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Diamond abrasion properties of plasma-deposited silicon oxide films
Wear, 112 (1986)
29 - 37
29
DI~~ND ABRASION PROPERTIES OF PLASMA-DEPOSITED SILICON OXIDE FILMS G. KAGANOWICZ RCA Laboratories,
Princeton,
NJ 08540
(Received November 21,1985;accepted
(U.S.A.) February 11,1986)
Summary Diamond abrasion properties of plasma-deposited silicon oxide thin films are described. Abrasive coatings are made only when excess of oxygen is present in the plasma during deposition. It is shown that diamond abrasion on plasma-deposited silicon oxide is a chemical process in which diamond is oxidized by oxygen present in the film in excess of the SiOz stoichiometry. Surface contamination such as some metals, that can tie up the oxygen, or water, that can prevent it from getting into close contact with diamond, decreases the abrasion rate.
1. Introduction In two recent papers [ 1, 21 the deposition of SiOz using a m~etic~ly enhanced glow discharge has been described. The main application of the Si02 layers described in these papers was as an abrasive medium to shape the diamond stylus for the RCA Videodisc@ system. A completed videodisc stylus tip is shown in Fig. 1. In order to make a diamond tip with such a shape, a blunt-ended diamond is first facet machined on a scaife to form a sharp tip as shown in Fig. 2. Then it is placed in a cartridge and played on a vinyl record with a spiral groove of cross section shown in Fig. 3. The record is coated with about 500 A of metal and about 1000 a of SiO, (where x = 2.0 + 0.1). The diamond shaping process is referred to as lapping or micromachining. The purpose of this paper is to describe the abrasion (lapp~g) properties of SiOz coatings and to link the deposition parameters and chemical composition of the coating with its lapping properties. 2. Experimental details 2.1. Coating deposition All the coatings tested in this study were deposited in a custom vacuum system. The system and deposition procedures have been described previously [ 1, 21. @ Elsevier Sequoia/Printed
in The Netherlands
Fig. 1. Scanning
electron
Fig. 2. SEM photograph
Fig. 3. Schematic
diagram
microscopy
(SEM)
photograph
of Videodisc
stylus
before
1lOOi
So02 COATING
500i
METAL
VINYL
SUBSTRATE
of Videodisc
stylus.
lapping.
UNDERCOAl
of a lapping
groove
showing
coatings.
2.2. Stylus lapping Shaping of the diamond tip was done on a standard production lapper with dry deionized air flowing into the lapping area. Because abrasion is dependent on relative humidity during the lapping, care was taken to make all the measurements for a data set at the same relative humidity. However, since not every set of data was taken at the same humidity, not all the sets of data presented here can be intercompared. Before the lapping operation the coated records were stored in a dry box under nitrogen. Three to four styli were lapped and measured for each data point. The lapping time was 35 s. It was found experimentally that for that time enough material was removed to make reproducible measurements. All the lapping was done at the same radius on the lapping disk. Each stylus was preset at 65 mgf before the lapping. The product of the stylus shoe length SL and the shoulder width SW shown in Fig. 1 was used as a measure of the volume of diamond removed during lapping. We have found that this value, which can be easily measured, corresponds best to the actual volume of the diamond removed during the stylus shaping process. The measurements of the tip were made with a Vickers’ eyepiece. A precision of +O.l pm was established by measuring the same stylus ten times in a blind test. 3. Results and discussion 3.1. Lapping properties 0 f SiO, coating The lapping properties of SiO, coatings have been evaluated as a function of various physical and chemical parameters of the coating and as a function of deposition conditions.
31 N20 :Si Ii,
RATIO
SiH, flow (standard cm’)
Fig. 4. Lapping rate as a function N20; r.f. power, 300 W).
