 100) and (006) peaks of graphite (at around 42” and 90” respectively). Data taken at a higher sensitivity also shows presence of indium oxide and the SiO, cap. Surprisingly, in this spectrum the substrate (SiCdH) peaks are not apparent. This effect can be explained only by assuming that the top surface of the substrate has been completely amorphized, or that the film is now so thick that it significantly reduced the X-rays penetration. Unlike the previous cases in which the liquid medium has either been buried
under the growing film or has completely vanished, it is clear that in this case In has remained on the substrate. It should be noted that the implantation was performed in the same run for all the In samples and the fact that In disappeared from one sample and not from the other indicates that the films were probably not of uniform thickness. Based on the appearance of the sample and on the XRD data it was concluded that in this case graphite has grown on the hexagonal substrate. The history of the In-on-Si sample can be derived from the AES data. The variable concentration of C shows that loss of the In solvent was gradual and not instantaneous. ProfileCodeTM [ 61 calculations indicate that without coverage the
I
60.0
.I
100.0
120.0
lOO.O! 60.0 :
cl+ sic
60.0: 40.0: 20.0. 20.0
29-1128 I 40.0
Fig. 9. XRD spectrum for sample 8 (In on SiCdH)
I 60.0
and comparison
60.0
100.0
120.0
with possible spectra. Preparation
conditions
as per Table 1.
94
R.R.Manoryet al. /Thin Solid Films278 (1996) 87-95
C depth profile of a 190 keV implant at 2x10’* C+ cm-’ could not have reached the concentration level of nearly 90 at.%, as measured, and that the C peak would have still been buried several thousand angstroms below the Si surface. Thus, it must be assumed that the In solvent survived for perhaps about half of the run and was being lost gradually, rather than in a catastrophic event. As thinning took place, the beam began to first bombard and thicken the initially grown layer of C and later passed through that layer and began adding C in the Si substrate beneath. This would account for the lack of regularity of the AES C distribution. On the other hand, in the case of the Sic substrate it seems that the In layer was thicker and therefore it was still present after the process. 4.4. General discussion The technique of ion implanting into a liquid metal did not produce a crystalline diamond film as desired. This study has shown, however, that a carbonaceous, non-graphitic layer was formed on Si, when either Al or In were used. The interpretation that diamond has formed and was subsequently amorphized is also supported by the loss of solvent. The metallic layer did not cover the growing film in either case, and the beam amorphized it. As mentioned, authors using other growth methods have shown that diamond crystals can grow on Si forming an Sic interface [2,3]. The technique presented here has definitely caused formation of B-Sic, which consistently appeared in XRD data when the substrate was Si. This is the required initial step in having the right base for possible diamond growth. Based on AES data, it is clear that a carbonaceous, (non-graphite) layer was formed in a mixture with Sic, particularly when Al was the solvent. In both cases, the C concentration is much greater than that in Sic. Since discrete layers of substrate/Sic/C were not observed, this implies that some Si must either have dissolved in the solvent metal in both cases in order to bring the Si out from the substrate, or that the solvent evaporated extremely quickly and most of what is observed is just a very high dose implant of C into first Si and later C-in-Si. The former appears to be the likely scenario in the case of the Al solvent because the concentration of the Si is so constant in the C layer and this is consistent with the fact that Si was floated in the liquid and thus a constant concentration was exposed to implantation. The latter model appears to apply to the In case. The Sic layer observed by XRD is consistent with growth from either dissolved Si or from C implanted into Si. It has long been known that C implanted into Si makes Sic [lo] and research was previously done at Implant Sciences to demonstrate that highly ordered crystalline silicon carbide could be produced if the temperature of the substrate was sufficiently high ( > 1000 “C) . In the case of the In layer, it is clear that the SIC found on the surface originates from direct implantation into the substrate, since In does not dissolve Si, and could not grow from dissolved Si.
