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Glass-ceramics for the coating and bonding of silicon semiconductor material
Microelectronics and Reliability
Pergamon Press 1969. Vol. 8, pp. 113-119.
Printed in Great Britain
GLASS-CERAMICS FOR THE COATING AND BONDING OF SILICON SEMICONDUCTOR MATERIAL P. W. M c M I L L A N , G. P A R T R I D G E a n d F. R. W A R D
Nelson Research Laboratories, The English Electric Co. Ltd., Beaconhill, Stafford Abstract--Workers in the field of microminiature solid state electronics are interested in the possible use of glass-ceramics for the coating and bonding of silicon semiconductor material in the fabrication of integrated circuit devices. The glass-ceramics must be able to withstand temperatures of 11001200°C and should be reasonably well matched in expansion to silicon. It has been shown that glass-ceramics of the ZnO-AlaOs-SiOI type containing proportions of CaO, BaO or BiOs give materials which are matched in linear thermal expansion characteristics to silicon and which can be applied to pre-oxidized silicon at temperatures in the region of 1200°C to give smooth, adherent coatings on the silicon, the coatings being refractory at temperatures in excess of ll00°C. The glassceramics can also be used to bond together pieces of silicon using similar firing schedules to those used to provide the coatings on silicon. By suitable choice of the firing conditions and control of the thickness of the silicon oxide interlayer, the attack by the glass-ceramic on the underlying silicon can be reduced to a negligible amount. The glass-ceramics of the types examined have been shown to have high volume resistivities (greater than 108 fl em at 500°C) and can resist the environmental conditions likely to be encountered in diffusion processes. 1. INTRODUCTION ELECTRONIC engineers are working intensively in the field of micro-miniature solid state electronics. I n many cases technological difficulties arise owing to possible interaction between adjacent circuitry units and this can be particularly troublesome in the case of the manufacture of complementary pair circuits, in which two pieces of silicon (n-type and p - t y p e respectively) have to be arranged together in a p r e - d e t e r m i n e d geometrical relationship. I n order to do this a material is required which can be used to b o n d the pieces o f silicon together and at the same time insulate t h e m from each other. Figure 1 illustrates such a possible arrangement. T h e material could also, where necessary, provide an insulating coating over the resultant unit. I t is essential that the insulating material should wet and adhere strictly to the silicon and also it should be sufficiently refractory to withstand subsequent processing conditions on the silicon. Conventional ceramic materials, whilst they would be sufficiently refractory, would not wet and adhere to the silicon. Glasses could be prepared which would wet and adhere to the silicon but these
///v///= //A/
/I
/i/ FIG. 1. Bonded assembly of p-type and n-type silicon. would not be sufficiently refractory for the required purpose. I t was considered, however, that a glass-ceramic material, prepared b y the controlled crystallization of a special glass, might be suitable for this application. Materials of this type have been prepared which can wet and adhere strongly to various substrates and also materials of this type have been prepared which are sufficiently refractory for the required application. T h e requirements for the glass-ceramic can be set out as follows: (a) T h e linear thermal expansion characteristics of the glass-ceramic should be similar to 113
114
P. W. M c M I L L A N , G. P A R T R I D G E and F. R. WARD
those of silicon in order to ensure that the barrier to the diffusion of the majority of ions silicon is not strained or damaged by the (excepting alkali ions) which could be present in coating and also to ensure satisfactory the glass-ceramic. adhesion to the silicon. The linear thermal Previous work carried out in the Nelson expansion coefficient of silicon is in the Research Laboratories on alkali-free aluminorange 32-39×10 -7 per °C (20-500°C), silicate glass-ceramics indicated that materials of depending on crystallographic direction ~1, 2) the zinc alumino-silicate type might be suitable and it is thought that the linear thermal for the required application. Glass-ceramics of the expansion coefficient of the glass-ceramic ZnO-AI203-SiO 2 type containing proportions of should not differ by more than 5 × 10-7 per other oxides, such as MgO, CaO, BaO, B20 a and °C from this and should preferably be lower GeO 2 were prepared as given in Table 1. The than that of the