of gas flow during the deposition
(85 standard cm3
Figure 4 shows the lapping rate as a function of the composition of the reactant gases. The composition of the coating as a function of varying N*O: SiH4 ratio has been described previously [ 21. It can be seen from Fig. 4 that the lapping rate changes with composition non-linearly. The rate is very low for N20:SiH4 ratios below 2.8 then increases rapidly and becomes stable at a ratio of 3.2. For the two highest N20:SiH4 ratios evaluated (10.6 and 7.1) a slight drop in lapping rate is observed. The decrease is due to nucleation and deposition in the gas phase, resulting in a rough surface on the coating [2]. A possible reason for the non-linear dependence of the lapping rate on coating composition is shown in Fig. 5. Figure 5 shows the lapping rate of the coating and free oxygen present in the plasma during the deposition as
N20:
Siti,
RATIO
SiH, flow (standard cm))
Fig. 5. Free oxygen in plasma (0) and lapping rate (x) as a function deposition (85 standard cm3 NzO; r.f. power, 300 W).
of gas flow during the
32
a function of the N20:SiH4 ratio. Below an N,0:SiH4 ratio of about 3.0 no oxygen is present in the plasma since it all is consumed in reaction with s&me. When the ratio increases, so does the amount of oxygen. Above an N,0:SiH4 ratio of 3.0 more oxygen is available than is required to react with all available silane. From Fig. 5 we can see that the lapping rate of an SiOz coating is high when excess oxygen is present in the plasma during coating deposition and low when oxygen is absent. The above results suggested that the amount of oxygen in the coating may be the critical factor in the stylus lapping. We have evaluated 0:Si ratios, nitrogen concentration and binding energies between silicon and oxygen on the different coating surfaces as measured by X-ray photoelectron spectroscopy (XPS) [3]. The results presented in Fig. 6 show that all the coatings which were evaluated can be divided into two groups: first, when nitrogen is present on the surface, the 0:Si ratio is below 2, and the 0-Si binding energy is 429.5 eV; second, when no nitrogen is present on the surface, the 0:Si ratio is above 2 and the 0-Si binding energy is 429.3 eV. NzO: SiH4 RATIO
2 20r
2 loz 6
zoo-
1.90-
I
0
5
I
1
10
15
I
I
20 25 SI H4 FLOW
1
I
/
30
35
40
Fig. 6. Surface nitrogen (x), 0:Si ratio N20:SiH4 ratio during the deposition.
(A) and Si-0
bond
energy
(0) as a function
of
Coatings from the first group show poor lapping performance (low abrasion) while the coatings from the second group show good lapping performance (high abrasion). The analytical results confirm the fact that oxygen plays a critical role in the lapping process. Thus, when an excess of oxygen is present in the plasma, a coating having oxygen in excess of stoichiometry on the coating surface is formed and only such coatings have good lapping properties. The relation between lapping rate and free oxygen in the plasma is shown in Fig. 7. The XPS results were confirmed by Rutherford backscattering (RBS) analysis of good and poor lapping coatings in comparison with a thermally grown SiOz coating. The RBS results are shown in Table 1.
33
02
PEAK
IN PLASMA
(ARE. UNITS)
Fig. 7. Lapping rate us. oxygen peak, measured by mass spectroscopy
in the plasma.
TABLE 1 Relative 0:Si ratio in non-abrasive and highly abrasive SiO, coatings as determined by Rutherford backscattering SiO,
coating
Poor lapping coating Thermally grown Good lapping coating
0:Si
ratioa
1.87 189 2.03
aThese values are the ratios of the integrated areas of the oxygen and silicon signals in the respective coatings.
In order to verify experimentally that the excess of free oxygen in the plasma is crucial to the deposition of good lapping coatings, an experiment was conducted in which varying amounts of oxygen were introduced into the plasma which normally produced a poor lapping coating. In the second part of this experiment we introduced nitrogen and hydrogen into a plasma which normally produced a good lapping coating. The purpose of this was to see whether an excess of nitrogen and hydrogen normally observed in the plasma during the deposition of poor lapping coatings is related to the coating lapping performance. The results are summarized in Table 2. The results from Table 2 show that, while the addition of hydrogen and/or nitrogen to the plasma does not affect the coating lapping properties, addition of oxygen to a plasma which normally produces poor lapping coatings substantially improves the coating lapping performance. Mass spectroscopic analyses showed that in experiments 175 - 178 an excess of oxygen is present in the plasma.