Another interesting observation in this study was that low temperature growth on a non-cubic substrate has produced graphite. When Al was used on non-cubic Sic, the higher temperature process produced aluminum carbide. These effects emphasize the importance of the presence of a cubic SIC interface for successful film growth. Several problems were encountered in applying the technique which had not been anticipated. Some of these problems were: 1. deposition and maintenance of oxide-free solvent metal layers; 2. rapid loss of the medium during ion bombardment; 3. amorphization of the resulting thin film. The first problem has been solved satisfactorily, but the loss of solvent was not revealed until the results were analyzed. All of the AES and XRD data indicated that very little solvent was still present on the surface of the samples after implantation. This unexpectedly allowed the ion beam to reach and bombard the substrate surface, damaging any grown layer. This seems to be main technical difficulty encountered with the technique. The rate of sputtering by the beam is predictable, and published data exist on evaporation rates. However, the constants used for calculation proved to be insufficient, as they refers to solid, not to molten metal. (In the case of Al and In, a measurement of loss rate was made during the program, but in view of the mentioned difficulties in determining thickness these measurements are only approximate). Nevertheless, even though attempts were made to provide a reasonable margin for the coating thickness, the results indicate that the solvent was not present in any significant concentration towards the end of the implantation. This problem has not been resolved despite efforts to vary the temperature, and thus the evaporation rate. Possible speculations regarding the loss mechanism include local ion beam charging effects and/or local high temperatures due to the thermal insulation properties of the substrate and the low mass of the solvent coating. For further utilization of this technique, a method will need to be developed for maintaining the solvent thickness throughout the bombardment. Such a method might consist of continuously co-evaporating or co-sputtering the solvent during the high temperature phase. Alternatively, the codeposit might only need to consist of a renewal of the cap material, such as SiO,, which is much easier to produce in a contaminant-free layer.
5. Summarizing remarks To the authors’ knowledge, this study is the first deliberate attempt to use a liquid medium in ion beam deposition. The results obtained with both Al and In on silicon substrates are promising, although not fully conclusive. The films produced were not of the quality needed for Raman measurements and even if some diamond had formed during the process, sub-
R.R. Manory et al. /Thin Solid Films 278 (1996) 87-95
sequent solvent loss guaranteed the amorphization of the film. On the other hand, this study revealed the importance of the substrate crystal structure for any film grown: It was shown that SC-6H substrates are completely unsuitable for diamond growth. The study showed that straightforward application of ion implantation into a solvent liquid is significantly more difficult than envisaged, and that the process needs refinement before it can produce useful films. A method for maintaining the solvent during ion bombardment will be necessary for any future studies.
95
References
[ 11 J.F. Prim and H.L. Geigher, in R. Messier, J.T. Glass, J.E. Butler and R. Roy (et%.), New Diamond Science and Technology, Mater. Res. Sot.. Pittsburgh, PA, 1991, p. 561; J.F. Prins, U.S. PatmtApplication 842457 (1990). [2] Y. Shimada, N. Mutsukura, Y. Machi, J. Appl. Phys., 71(1992)
4019.
[3] J.E. Yehoda, R.I. Fuentes, J.C. Tsang, S.J. Whitehair, CR. Guamieri, J.J. Cuomo, Appl. Phys. Z.&t., 60 (1992) 2865. [41 P.S. Weiser, S. Prawer, A. Hoffman, R. Manory, P.J.K. Paterson, S.A. Stuart, J. Appl. Phys., 72 (1992) 4643. [5] D.K. Fork,D.B. Fenner,G. Connel1,J.M. Phil1ipqT.H. Geballe,Appl.
Acknowledgements
Thanks are due to Prof. J. Laferski from Brown University and his staff for the successful deposition of indium films. XRD data was collected by Ed Werekhois at Manlabs, Cambridge, Mass., and the AES results were obtained by Joe Geller, Geller Analytical Labs, Peabody, MA. This work was supported by the DOE contract DAAL-03-92-C-0030.
Phys. ktt., 57 (1990) 1137. [61 S.N. Bunker, A.J. Armini, Nucl. Instrum. Methods B, 39 ( 1989) 7. VI Diffraction Data of Inorganic Compounds, Joint Committee for Powder Diffraction Standards, Swarthmore, PA, 1991, various cards, as listed. [gl M. Hansen, K. Anderko, Constitution of Binary Alloys, McGraw Hill, New York, NY, 1958, pp.75. [91 J.J. Hauser, J.R. Patel, Solid State Commun., 18 ( 1976) 789. [ 101 K.J. Reson, L.P. Hemment, 3. Stoemenos, J. Davis, C.E. Celler, Appl. Phys. Lett.. 51 (1987) 2242.