silicon. This requirement linear thermal expansion characteristics of the that the silicon should not be strained is materials were measured on samples which had important in connexion with semiconductor been formed by compaction of the glasses in technology, since excessive dislocations and powder form and then fired to a temperature at slip, caused in the silicon by the coating, which the compact first started to deform as would render the silicon unsuitable for use shown by rounding of the corners. Devitrification as a semiconductor. to form the glass-ceramic also occurred in this (b) The glass-ceramic must be able to withstand process which simulated fairly closely the firing of temperatures up to about 1000-1200°C with- the glass-ceramic coatings on to silicon. The comout significant deformation occurring. Tem- pact fusion temperatures for the various materials peratures in this region are used in the are given in Table 1. diffusion processes used to implant devices The linear thermal expansion coefficients, 20in the silicon. 500°C, of the various glass-ceramics determined (c) The glass-ceramic must be resistant to on fused compact samples are given in Table 1. chemical attack during the diffusion pro- It was clear from the expansion characteristics of cesses. the materials that the base ZnO-A12Oz-SiO 2 (d) The glass-ceramic must be capable of being materials and those containing MgO and GeOz, applied satisfactorily to silicon and be capable were unsuitable for the present application because of bonding pieces of silicon firmly together of the development of high expansion crystal at a temperature substantially below the phases in the glass-ceramics. Only those glassmelting point of silicon (1410°C). This ceramics could be considered which contained prorequirement may be even more stringent if portions of B203, BaO or CaO as given in Table 1. an intermediate layer of silica is provided Examples of the linear thermal expansion characbetween the glass-ceramic and the silicon teristics of glass-ceramics matched in expansion to since the oxide tends to be unstable and de- silicon are shown in Fig. 2. X-ray diffraction vitrify at temperatures above about 1300°C analysis showed that the glass-ceramics suitable and certainly above 1350°C. Is) for application to silicon contained major proportions of zinc orthosilicate and zinc aluminate. 2. SELECTION OF THE GLASS-CERAMIC TM Other properties have been determined on the The glass-ceramic selected must clearly fulfil glass-ceramics suitable for application to silicon. the above requirements but there are also certain The glass-ceramics have been shown to have high limitations on the chemical composition of the volume resistivities and at a temperature of 500°C material. The glass-ceramic must not contain any have resistivities in excess of 108 l) cm (see Table 1). ions, in particular alkali ions (e.g. Li +, Na ÷ and The glass-ceramics have also been shown to have K +) which could easily diffuse into the silicon high resistances to the environmental conditions during application and subsequent processing which occur during diffusion processes in the since this would clearly influence the devices fabrication of silicon semiconductor devices. For implanted in the silicon. In connexion with this, example, Glass-ceramic No. 18 fired to a temperaan interlayer of silicon dioxide would act as a ture of 1080°C for a period of 16 hr in an a t m o -
GLASS
CERAMICS
FOR COATING
AND BONDING
115
Table 1. Properties of the glass-ceramics and their estimated suitability for coating and bonding silicon
Glassceramic Ref. No.
ZnO/SiOz ratio
Weight % addition
Compact fusion temp. (°C)
Linear thermal expansion coeff, x 107 20-500°C
Volume resistivity at 500°C (t~ cm)
Firing temp. on silicon (°C)
Suitability for coating silicon based on expansion characteristics
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
1"08 1"43 1"70 1"11 0.96 0.84 0"72 0'96 0'84 0"72 0'60 1-03 0'95 0"76 0"96 1"35 1'50 0"75 1"18 1"17 1.33 0"82 0"88 0'75 1"43 0"96 0"84 0"72 0'60 1"55 1"22 1'70
-10% B~O3 15% B~Os 1% CaO 5% MgO 10% MgO 15% MgO 5% CaO 10% CaO 15% CaO 20% CaO 4% BaO 10% BaO ---30% B 2 0 3 10% BsO s 10% B,Os 15% B20, 16% B2Os 7% B208 5% B~Os 10% GeO~ 10% GeO~ 5% BaO 10% BaO 15% BaO 20% BaO 5% BaO 5% BaO --
1320 --1320 1280 1280 1260 1220 1220 1200 1160 1280 1240 1320 1320 1320
90"6 40'8 44'3 84.1 76-6 91.1 103.0 67.4 31"5 38'5 43"2 66"1 32-5 112.1 88'0 60"4
------------5"4 × 10 s ----
-1080 ------1170 1140 --1230 ----
No Yes No No No No No No Yes Yes No No Yes No No No
--
41"1
1220 ---1280 1320 1360 1320 1300 1260 1240 1280 1300 1310 1320
43"4 36'6 31"7 36-9 41"6 33"2 101.6 63.5 48.4 41"3 38'7 37"7 29"2 36.4 67'4