3.2. Lapping: surface phenomenon The role of coating thickness in the lapping process has been investigated and described previously [4]. In general, the lapping rate increases with coating thickness up to a value of about 600 A and then stabilizes
34 TABLE 2 Dependence of abrasive properties of SiO, coatings on plasma conditions Coating
(standard cm3) of the followinggases
Flows N20
SiH,
N20,
Lapping SL xsw
H2
N2
02
100 100 -
100 100 -
-
rate:
SiH4
163/164 166/165 1671168 1691170 1711172 1751177 1761178
170 85 170 170 170 85 85
200 SIO,
400 COATING
27.5 32 27.5 27.5 21.5 32 32
600
600
THICKNESS
6.2 2.7 6.2 6.2 6.2 2.7 2.7
1000
I
25 100
25.8 8.4 26.1 28.3 26.5 26.5 27.4
10
th
Fig. 8. Lapping rate us. SiO, coating thickness.
(Fig. 8). It was also known [4] that the abrasive properties of the SiO, coating degraded with use and that an additional overlayer of about 200 a of new SiO, coating restored its original abrasion properties. The above information raised the question of which parts of the coating are involved in the lapping process. To answer this question, good and poor lapping coatings 1000 A thick were overcoated with 20 a of poor and good coatings respectively and tested for lapping performance. The results, presented in Table 3, show that only the surface layer of the coating is active in the abrasion process. In a follow-up experiment, we have exposed a poor lapping coating to an oxygen glow. The results of this experiment show an increase in the lapping properties to the level of a “good” coating (Table 3). XPS measurements showed that the 0:Si ratio on the surface increased as a result of the post-glowing treatment from 1.9 to 2.15. Since post-glowing affects mainly the coating surface, the experiment showed that by changing only the surface properties of the coating we can change its lapping properties. The above results strongly suggest that only the very top surface of the SiO, coating is involved in the lapping process. There is still, however, a strong dependence of the lapping ratio on the coating thickness {Fig. 3). Two reasons for this dependence can be suggested. One is that the thick coating is necessary to prevent the sharp diamond tip from penetrating the
35 TABLE 3 Abrasion rate for coatings with different surfaces Coatinga
Lappingrate: SLXSW
1000 a of A 1OOOa ofB 1000 A of A + 20 A of B on surface 1000 w of B + 20 a of A on surface 1000 A of B post-glowed in O2 (30 s; 300 W)
30 7 7.5 31 28
aA, 16 standard cm3 of SiH4 + 85 standard cm3 of NzO (300 W); B, 36 standard cm’of SiH4 + 85 standard cm3 of NzO (300 W).
coating and scoring the record. It is possible that a hard metal undercoat and a thick SiO, coating are needed to support the stylus and to strengthen the groove. The other possibility is that the thick SiO, coating acts as a barrier and prevents the metal from the undercoat from migrating to the surface [5]. In order to evaluate the two proposed mech~isms, we performed the following experiments. First, a good lapping coating was overcoated with 1 - 3 A of copper and Inconel and their lapping efficiencies were tested. The results show that when a good lapping record is overcoated with small amounts (less than a monolayer) of copper or Inconel, the lapping rate drops substantially, from SL X SW = 30 to SL X SW = 14. This proves that the presence of a small qu~tity of metal can have a detr~en~l effect on the lapping. The second experiment was aimed at separating the mechanical support role of the SiOz coating from its role as a barrier to metal migration. On standard lapping disks (vinyl substrate, 500 a of Inconel, 1000 A of Si02) layers of 300 A of copper and 300 a of Inconel were deposited. SiO, layers of different thicknesses were deposited over these structures and the lapping rate as a function of the SiO, thickness was determined. Since the structures had all the mechanical and structural properties of a standard coating, the change in the lapping rate as a function of the thickness of the top SiOz layer could be directly related to the metal migration. The results (Fig. 9) show that for both metals the lapping rate is still dependent on the SiO, thickness but to much lesser degree, however, than without the mechanical support of a standard coating. In the case of copper about 500 A, and in the case of Inconel about 100 A, of the top SiO, coating is required to obtain a good lapping coating. The differences can be explained by the fact that copper is a faster migrating metal than the ~gredients in Inconel (nickel, chromium, iron). Another piece of evidence confirming that lapping is a surface phenomenon is its strong dependence on the relative humidity as shown in Fig. 10. The amount of diamond removed in air decreases fivefold when the
36
000
100
200
300
THICKNESS
Fig. 9. Lapping Fig. 10. Lapping
400
0
500
10
20
OF TOP SiO,t&)
rate us. thickness rate us. relative
30
40
50
SC 70
80
90
100 I
Relative humidity [‘%a)
of SiO, over standard humidity:
X,
structure
overcoated
with metal.
in air; 0, in nitrogen.
relative humidity is increased from 15% to 95%. Figure 10 also shows that in nitrogen the abrasion is almost non-existent for all humidities. This suggests that oxygen from the air also takes part in the abrasion process, in addition to oxygen incorporated in the disk surface.