Thin
Solid Films 278 ( 1996) 87-95
Ion beam assisted film growth by high dose implantation of carbon into a liquid medium R.R. Manory *, R. Sahagian *, S.N. Bunker, A.J. Armini Implant Sciences Corporation, 107 Audubon Road, 5 Corporate Place, WakeJSeld,MA 01880-1246, USA
Received 15 May 1995;accepted12 October 1995
Abstract
A method of thin film growth by ion implantation into a medium of liquid metal is presented. Thin layers of aluminum and indium deposited on silicon and silicon carbide (6H-type) substrates were liquified and implanted with carbon ions (at 190 keV). The resulting layers were characterized by X-ray diffraction, Auger electron spectroscopy and scanning electron microscopy. The results obtained on silicon substrate show growth of a carbonaceous layer on an intermediate layer of beta Sic when either Al or In were used. On Sic (6H-type) substrates no diamond growth was observed, with either Al or In. The experimental problems faced during this work are also presented and discussed. Keywords: Ion implantation;
Liquid metal; Diamond; Silicon carbide
1. Introduction Recently, a number of studies [l-3] reported diamond film growth by implantation of carbon (C) into a metallic medium to facilitate nucleation. These attempts can be classed into two distinct groups: Rrins and Geigher [ l] have implanted C + ions into a non-carbide-former metal such as copper or nickel, whereas Shimada et al. [ 21 and Yehoda et al. [ 31 have (surprisingly!) used thin films of iron to enhance diffusion of C species generated in an RF plasma towards an unscratched silicon (Si) substrate [2] or a silicon carbide (Sic) substrate [ 31. The latter technique can be defined as s surprising in its approach because it makes use of a strong carbide former, iron, which has been shown to obstruct diamond film formation [ 4 ] when used as a substrate. Shimada et al. [ 21 found that a precursor of the diamond layer growing under the iron was a layer of P-Sic. The process presented here is a new approach to diamond deposition in which a thin film of liquid metal-rather than a solid-is used as the intermediate medium. To our knowledge, prior to this work ion implantation into liquids has never been deliberately attempted, although it had, most probably, occurred accidentally when implant targets melted owing to * Corresponding author. ’ Permanent address: Department of Chemical and Metallurgical neering, Royal Melbourne Institute of Technology, bourne, Vie 3001, Australia.
EngiP.O.B. 2476V, Mel-
0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10040-6090(95)08187-9
overheating. Although unsuccessful for its original purpose of growing single crystal diamond, the experimental technique presented herein could be refined and used in other applications. 2. Principles of the technique Fig. 1 illustrates the concept of this technique. C is implanted into the solvent layer using a modified ion implanSINGLE
IMPLANTED ATOM FLUX
LIQUID SOL&NT
FILM
CRYSTAL SUBSTRATE
‘GROWING SINGLE CRYSTAL THIN FILM
Pig. 1. Schematic diagram of the technique: a thin film of molten metal covers the substrate; a capping layer is applied to prevent sputtering; implanted atoms come to rest within the liquid layer and migrate in two directions.
R.R. Munory et al. /Thin Solid Films 278 (1996) 87-95
88
ter which enables control of high substrate temperatures. As the carbon concentration exceeds its solubility limit, it is expected that atoms precipitate on the substrate and crystallize. The bombardment parameters and the thickness of the solvent layer can be selected such that the distribution peak of the implanted species can be placed close to the solventsubstrate interface to facilitate nucleation and growth without damage to the substrate or the growing crystal by the incoming beam. Some basic requirements from the medium material are: 1. a relatively low melting point; 2. the metallic liquid has to wet the substrate without agglomerating or “balling up” into discrete droplets; 3. the liquid should have low vapor pressure to prevent its evaporation in the vacuum system. A basic requirement regarding the process parameters for the technique is that the C ions must come to rest inside the solvent coating. As schematically shown in Fig. 1, it is expected that the undissolved C atoms will migrate in two directions, towards the liquid-substrate interface and towards the liquid-vacuum interface, with the former becoming the growing diamond layer and the latter forming a graphitic layer which is then sputtered away by the incoming beam. Capping of the liquid layer was done since sputtering of the liquid was expected, the capping layer is also depicted in Fig. 1. Such a cap should not completely cover the liquid film to allow for volume expansion underneath. Expansion of the liquid layer is expected to occur for two reasons: thermal expansion due to heating by the beam, and increase in the volume under the cap as a result of film growth. The purpose of this study was to explore whether the basic concept produces a film. Experiments aimed to test this concept were conducted on silicon and Sic wafers using Ga, Al and In, and the results are presented herein.