s p h e r e o f d r i e d h y d r o g e n c h l o r i d e gas lost o n l y 0-29 p e r c e n t o f its original w e i g h t ; t h e s u r f a c e lost s o m e o f its original lustre b u t n o d e f o r m a t i o n occurred. 3. APPLICATION
OF TO
THE
SILICON
GLASS-CERAMICS ~4~
P r e l i m i n a r y trials s h o w e d t h a t t h e c o m p a c t f u s i o n t e m p e r a t u r e , as d e t e r m i n e d above, a n d t h e f u s i o n t e m p e r a t u r e for a p a r t i c u l a r g l a s s - c e r a m i c o n silicon w e r e g e n e r a l l y similar as g i v e n in T a b l e 1. T h e c o m p a c t f u s i o n t e m p e r a t u r e s w e r e a c c o r d i n g l y u s e d to o b t a i n an initial i d e a o f t h e
--
-1'3 X 108 -6'3 x 10 s --------1'4 × 108 ---
--
No
1220 1180 1180 1170 1260 1260 ---1240 1200 1220 1300 1280 --
Yes Yes Yes Yes Yes Yes No No No Yes Yes Yes Yes Yes No
f u s i o n t e m p e r a t u r e s for t h e m a t e r i a l s t h o u g h t t o b e s u i t a b l e for a p p l i c a t i o n to p r e - o x i d i z e d silicon. N - t y p e silicon slices, 1 in. dia. × 0 . 0 1 0 i n . thick, w e r e d e g r e a s e d a n d p r e - o x i d i z e d b y h e a t i n g at 1100°C for 3 h r in an a t m o s p h e r e o f w e t argon, u s i n g t h e m e t h o d d e s c r i b e d b y A i n g e r . TM S u s p e n s i o n s o f t h e p o w d e r glasses w e r e a p p l i e d to t h e p r e - o x i d i z e d silicon, u s i n g a flow c o a t i n g t e c h n i q u e . T h e p o w d e r c o a t i n g o n t h e silicon w a s a l l o w e d to d r y a n d t h e p o w d e r c o a t e d slices w e r e fired to t h e a p p r o p r i a t e f u s i o n t e m p e r a t u r e s in an a t m o s p h e r e o f d r i e d , h i g h p u r i t y a r g o n (see T a b l e 1), for a p e r i o d o f 5 m i n to fuse t h e
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P . W . M c M I L L A N , G. P A R T R I D G E and F. R. WARD
0.4
,39 Z 'si
73o
g
"~ 0.3 o
g 2 i 0.2 o.
Ol
200
400
600 Tsmperoture, "C
800
IO00
FIO. 2. Linear thermal expansion characteristics of glass-ceramics Nos. 9, 13 and 30 fired to compact fusion temperatures. coatings. The best coatings were obtained when the firing temperature was a little lower than the compact fusion temperature. Too high a firing temperature resulted in blistering of the coatings and this was believed to result from the glassceramic penetrating the silicon dioxide interlayer and reacting with the underlying silicon. The glassceramic coatings on the pre-oxidized silicon were white, smooth, free from cracks and were adherent to the silicon. Further coating trials with the various glass-ceramics showed that the firing temperatures could be reduced significantly (by up to 50°C) although it was necessary to increase the time held at this lower temperature to half an hour in order to obtain satisfactory coatings on the preoxidized silicon. Examples of silicon slices coated with glass-ceramic are shown in Fig. 3. The coating thicknesses were generally 0.002-0.003 in. but this could be reduced to 0.001 in. or less by control of the powder glass particle size. Concurrently with the coating work described above, bonding trials were also carried out. Powder coated silicon slices were stacked in a suitable jig and fired under a suitable load to the conditions determined in the coating trials. The silicon slices were found to be firmly bonded together and the bonding na~teri~l was almost completely free of
bubbles. Examples of silicon slices bonded with glass-ceramic are shown in Fig. 4. 4. E X A M I N A T I O N OF THE COATED A N D B O N D E D SILICON
4.1 Techniques used in the examinations N-type silicon slices were thoroughly cleaned and pre-oxidized in wet argon, as described above, to give oxide thicknesses on the slices ranging from 0.5 to 1.7 t~. The oxide-layers on one half of each of the silicon slices were removed by etching in 40 per cent hydrofluoric acid. The slices were then coated over half their surfaces with each glass in powder form, the half of the disc coated being at right angles to the half of the disc pre-coated with oxide. This gave each disc the appearance shown in Fig. 5, each disc being divided into quadrants. The discs were then fired to the appropriate fusion temperature given in Table 1. The whole slice was etched in 40 per cent hydrofluoric acid to remove the oxide and glass-ceramic coatings and expose the silicon beneath, thoroughly washed in distilled water, acetone and dried. The four regions on each slice were examined for damage and the presence of a p - n junction. A preliminary examination for a p - n junction was carried out using a localized heating technique
FIG. 3. Silicon slices coated with Glass-ceramic
No. 18.