3.3. Lapping mechanism The mechanism by which SiO, abrades diamond is not known. Initially it was thought that the abrasion was purely mechanical, with the removal of large pieces of diamond, Examination by high-resolution high-magnification SEM showed that the SiOz-abraded diamond surface was smooth to within 20 A or less, which might suggest an abrasion process consisting of graphitization of the diamond and later removal of graphite. The data presented in this paper, however, clearly support the concept that the abrasion mechanism is a chemical oxidation of the diamond. The relation of both the 0:Si ratio and the Si-0 binding energy to the lapping rate and the abrupt change in lapping rate as a function of the coating composition bear this out. We have shown that, depending on deposition conditions, two different phases of SiOZ are formed. One has an excess of oxygen and is involved in the diamond oxidation. The other does not have an excess of oxygen and does not abrade diamond. The fact that the lapping rate is substantially lower in nitrogen and that no lapping occurs in vacuum [6] shows that oxygen from the atmosphere is also involved in the lapping process. A possible explanation of the need for two sources of oxygen for lapping to occur may be that a freshly abraded diamond surface leaves a pair of dangling bonds at each carbon atom. In the presence of oxygen, the dangling bonds react to form groups such as > C=O [7]. The excess oxygen in the SiOz + Y may react only with these partially oxidized groups but not with fully self-bonded carbon atoms.
31
4. Conclusions (1) Abrasion of diamond on plasma-deposited SiOz is a chemical process which occurs mainly on the surface of the coating. (2) Highly abrasive SiOz surfaces have oxygen present in excess of stoichiometry. The bonding energy between oxygen and silicon for the efficient SiOz layer is less than that for the inefficient layers. (3) In order to produce highly abrasive SiOz surfaces, free oxygen has to be present in the plasma during the deposition. (4) The presence of substances on the SiOz surface that can tie up oxygen, such as copper or Inconel, or prevent it from getting into close contact with diamond, such as water vapor, decreases the lapping efficiency. (5) Lapping occurs only in oxygen-containing ambient and not in nitrogen or vacuum. Acknowledgments All the coatings used in this study were made by J. W. Robinson and all the stylus lapping and measurements were performed by E. Holub. The XPS analysis and interpretation was done by J. H. Thomas III and RBS analysis was done by C. Magee. Their effort and cooperation is greatly appreciated. During the course of this investigation discussions on the subject of stylus lapping were held with numerous people and their contribution is here acknowledged. Among them were V. S. Ban, J. Blanc, P. Datta, A. Dholakia, R. Goldberger, G. Kim, R. McCoy, M. Mindel, A. G. Moldovan, R. Nosker, H. L. Pinch, J. W. Robinson, J. A. van Raalte and S. Verma. The author is grateful to H. L. Pinch for reading and commenting on the manuscript.
References 1 G. Kaganowicz, V. S. Ban and J. W. Robinson, Spatial effects in plasma deposition of SiO, using a magnetically enhanced glow discharge, Int. Symp. on Plasma Chem-
istry 6, Montreal, July 1983. 2 G. Kaganowicz, V. S. Ban and J. W. Robinson,
3 4 5 6 7
Room temperature glow discharge deposition of silicon oxides from SiH4 and HzO, J. Vat. Sci. Technol. A, 2 (3) (1984) 1233. J. H. Thomas III and G. Kaganowicz, Materials Research Society Symp. Proc., Vol. 38, Elsevier, New York, 1985, p. 293. G. Kaganowicz and J. W. Robinson, U.S. Patent 4,355,052, 1982. G. Kim, personal communication, 1983. S. Verma, personal communication, 1983. H. P. Boehm, Adv. Catal., 16 (1966) 179.