3. Experimental The substrates selected for this series of trials were ( 111) Si and Sic wafers (6H type, supplied by CREE Research Inc., NC). The latent oxide was removed prior to the deposition of the solvent layer in order to ensure good adhesion of the metallic solvent film to the Si substrate, and also to facilitate the expected nucleation of diamond at the metalsilicon interface. A procedure developed by Fork et al. [5] to etch Si and cap it with a temporary hydrogen monolayer Table 1 List of samples and the experimental
conditions
was used. This technique terminates the Si surface with hydrogen, causing its passivation. Hydrogen is removed at a later stage by heating above 500 “C,. and the wafer is then covered with the appropriate metallic medium. The capped substrates can be exposed to air for a short time with no detrimental effects. The procedure was performed in a nitrogen filled positive pressure glove box and the samples were transferred to the process chamber in a nitrogen filled vessel. A number of methods were used for the deposition of the solvent layers: Thermal evaporation of Ga was proven unsuccessful, as evaporated Ga was found to form small discontinuous droplets which did not entirely wet the surface. This effect could not be improved after a number of attempts, and was abandoned as a candidate for the solvent layer. Al, on the other hand, was successfully deposited by ion beam sputtering, with a typical thickness of 1 pm. Indium deposition by evaporation could not produce the desired silvery metallic films because the films were oxidized. Successful indium layers were finally deposited using a highly modified, low power, low temperature RF sputter system at Brown University’s Thin Film Laboratory. In view of the fact that indium is a very soft material, its thickness could not be measured by surface profilometry and the thickness prior to implantation was assessed by electron spectroscopy in one spot to be 350 nm. Calculations using ProfileCodeTM [6] showed that indium would require a capping layer of silica (SiO,), which was deposited using reactive ion sputtering. The In films produced were circular spots about 5 mm in diameter, which were subsequently covered by SiO* spots of 4 mm diameter. (The difference in size between the metal and the cap was intentional, to allow for expansion of the liquid, as indicated). Implantation was performed using a mass analyzed ion beam of C directed at the sample at a 0” incidence angle. Implantation parameters were selected such that the concentration peak of the profile would be at an approximate depth of 213 of the layer thickness, but because of many unknowns these calculations have proved to be only approximate. For the SIC substrates a C 13+ ion beam was used, to enable differentiation between the C from the base material and implanted C in various analytical techniques. The Al coated samples were implanted above the Al melting point at 700 “C. The implanted doses are given in Table 1. All implants were performed at 190 keV. A specially designed tube furnace was used to heat the substrate. A thermocouple mounted under the substrate was used for temperature control. The temper-
for implantation
Sample
Solvent
Substrate
Ion
Dose
Temp (“C)
Tests
3a.b 5a,b 6 7 8
Al Al In In In
(111)Si 6H-SiC (lll)Si (1ll)Si 6H Sic
Cl2 Cl3 Cl2 None Cl3
1.8/3.6X 10” 0.91/1.7x 1018 2.0x lo’*
700 700 500 500 400
XRD, AES Microprobe XRD, AES Control (evaporation XRD
2.0 x 1OL8
test
R.R. Manory et al. /Thin Solid Films 278 (1996) 87-95
ature reported is the actual measured temperature including beam heating. To obtain variation in the dose on the same sample wafer, groups of deposited spots were masked after the initial dose. The implant was continued and unmasked areas of the sample wafer received an additional dose. The techniques used to analyze the samples were X-ray diffraction (XRD) ( using copper radiation), Auger electron spectroscopy ( AES) , and scanning electron microscopy (SEM) and were performed by specialist contractors.
89
4. Results and discussion 4.1. UsingAl as the liquid layer on silicon The first results to be discussed here are those obtained from Al layers on Si. The two parts of sample 3, one receiving a 10~ dose of 1.8~10” C+ cm-‘and one receiving twice that dose were examined by XRD and the results are shown in Fig. 2 and Fig. 3 respectively, with d-spacings shown in
*lo6 J
2.00
1.62 1.26 0.96 0.72 0.50 0.32 0.16 0.06 0.02 10.0 2.00
15.0
20.0
25.0
‘30.0
35.0
40.0
45.0
50.0
05.0
90.0
-
1.621.26 0.960.72’
-
0.50 0.320.16.
+
0.06 0.02
50.0
I 55.0
i!
6
i .i L
zI 8 .i
60.0
I *
;
65.0
70.0
75.0
a ?
t *
1 60.0
Fig. 2. XRD spectrum for sample 3a. (Low dose implantation; Al on Si). Preparation conditions as per Table 1.
X104 1.00 0.01-
a
0.64’ 0.49-
I
0.360.25
2
-
c: :
0.160.0§-
10.0 1.00 0.61. 0.640.490.36 0.25 0.16-
15.0
20.0
25.0
30.0
65.0
70.0
35.0
40.0
75.0
60.0
45.0
50.0
-
k ;
0.090.040.01
-7 -4 50.0
65.0
60.0
Fig. 3. XRD spectrum for sample 3b. (High dose implantation;
Al on Si). Preparation
65.0 conditions
90.0 as per Table 1.