FIG. 4. Silicon slices bonded together with Glass-ceramic
No. 18.
!b)
FIG. 6.
Glass-ceramic
(a) After
removal
No.
13 on silicon
of glass-ceramic
(coated
at llSO’C!& surface. with hydrofluoric acid.
hr).
Photomicrographs
(h) Exposed
silicon
of the surface
underlying
copper
stained.
silicon
FIG. 7. Scanning by Glass-ceramic
electron micrograph of section through p-n junction produced No. 30 fired on at 128OT/5 min with an oxide barrier 1.3 IL thick.
t,
Frc.
8.
Glass-ceramic
18 on oxide
No. (a)
Surface
on silicon
copper
(coated at 1180 silicon surface. stained. (h) Surface
C, $ hr). after
Photomicrographs
“Dash”
etch.
of
the
undtxlyiny
G L A S S C E R A M I C S FOR C O A T I N G AND B O N D I N G (a) Planvlew
oxide layers had been removed were etched in fast "Dash" etch (1 pt 48 per cent hydrofluoric acid: 3 pt concentrated nitric acid, 6 pt glacial acetic acid) for periods of up to ½ hr, after which time the slices were thoroughly washed in distilled water, acetone and allowed to dry. The various regions were then examined microscopically for dislocations and the presence of slip damage. In the above microscopical examinations, both optical and electron microscopy were employed. Several slices were examined using an electron probe microanalyser to see if any of the glassceramic constituents were present in the silicon surface layer. In all cases nothing was detected, so that the concentration of any diffusive elements was less than the sensitivity of the instrument (30 pts/million).
(b) Sectionalviews
jGlass
l///////q~SUicon A
Glass_
Silicon.-"V/ / / / / / I S
.~Oxide
V / / / / / A-"silicon D
117
silicon~L// / / / / A C
FIG. 5. Plan and sectional view of oxidized and coated silicon disc in quadrants. involving a hot soldering iron as one probe and an auxiliary probe close to the hot iron, the two being connected to a sensitive galvanometer. The hot soldering iron started the diffusion process and the direction of throw of the galvanometer indicated the type of silicon in the surface layer. More detailed examination was carried out using a grooving and staining technique. Grooves were cut in the surface of the slices and the grooves and surrounding areas stained with a copper sulphate/ hydrofluoric acid stain. The stain is preferential and stains only n-type silicon so any p-type silicon is unstained and easily visible under a microscope. The depth of any p-type layer was determined by microscopical examination. Resistivity measurements were also carried out on the silicon slices. The slices used in the experiments were n-type and had resistivities of 0-5-1,5 ~ cm. If a p-type layer were present the resistivities were appreciably greater than 1.0 fl cm. The slices from which the glass-ceramic and
4.2 Results of the examinations The results of the examinations of the silicon slices coated with selected glass-ceramics are summarized in Appendix 1. In all cases the section of the silicon slice on which the glass-ceramic had been applied without the protection of an intermediate oxide layer, showed the development of a p-type surface layer. Damage to the silicon was also heavy in this region (Fig. 6). On the sections of the slices where an intermediate oxide layer had been present and attack of the underlying silicon had