29 - 37
29
DI~~ND ABRASION PROPERTIES OF PLASMA-DEPOSITED SILICON OXIDE FILMS G. KAGANOWICZ RCA Laboratories,
Princeton,
NJ 08540
(Received November 21,1985;accepted
(U.S.A.) February 11,1986)
Summary Diamond abrasion properties of plasma-deposited silicon oxide thin films are described. Abrasive coatings are made only when excess of oxygen is present in the plasma during deposition. It is shown that diamond abrasion on plasma-deposited silicon oxide is a chemical process in which diamond is oxidized by oxygen present in the film in excess of the SiOz stoichiometry. Surface contamination such as some metals, that can tie up the oxygen, or water, that can prevent it from getting into close contact with diamond, decreases the abrasion rate.
1. Introduction In two recent papers [ 1, 21 the deposition of SiOz using a m~etic~ly enhanced glow discharge has been described. The main application of the Si02 layers described in these papers was as an abrasive medium to shape the diamond stylus for the RCA Videodisc@ system. A completed videodisc stylus tip is shown in Fig. 1. In order to make a diamond tip with such a shape, a blunt-ended diamond is first facet machined on a scaife to form a sharp tip as shown in Fig. 2. Then it is placed in a cartridge and played on a vinyl record with a spiral groove of cross section shown in Fig. 3. The record is coated with about 500 A of metal and about 1000 a of SiO, (where x = 2.0 + 0.1). The diamond shaping process is referred to as lapping or micromachining. The purpose of this paper is to describe the abrasion (lapp~g) properties of SiOz coatings and to link the deposition parameters and chemical composition of the coating with its lapping properties. 2. Experimental details 2.1. Coating deposition All the coatings tested in this study were deposited in a custom vacuum system. The system and deposition procedures have been described previously [ 1, 21. @ Elsevier Sequoia/Printed
in The Netherlands
Fig. 1. Scanning
electron
Fig. 2. SEM photograph
Fig. 3. Schematic
diagram
microscopy
(SEM)
photograph
of Videodisc
stylus
before
1lOOi
So02 COATING
500i
METAL
VINYL
SUBSTRATE
of Videodisc
stylus.
lapping.
UNDERCOAl
of a lapping
groove
showing
coatings.
2.2. Stylus lapping Shaping of the diamond tip was done on a standard production lapper with dry deionized air flowing into the lapping area. Because abrasion is dependent on relative humidity during the lapping, care was taken to make all the measurements for a data set at the same relative humidity. However, since not every set of data was taken at the same humidity, not all the sets of data presented here can be intercompared. Before the lapping operation the coated records were stored in a dry box under nitrogen. Three to four styli were lapped and measured for each data point. The lapping time was 35 s. It was found experimentally that for that time enough material was removed to make reproducible measurements. All the lapping was done at the same radius on the lapping disk. Each stylus was preset at 65 mgf before the lapping. The product of the stylus shoe length SL and the shoulder width SW shown in Fig. 1 was used as a measure of the volume of diamond removed during lapping. We have found that this value, which can be easily measured, corresponds best to the actual volume of the diamond removed during the stylus shaping process. The measurements of the tip were made with a Vickers’ eyepiece. A precision of +O.l pm was established by measuring the same stylus ten times in a blind test. 3. Results and discussion 3.1. Lapping properties 0 f SiO, coating The lapping properties of SiO, coatings have been evaluated as a function of various physical and chemical parameters of the coating and as a function of deposition conditions.
31 N20 :Si Ii,
RATIO
SiH, flow (standard cm’)
Fig. 4. Lapping rate as a function N20; r.f. power, 300 W).
of gas flow during the deposition
(85 standard cm3
Figure 4 shows the lapping rate as a function of the composition of the reactant gases. The composition of the coating as a function of varying N*O: SiH4 ratio has been described previously [ 21. It can be seen from Fig. 4 that the lapping rate changes with composition non-linearly. The rate is very low for N20:SiH4 ratios below 2.8 then increases rapidly and becomes stable at a ratio of 3.2. For the two highest N20:SiH4 ratios evaluated (10.6 and 7.1) a slight drop in lapping rate is observed. The decrease is due to nucleation and deposition in the gas phase, resulting in a rough surface on the coating [2]. A possible reason for the non-linear dependence of the lapping rate on coating composition is shown in Fig. 5. Figure 5 shows the lapping rate of the coating and free oxygen present in the plasma during the deposition as
N20:
Siti,
RATIO
SiH, flow (standard cm))
Fig. 5. Free oxygen in plasma (0) and lapping rate (x) as a function deposition (85 standard cm3 NzO; r.f. power, 300 W).