R.R. Manmy et al. /Thin Solid Films 278 (19%) 87-95
90
100.0
10.0
(d= 1.263 A) can also be distinguished. A peak which appears as a small “hump” at 35” in Fig, 2 becomes the second highest in Fig. 3 and is clearly attributed to SiC( 111). The relative intensity (Z/XI) of this peak has increased markedly because of the combined effect of the film growing on the surface and increasing its relative intensity while in the same time reducing the relative intensity of the Si substrate being buried underneath. Fig. 4 shows some relevant comparison spectra from the diffraction data file [7 1. Comparison of the spectra in Fig. 3 with those in Fig. 4 shows no indication of graphite formation. (Not included in Fig. 4 is data for aluminum carbide, which was also not observed in this case). The P-Sic layer appears to have grown well, and a very strong peak is observed for ( 11I), the highest intensity peak of this structure (d = 2.520 A). Other peaks are not observed, most probably indicating that this growth is preferentially (111) oriented. This is the expected result on a ( 111) Si substrate. The peak observed at 75” can be attributed to p-SiC(222) (d= 1.258 A> (which would be consistent with the ( 111) preferred orientation), or diamond (220) (d = 1,261 A), or both. Surface analysis was also performed on sample 3 to determine its composition with depth, and the AES results are shown in Fig. 5. As also shown by XRD, no Al is observed on the surface, and its concentration reaches only a modest 20 at.% deep below the sample surface, suggesting that it was either lost during the implantation or that it was indeed completely covered by the grown layer. These results indicate that film growth has occurred, and that the sample is covered with a layer consisting primarily of carbon mixed with Sic. The Si curve is especially revealing: it shows a nearly constant distribution of about 8 at.% until the substrate Si is encountered.
DIAMHMIO-JC 6- 67s
I 20.0
40.0 20.0
30.0
40.0
100.0 80.0 60.0 40.0 20.0
70.0
60.0
00.0
90.0
Al
I
60.0 50.0
20.0
50.0
40.0
30.0
ioo.0
10.0
C
I
1 42::: 60.0
60.0
I,
50.0
.
! 50.0
AL”&“M 4- 797
.I:.
70.0
.
20.0
90.0
27-i2 I, m.0
40.0
._,,I;,
60.0
100 .o BO.0 SO.0 40.0
,I, 00.0
100.0
PSiC 29-5329
I
20.0
I io.0
‘IiE/ 10.0
( 120.0
20.0
30.0
. .j 20.0
40.0
, 30.0
I 50.0
60.0
,j, ~ [ ,. 40.0
50.0
60.0
70 .o
. 70.0
. 60.0
90.0
J yyy$ BO.0
90.0
Fig. 4. Comparison of powder XRD spectra of diamond, aluminum, silicon, j3-Sic and graphite (from Ref. 171).
zlngstroms. It should be noted that the peak intensities in these figures differ significantly as the sensitivity has been increased for Fig. 3 to reveal peaks of low intensity. The spectrum in Fig. 2 basically shows the presence of Al and Si. Two additional low intensity peaks, at about 35” and 75” were identified by the computer. The relative intensities (Z/XI) of these two peaks increase significantly when the dose is doubled, as shown in Fig. 4. The maximum peak intensity in the latter figure is much lower than in the former. The lower overall intensity of the Si peak in this figure is attributed to the presence of a film on top of the wafer. The previously sharp ( 111) Si peak has also broadened, indicating damage to the single crystal. A broad peak at 75” AL/S1 121-ALlS.DEP
94x3
..“‘....’
,,,,,,,,,,,__,,.,).......,.,.........,.....
2000
40 4000 SPUTTERING
“.‘..
60 6000 m
Fig. 5. AES depth profile for sample 3b.
. ...,.
PEAK
SCALING FACTOR
ALL SISIC csc OS12
li-.--. l---1”““““”
R. R. Manory et al. /Thin Solid Films 278 (1996) 87-95
The picture revealed by both surface analysis and XRD data suggests the following sequence of events: liquid Al has dissolved the Si substrate underneath (the Al-Si phase diagram predicts about 20 at% Si at 700 “C [ 81) , and C atoms from the incoming beam have preferentially reacted with solid Si to form cubic Sic. There is definitely a layer of a carbonaceous material on top of this sample which is clearly shown by the AES data, and we believe that this layer has originally grown as diamond. As indicated, this experiment was intended to cause diamond growth inside the Al layer, apparently this has not happened. Instead, Si from the substrate was “floated” on the liquid and formed Sic on which the carbonaceous film grew. The Al layer has been buried under the film, exposing the latter directly to the beam. Thus, assuming that diamond had grown, the beam subsequently partially amorphized the layer. Diamond’s structure is known to be highly sensitive to ion bombardment [9] and if this model is correct the XRD data in Fig. 3 reflects its damaged structure: The broad peak at 75” suggests that a layer of small diamond crystallites growing with a (220) preferred orientation has apparently formed on the (222) planes of P-Sic, for which the d-spacing is very close. (This actually means that the habit planes of this growth is (11 l), as the (110) plane in diamond does not have a positive reflection in XRD.) This sequence of events makes sense particularly in view of recently published data [2] in which nucleation of diamond on Si was observed on particles consisting of Sic on the bottom and diamond on top. This model is also supported by the fact that no graphite appears in the XRD data. AES reveals that the top layers of this sample consist of C and a small amount of Si. Had this layer been SIC only, Auger analysis would have shown a Si content of about 50 at%. Had the Sic been completely buried under the C film, Si would not have been detected so close to the top surface. The low intensity broad peak observed in the XRD spectrum at 75” can most probably be interpreted as an indication of a diamond layer crystallizing on the (222) planes of the cubic Sic, since the closest lattice match between P-Sic and diamond is on these planes. The fact that the peaks in Fig. 3 in general are not sharp can be directly attributed to a combination of ion beam damage to the crystal structures of Sic, Si and diamond. It should be noted that the relatively high temperature at which the process was conducted, 700 “C, makes it very unlikely that the carbonaceous layer has grown originally as amorphous on a cubic SIC base. Moreover, because SIC has grown first, and diamond has been shown to grow preferentially on Sic under conditions of fast diffusion [ 2,3] even at temperatures as low as 350 “C, we believe that diamond was originally formed but was then destroyed. Despite the fact that a good diamond film was not grown, the results obtained were encouraging in the sense that a nongraphitic film was shown to grow under these conditions. The bombardment of the growing film by the incoming beam was unexpected because the molten metal layer was expected to cover the film at all times. The disappearance of the aluminum layer can be attributed to two reasons: sputter-
Fig. 6. SEM micrograph of a large aluminum carbide crystal obser ved in sample 4b (Al on Sic-6H). Preparation conditions as per Table 1.