occurred it was found, in certain cases, that the p-type layer in the silicon was non-uniform having several deep troughs as shown in Fig. 7. It is believed that this result indicates that the oxide layer was pinholed and attack on the underlying silicon by the glassceramic was heavier through these pinholes than over the remainder of the surface. The various glass-ceramics attacked the silicon underlying the oxide layer to different extents, depending on composition of the glass-ceramic, fusion temperature of the glass-ceramic on to the pre-oxidized silicon and the thickness of the intermediate oxide layer. It was clear that reduction in the fusion temperature of a particular glass-ceramic on pre-oxidized silicon decreased the extent of the attack by the glass-ceramic on the underlying silicon even though the fusion time had to be increased at the lower temperatures. Thinner oxide layers were also more effective as barriers at the lower fusion temperatures than at the higher fusion
118
P. W. M c M I L L A N , G. P A R T R I D G E and F. 1t. W A R D
temperatures. For example, at the temperatures where fusion was satisfactorily completed in 5 m i n an oxide thickness of 1 Ez was necessary and in m a n y cases a thickness of 1.3 Ezwas required, but with the lower firing temperatures for which a 30 m i n fusion period was necessary an oxide thickness of 0.5 ~ was adequate for some glass-ceramics. This was the case for Glass-ceramics Nos. 13 and 18 which were fired on to pre-oxidized silicon at a temperature of 1180°C maintained for 3 0 m i n . For these glass-ceramics, when a suitable oxide barrier was present, the extent of damage of the underlying silicon was very light. I n general it appears that for all the range of glass-ceramics applied to silicon it is advantageous to employ as low a firing temperature as possible since this leads to less attack on the silicon and enables t h i n n e r intermediate oxide layers to be used. Although firing temperatures below the 1180°C used for Glass-ceramics Nos. 13 and 18 might be better from the above viewpoint such fusion temperatures would mean that the glassceramic would be less refractory and less able, therefore, to withstand the temperatures involved in subsequent diffusion processes.
convert n-type silicon to p-type) and damage to the silicon is at an acceptable level. T h e glass-ceramics have high volume resistivities (greater than l0 s f~ cm at 500°C) and can resist the environmental conditions likely to be encountered in diffusion processes.
Acknowledgements--The authors thank Dr. E. Eastwood, Director of Research, the English Electric Company Limited, for permission to publish this paper. They would also like to thank the Ministry of Technology for their financial support of the work described in the present paper and Dr. S. Nielsen and colleagues of the Royal Radar Establishment for carrying out some of the examinations of the coated and bonded silicon slices.
REFERENCES
1. Handbook of Thermophysical Properties of Solid Materials, Vol. 1. Macmillan (1961). 2. L. MAISSEL,J. Appl. Phys. 31, 211 (1960). 3. F. W. AINGEa, The formation and devitrification of oxides on silicon..7. Matl. Sci. 1, 1-13 (1966). 4. Detailed account submitted for publication in the ft. Marl. Sci.