of gas flow during the
32
a function of the N20:SiH4 ratio. Below an N,0:SiH4 ratio of about 3.0 no oxygen is present in the plasma since it all is consumed in reaction with s&me. When the ratio increases, so does the amount of oxygen. Above an N,0:SiH4 ratio of 3.0 more oxygen is available than is required to react with all available silane. From Fig. 5 we can see that the lapping rate of an SiOz coating is high when excess oxygen is present in the plasma during coating deposition and low when oxygen is absent. The above results suggested that the amount of oxygen in the coating may be the critical factor in the stylus lapping. We have evaluated 0:Si ratios, nitrogen concentration and binding energies between silicon and oxygen on the different coating surfaces as measured by X-ray photoelectron spectroscopy (XPS) [3]. The results presented in Fig. 6 show that all the coatings which were evaluated can be divided into two groups: first, when nitrogen is present on the surface, the 0:Si ratio is below 2, and the 0-Si binding energy is 429.5 eV; second, when no nitrogen is present on the surface, the 0:Si ratio is above 2 and the 0-Si binding energy is 429.3 eV. NzO: SiH4 RATIO
2 20r
2 loz 6
zoo-
1.90-
I
0
5
I
1
10
15
I
I
20 25 SI H4 FLOW
1
I
/
30
35
40
Fig. 6. Surface nitrogen (x), 0:Si ratio N20:SiH4 ratio during the deposition.
(A) and Si-0
bond
energy
(0) as a function
of
Coatings from the first group show poor lapping performance (low abrasion) while the coatings from the second group show good lapping performance (high abrasion). The analytical results confirm the fact that oxygen plays a critical role in the lapping process. Thus, when an excess of oxygen is present in the plasma, a coating having oxygen in excess of stoichiometry on the coating surface is formed and only such coatings have good lapping properties. The relation between lapping rate and free oxygen in the plasma is shown in Fig. 7. The XPS results were confirmed by Rutherford backscattering (RBS) analysis of good and poor lapping coatings in comparison with a thermally grown SiOz coating. The RBS results are shown in Table 1.
33
02
PEAK
IN PLASMA
(ARE. UNITS)
Fig. 7. Lapping rate us. oxygen peak, measured by mass spectroscopy
in the plasma.
TABLE 1 Relative 0:Si ratio in non-abrasive and highly abrasive SiO, coatings as determined by Rutherford backscattering SiO,
coating
Poor lapping coating Thermally grown Good lapping coating
0:Si
ratioa
1.87 189 2.03
aThese values are the ratios of the integrated areas of the oxygen and silicon signals in the respective coatings.
In order to verify experimentally that the excess of free oxygen in the plasma is crucial to the deposition of good lapping coatings, an experiment was conducted in which varying amounts of oxygen were introduced into the plasma which normally produced a poor lapping coating. In the second part of this experiment we introduced nitrogen and hydrogen into a plasma which normally produced a good lapping coating. The purpose of this was to see whether an excess of nitrogen and hydrogen normally observed in the plasma during the deposition of poor lapping coatings is related to the coating lapping performance. The results are summarized in Table 2. The results from Table 2 show that, while the addition of hydrogen and/or nitrogen to the plasma does not affect the coating lapping properties, addition of oxygen to a plasma which normally produces poor lapping coatings substantially improves the coating lapping performance. Mass spectroscopic analyses showed that in experiments 175 - 178 an excess of oxygen is present in the plasma.