ing by the beam and lack of coverage by the film. It should be noted that initial calculations by ProfileCodeTM [6] did not predict loss of Al due to evaporation or sputtering and thus a cap was not used. It is interesting to compare the effects observed on the Al-on-Si sample in which SIC has grown during the process, with those observed when a similar Al layer was used with a SIC (6H) substrate. The results differed significantly and will be discussed in the next section. 4.2. Using Al as a liquid medium on Sic Examination of the Al-on-Sic sample by optical microscopy showed large crystals protruding out of the surface. An electron micrograph of a typical crystal is shown in Fig. 6. The composition of these crystals was determined using electron probe microanalysis (EPMA) . They were shown to contain Al and C, with a small amount of Si, which most likely originated from the substrate. It was concluded that, in this case, formation of aluminum carbide crystals was observed. The difference between these results and those of the Alon-Si sample discussed before is striking and it stems from the fact that the SIC grown on Si was cubic P-Sic, whereas in sample 4 the substrate was hexagonal Sic. No interaction occurred between the liquid aluminum and its substrate, and the result was significant growth of aluminum carbide crystals, which did not occur on the Si substrate. Surprisingly, no carbonaceous layer was observed in this case, probably indicating that all the available carbon was consumed in the growth of these crystals while the remainder may have been sputtered away. In previous studies [ 2,3] iron, a well known carbide former, was used as an intermediate layer for enhancing diamond nucleation. The fact that Al forms a carbide indicated that it
R. R. Manory et al. /Thin Solid Films 278 (1996) 87-95
92
il/:;J[. , 1,._, )L , ;., X103
20.0 ioo.0.‘ 60.0: 60.0: 40.0: 20.0:
40.0
60.0
20.0 100.0: 60.0: 60.0: 40.0: 20.0: v 20.0
40.0
60.0
ioo.0
120.0 &Ggc
I I
I. 60.0
60.0
100.0
120.0 6H.SiC 29-1128
I
I
40.0
60.0
60.0
100.0
i20.0
100.0 60.0
33RSlC
60.0
22-1316
40.0 20.0 20.0
II 40.0
IllIll 60.0
II I
II 60.0
100.0
II I I 120.0
Fig. 7. XRD spectrum for sample 6 (In on Si) and comparison with possible spectra. Preparation conditions as per Table 1.