3.2. Lapping: surface phenomenon The role of coating thickness in the lapping process has been investigated and described previously [4]. In general, the lapping rate increases with coating thickness up to a value of about 600 A and then stabilizes
34 TABLE 2 Dependence of abrasive properties of SiO, coatings on plasma conditions Coating
(standard cm3) of the followinggases
Flows N20
SiH,
N20,
Lapping SL xsw
H2
N2
02
100 100 -
100 100 -
-
rate:
SiH4
163/164 166/165 1671168 1691170 1711172 1751177 1761178
170 85 170 170 170 85 85
200 SIO,
400 COATING
27.5 32 27.5 27.5 21.5 32 32
600
600
THICKNESS
6.2 2.7 6.2 6.2 6.2 2.7 2.7
1000
I
25 100
25.8 8.4 26.1 28.3 26.5 26.5 27.4
10
th
Fig. 8. Lapping rate us. SiO, coating thickness.
(Fig. 8). It was also known [4] that the abrasive properties of the SiO, coating degraded with use and that an additional overlayer of about 200 a of new SiO, coating restored its original abrasion properties. The above information raised the question of which parts of the coating are involved in the lapping process. To answer this question, good and poor lapping coatings 1000 A thick were overcoated with 20 a of poor and good coatings respectively and tested for lapping performance. The results, presented in Table 3, show that only the surface layer of the coating is active in the abrasion process. In a follow-up experiment, we have exposed a poor lapping coating to an oxygen glow. The results of this experiment show an increase in the lapping properties to the level of a “good” coating (Table 3). XPS measurements showed that the 0:Si ratio on the surface increased as a result of the post-glowing treatment from 1.9 to 2.15. Since post-glowing affects mainly the coating surface, the experiment showed that by changing only the surface properties of the coating we can change its lapping properties. The above results strongly suggest that only the very top surface of the SiO, coating is involved in the lapping process. There is still, however, a strong dependence of the lapping ratio on the coating thickness {Fig. 3). Two reasons for this dependence can be suggested. One is that the thick coating is necessary to prevent the sharp diamond tip from penetrating the
35 TABLE 3 Abrasion rate for coatings with different surfaces Coatinga
Lappingrate: SLXSW
1000 a of A 1OOOa ofB 1000 A of A + 20 A of B on surface 1000 w of B + 20 a of A on surface 1000 A of B post-glowed in O2 (30 s; 300 W)
30 7 7.5 31 28
aA, 16 standard cm3 of SiH4 + 85 standard cm3 of NzO (300 W); B, 36 standard cm’of SiH4 + 85 standard cm3 of NzO (300 W).
coating and scoring the record. It is possible that a hard metal undercoat and a thick SiO, coating are needed to support the stylus and to strengthen the groove. The other possibility is that the thick SiO, coating acts as a barrier and prevents the metal from the undercoat from migrating to the surface [5]. In order to evaluate the two proposed mech~isms, we performed the following experiments. First, a good lapping coating was overcoated with 1 - 3 A of copper and Inconel and their lapping efficiencies were tested. The results show that when a good lapping record is overcoated with small amounts (less than a monolayer) of copper or Inconel, the lapping rate drops substantially, from SL X SW = 30 to SL X SW = 14. This proves that the presence of a small qu~tity of metal can have a detr~en~l effect on the lapping. The second experiment was aimed at separating the mechanical support role of the SiOz coating from its role as a barrier to metal migration. On standard lapping disks (vinyl substrate, 500 a of Inconel, 1000 A of Si02) layers of 300 A of copper and 300 a of Inconel were deposited. SiO, layers of different thicknesses were deposited over these structures and the lapping rate as a function of the SiO, thickness was determined. Since the structures had all the mechanical and structural properties of a standard coating, the change in the lapping rate as a function of the thickness of the top SiOz layer could be directly related to the metal migration. The results (Fig. 9) show that for both metals the lapping rate is still dependent on the SiO, thickness but to much lesser degree, however, than without the mechanical support of a standard coating. In the case of copper about 500 A, and in the case of Inconel about 100 A, of the top SiO, coating is required to obtain a good lapping coating. The differences can be explained by the fact that copper is a faster migrating metal than the ~gredients in Inconel (nickel, chromium, iron). Another piece of evidence confirming that lapping is a surface phenomenon is its strong dependence on the relative humidity as shown in Fig. 10. The amount of diamond removed in air decreases fivefold when the
36
000
100
200
300
THICKNESS
Fig. 9. Lapping Fig. 10. Lapping
400
0
500
10
20
OF TOP SiO,t&)
rate us. thickness rate us. relative
30
40
50
SC 70
80
90
100 I
Relative humidity [‘%a)
of SiO, over standard humidity:
X,
structure
overcoated
with metal.
in air; 0, in nitrogen.
relative humidity is increased from 15% to 95%. Figure 10 also shows that in nitrogen the abrasion is almost non-existent for all humidities. This suggests that oxygen from the air also takes part in the abrasion process, in addition to oxygen incorporated in the disk surface.
3.3. Lapping mechanism The mechanism by which SiO, abrades diamond is not known. Initially it was thought that the abrasion was purely mechanical, with the removal of large pieces of diamond, Examination by high-resolution high-magnification SEM showed that the SiOz-abraded diamond surface was smooth to within 20 A or less, which might suggest an abrasion process consisting of graphitization of the diamond and later removal of graphite. The data presented in this paper, however, clearly support the concept that the abrasion mechanism is a chemical oxidation of the diamond. The relation of both the 0:Si ratio and the Si-0 binding energy to the lapping rate and the abrupt change in lapping rate as a function of the coating composition bear this out. We have shown that, depending on deposition conditions, two different phases of SiOZ are formed. One has an excess of oxygen and is involved in the diamond oxidation. The other does not have an excess of oxygen and does not abrade diamond. The fact that the lapping rate is substantially lower in nitrogen and that no lapping occurs in vacuum [6] shows that oxygen from the atmosphere is also involved in the lapping process. A possible explanation of the need for two sources of oxygen for lapping to occur may be that a freshly abraded diamond surface leaves a pair of dangling bonds at each carbon atom. In the presence of oxygen, the dangling bonds react to form groups such as > C=O [7]. The excess oxygen in the SiOz + Y may react only with these partially oxidized groups but not with fully self-bonded carbon atoms.
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4. Conclusions (1) Abrasion of diamond on plasma-deposited SiOz is a chemical process which occurs mainly on the surface of the coating. (2) Highly abrasive SiOz surfaces have oxygen present in excess of stoichiometry. The bonding energy between oxygen and silicon for the efficient SiOz layer is less than that for the inefficient layers. (3) In order to produce highly abrasive SiOz surfaces, free oxygen has to be present in the plasma during the deposition. (4) The presence of substances on the SiOz surface that can tie up oxygen, such as copper or Inconel, or prevent it from getting into close contact with diamond, such as water vapor, decreases the lapping efficiency. (5) Lapping occurs only in oxygen-containing ambient and not in nitrogen or vacuum. Acknowledgments All the coatings used in this study were made by J. W. Robinson and all the stylus lapping and measurements were performed by E. Holub. The XPS analysis and interpretation was done by J. H. Thomas III and RBS analysis was done by C. Magee. Their effort and cooperation is greatly appreciated. During the course of this investigation discussions on the subject of stylus lapping were held with numerous people and their contribution is here acknowledged. Among them were V. S. Ban, J. Blanc, P. Datta, A. Dholakia, R. Goldberger, G. Kim, R. McCoy, M. Mindel, A. G. Moldovan, R. Nosker, H. L. Pinch, J. W. Robinson, J. A. van Raalte and S. Verma. The author is grateful to H. L. Pinch for reading and commenting on the manuscript.
References 1 G. Kaganowicz, V. S. Ban and J. W. Robinson, Spatial effects in plasma deposition of SiO, using a magnetically enhanced glow discharge, Int. Symp. on Plasma Chem-
istry 6, Montreal, July 1983. 2 G. Kaganowicz, V. S. Ban and J. W. Robinson,
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Room temperature glow discharge deposition of silicon oxides from SiH4 and HzO, J. Vat. Sci. Technol. A, 2 (3) (1984) 1233. J. H. Thomas III and G. Kaganowicz, Materials Research Society Symp. Proc., Vol. 38, Elsevier, New York, 1985, p. 293. G. Kaganowicz and J. W. Robinson, U.S. Patent 4,355,052, 1982. G. Kim, personal communication, 1983. S. Verma, personal communication, 1983. H. P. Boehm, Adv. Catal., 16 (1966) 179.