would prevent formation of diamond. It seems that the lack of diamond growth in this case was due to the incompatibility between the hexagonal substrate and the diamond structure. Some graphite may have formed, which could have been sputtered away by the beam, since it was not observed in the analysis. The data therefore suggest that the presence of cubic Sic at the interface is crucial for any possible diamond formation in this technique. In the next series of experiments In was used. Better results were expected because of its lower melting point, it is not a carbide former and it does not interact with Si. The results, which showed marked similarities, particularly with the Alon-Si case, are summarized in the next section. 4.3. Using indium as the intermediate layer For all In samples an SiO, cap, nominally 1250-1500 A thick was utilized. The XRD analysis performed on sample 6 (In on Si) is shown in Fig. 7. Metallic In is not observed in this spectrum indicating that it has either been completely covered by a growing film, or lost by evaporation or sputtering. In order to better show the smaller peaks the Si peak has been truncated in this figure. The most readily identifiable peaks are attributable to the Si substrate and to Sic. The formation of the latter phase indicates that despite the presence of an intermediate medium (and a capping layer) the beam has penetrated into the substrate. This may be due to the thinness of the In layer (which could not be measured, as mentioned) or to the fact that the indium layer was lost during the process despite the capping. There are a number of overlapping peaks between the various possible SIC structures, as indicated by the comparison spectra in Fig. 7 (from [7] ) : two sharp peaks at around 35” and 60” are common to a number of such structures. Despite this however, we can state with certainty that the phase formed here is the (cubic) j3 phase, because non-cubic phases
exhibit many additional peaks, none of which are observed, and also because the cubic form is commonly obtained by implanting Si single crystal with carbon [ lo]. The XRD data again suggests diamond formation. The large sharp peak at 95” has not been identified, but at its base it is very broad and it seems to include the (3 11) diamond peak at 9 1.5”. As well, the small peak at 75” can be treated in the same manner as in the previous discussion of the Al on Si case. This sample was also analyzed by AES, and the result is shown in Fig. 8. In was not detected, confirming the XRD finding that it was no longer on the substrate. The C deposit has a variable and small amount of Si, indicating that the SIC is again mixed in a C matrix. The SiOZ cap was still present on the surface, but the In has nevertheless disappeared. It should be noted that the capping layer was applied on top of the In film and deliberately allowed for expansion of the metal to the side to prevent cracking of the cap. Nevertheless, such cracks were observed to have formed during the implantation and they have apparently enabled significant evaporation of the metal. A possible explanation for the formation of these cracks is that during film growth under the cap, the liquid metal layer is squeezed out to make room for the growing film. Lateral expansion was apparently not sufficient for this purpose. As encountered in the Al-on-Si case, the overall analysis from AES and XRD data indicates the presence of a carbonaceous layer which was not well crystallized. This lack of crystallinity is again due to two possible causes: either the film grows as amorphous, or it grows as diamond and is then amorphized by the beam. Because of the nature of the process however, we are inclined towards the latter interpretation. Unlike the case with Al however, where SIC has formed from Si dissolved in liquid Al and has covered the Al layer, in this case Sic has formed directly in the substrate, basically showing that the In layer has not served its original purpose of being a medium for film growth.
R.R. Manory et al. /Thin Solid Films 278 (1996) 87-95
93
IN/S1 121-INiS.DEP
SMx3 PEAK
loo1
SCALING FACTOR l-
I--.-, 1---
0
3000
2000
1000
SPUTTERING
4000
Tw
Fig. 8. AES depth profile for sample 6 ( In-on-Si).
Fig. 9 shows the XRD data for the In-on-Sic sample. Peaks identified in this spectrum include the ( 101) and (202) peaks of indium (most probably indicating preferred orientation of the In layer, a commonly encountered feature in sputtered films), as well as the ( 100) and (006) peaks of graphite (at around 42” and 90” respectively). Data taken at a higher sensitivity also shows presence of indium oxide and the SiO, cap. Surprisingly, in this spectrum the substrate (SiCdH) peaks are not apparent. This effect can be explained only by assuming that the top surface of the substrate has been completely amorphized, or that the film is now so thick that it significantly reduced the X-rays penetration. Unlike the previous cases in which the liquid medium has either been buried
under the growing film or has completely vanished, it is clear that in this case In has remained on the substrate. It should be noted that the implantation was performed in the same run for all the In samples and the fact that In disappeared from one sample and not from the other indicates that the films were probably not of uniform thickness. Based on the appearance of the sample and on the XRD data it was concluded that in this case graphite has grown on the hexagonal substrate. The history of the In-on-Si sample can be derived from the AES data. The variable concentration of C shows that loss of the In solvent was gradual and not instantaneous. ProfileCodeTM [ 61 calculations indicate that without coverage the
I
60.0
.I
100.0
120.0
lOO.O! 60.0 :
cl+ sic
60.0: 40.0: 20.0. 20.0
29-1128 I 40.0
Fig. 9. XRD spectrum for sample 8 (In on SiCdH)
I 60.0
and comparison
60.0
100.0
120.0
with possible spectra. Preparation
conditions
as per Table 1.
94
R.R.Manoryet al. /Thin Solid Films278 (1996) 87-95
C depth profile of a 190 keV implant at 2x10’* C+ cm-’ could not have reached the concentration level of nearly 90 at.%, as measured, and that the C peak would have still been buried several thousand angstroms below the Si surface. Thus, it must be assumed that the In solvent survived for perhaps about half of the run and was being lost gradually, rather than in a catastrophic event. As thinning took place, the beam began to first bombard and thicken the initially grown layer of C and later passed through that layer and began adding C in the Si substrate beneath. This would account for the lack of regularity of the AES C distribution. On the other hand, in the case of the Sic substrate it seems that the In layer was thicker and therefore it was still present after the process. 4.4. General discussion The technique of ion implanting into a liquid metal did not produce a crystalline diamond film as desired. This study has shown, however, that a carbonaceous, non-graphitic layer was formed on Si, when either Al or In were used. The interpretation that diamond has formed and was subsequently amorphized is also supported by the loss of solvent. The metallic layer did not cover the growing film in either case, and the beam amorphized it. As mentioned, authors using other growth methods have shown that diamond crystals can grow on Si forming an Sic interface [2,3]. The technique presented here has definitely caused formation of B-Sic, which consistently appeared in XRD data when the substrate was Si. This is the required initial step in having the right base for possible diamond growth. Based on AES data, it is clear that a carbonaceous, (non-graphite) layer was formed in a mixture with Sic, particularly when Al was the solvent. In both cases, the C concentration is much greater than that in Sic. Since discrete layers of substrate/Sic/C were not observed, this implies that some Si must either have dissolved in the solvent metal in both cases in order to bring the Si out from the substrate, or that the solvent evaporated extremely quickly and most of what is observed is just a very high dose implant of C into first Si and later C-in-Si. The former appears to be the likely scenario in the case of the Al solvent because the concentration of the Si is so constant in the C layer and this is consistent with the fact that Si was floated in the liquid and thus a constant concentration was exposed to implantation. The latter model appears to apply to the In case. The Sic layer observed by XRD is consistent with growth from either dissolved Si or from C implanted into Si. It has long been known that C implanted into Si makes Sic [lo] and research was previously done at Implant Sciences to demonstrate that highly ordered crystalline silicon carbide could be produced if the temperature of the substrate was sufficiently high ( > 1000 “C) . In the case of the In layer, it is clear that the SIC found on the surface originates from direct implantation into the substrate, since In does not dissolve Si, and could not grow from dissolved Si.
Another interesting observation in this study was that low temperature growth on a non-cubic substrate has produced graphite. When Al was used on non-cubic Sic, the higher temperature process produced aluminum carbide. These effects emphasize the importance of the presence of a cubic SIC interface for successful film growth. Several problems were encountered in applying the technique which had not been anticipated. Some of these problems were: 1. deposition and maintenance of oxide-free solvent metal layers; 2. rapid loss of the medium during ion bombardment; 3. amorphization of the resulting thin film. The first problem has been solved satisfactorily, but the loss of solvent was not revealed until the results were analyzed. All of the AES and XRD data indicated that very little solvent was still present on the surface of the samples after implantation. This unexpectedly allowed the ion beam to reach and bombard the substrate surface, damaging any grown layer. This seems to be main technical difficulty encountered with the technique. The rate of sputtering by the beam is predictable, and published data exist on evaporation rates. However, the constants used for calculation proved to be insufficient, as they refers to solid, not to molten metal. (In the case of Al and In, a measurement of loss rate was made during the program, but in view of the mentioned difficulties in determining thickness these measurements are only approximate). Nevertheless, even though attempts were made to provide a reasonable margin for the coating thickness, the results indicate that the solvent was not present in any significant concentration towards the end of the implantation. This problem has not been resolved despite efforts to vary the temperature, and thus the evaporation rate. Possible speculations regarding the loss mechanism include local ion beam charging effects and/or local high temperatures due to the thermal insulation properties of the substrate and the low mass of the solvent coating. For further utilization of this technique, a method will need to be developed for maintaining the solvent thickness throughout the bombardment. Such a method might consist of continuously co-evaporating or co-sputtering the solvent during the high temperature phase. Alternatively, the codeposit might only need to consist of a renewal of the cap material, such as SiO,, which is much easier to produce in a contaminant-free layer.
5. Summarizing remarks To the authors’ knowledge, this study is the first deliberate attempt to use a liquid medium in ion beam deposition. The results obtained with both Al and In on silicon substrates are promising, although not fully conclusive. The films produced were not of the quality needed for Raman measurements and even if some diamond had formed during the process, sub-
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sequent solvent loss guaranteed the amorphization of the film. On the other hand, this study revealed the importance of the substrate crystal structure for any film grown: It was shown that SC-6H substrates are completely unsuitable for diamond growth. The study showed that straightforward application of ion implantation into a solvent liquid is significantly more difficult than envisaged, and that the process needs refinement before it can produce useful films. A method for maintaining the solvent during ion bombardment will be necessary for any future studies.
95
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Acknowledgements
Thanks are due to Prof. J. Laferski from Brown University and his staff for the successful deposition of indium films. XRD data was collected by Ed Werekhois at Manlabs, Cambridge, Mass., and the AES results were obtained by Joe Geller, Geller Analytical Labs, Peabody, MA. This work was supported by the DOE contract DAAL-03-92-C-0030.
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