5. C O N C L U S I O N S
It has been shown that glass-ceramics can be prepared, basically of the zinc alumino-silicate type with additions of calcium oxide, barium oxide or boric oxide which are matched in linear thermal expansion characteristics to silicon and which can be applied satisfactorily to pre-oxidized silicon at temperatures in the region of 1200°C to give smooth, adherent coatings on the silicon, the coatings being refractory at temperatures of at least ll00°C. T h e glass-ceramics can also be used to bond together silicon pieces using similar firing schedules to those used to provide the coatings on preoxidized silicon. I t has been shown that by suitable choice of the firing conditions and the thickness of the silicon dioxide interlayer the attack b y the glass-ceramic on the underlying silicon can be reduced to a negligible quantity. Where o p t i m u m conditions are employed there is no evidence of any diffusion of any of the glass-ceramic constituents, including boron, into the underlying silicon (which would
APPENDIX 1
Detailed results of the examination of the underlying silicon which has been pre-oxidised and coated with glass-ceramic Glass-ceramic No. 13 The effectiveness of the oxide layer as a barrier was determined both by its thickness and the temperature of the glass-ceramic fusion process. When coated at 1180°C for 30 min, the surface of the silicon remained n-type for a 0"5 ~ thickness of oxide, whereas 0.9 ~t of oxide did not prevent a p-type diffusion when fired at 1230°C for 5 min, a p-type layer 0.9 ~ deep being formed. Oxides of greater than 1.3 ~ thick were effective barriers against attack on the silicon when fired to 1230°C for 5 rain. The damage to the silicon was light and no pitting was evident. No slip damage was revealed when dislocation etched with fast "Dash" etch for periods of up to 30 min. Glass-ceramic No. 18 An oxide layer of 0.5 ~ thickness gave adequate protection when the coating was applied at 1180°C for 30 min, but 0.9 ~ of oxide was ineffective when the coating was applied at 1220°C for 5 min, a p-type layer
GLASS
CERAMICS
FOR COATING
AND BONDING
119
1'1 g thick being formed in the surface of the silicon. Oxide thicknesses greater than 1"3 g were effective barriers against attack on the silicon when fired to 1220°C for 5 rain. On surfaces which had no junction, scattered "star-shaped" pitted areas were evident (Fig. 8), although these were less pronounced on samples fired to the lower temperatures, and were thought to be due to pinholes in the oxide barrier. " D a s h " etching revealed a high density of dislocations in all cases. Slip damage was seen in only one case on a slice with an oxide layer of 0'9 g and coated at 1220°C for 5 rain, but this was very faint and disappeared on dislocation etching.
locations. Slip damage was also observed in the boundary area between the quadrants silicon and silicon/ceramic removed on a quartered slice and there was a possibility of slip in the silicon/silica/glass-ceramic region.
Glass-ceramic No. 27 When the coating was applied on to an oxide coating of 0-9 g thick at a temperature of 1240°C for 5 rain a shallow junction 0'4 g deep was formed. Oxides greater than 1.3 g thick were effective barriers under these firing conditions. The silicon surface had a very low level of damage, consisting of scattered clusters of small pits which, it was thought, were due to pinholes in the oxide layer. "Dash" etching revealed a high density of dis-
Glass-ceramic No. 30 An oxide thickness of 1.7 tt did not prevent the formation of a p-type layer in the silicon when coated at 1280°C for 5 rain or 1260°C for 30 rain. With the former schedule a junction 1.4 tt deep was formed. A large number of dislocations was revealed by "Dash" etching for periods up to 30 min. Line defects were also seen which seemed to originate from a centre point, indicating the possibility that the oxide layer contained pinholes.
Glass-ceramic No. 28 An oxide thickness of 0.9 ~ was ineffective as a barrier when coating with this glass-ceramic was carried out at 1240°C for 5 rain but oxide thicknesses greater than 1.3 g appeared effective with this coating schedule. " D a s h " etching, however, revealed a large number of dislocations and slip damage in the silicon.
Pergamon Press 1969. Vol. 8, pp. 113-119.
Printed in Great Britain
GLASS-CERAMICS FOR THE COATING AND BONDING OF SILICON SEMICONDUCTOR MATERIAL P. W. M c M I L L A N , G. P A R T R I D G E a n d F. R. W A R D
Nelson Research Laboratories, The English Electric Co. Ltd., Beaconhill, Stafford Abstract--Workers in the field of microminiature solid state electronics are interested in the possible use of glass-ceramics for the coating and bonding of silicon semiconductor material in the fabrication of integrated circuit devices. The glass-ceramics must be able to withstand temperatures of 11001200°C and should be reasonably well matched in expansion to silicon. It has been shown that glass-ceramics of the ZnO-AlaOs-SiOI type containing proportions of CaO, BaO or BiOs give materials which are matched in linear thermal expansion characteristics to silicon and which can be applied to pre-oxidized silicon at temperatures in the region of 1200°C to give smooth, adherent coatings on the silicon, the coatings being refractory at temperatures in excess of ll00°C. The glassceramics can also be used to bond together pieces of silicon using similar firing schedules to those used to provide the coatings on silicon. By suitable choice of the firing conditions and control of the thickness of the silicon oxide interlayer, the attack by the glass-ceramic on the underlying silicon can be reduced to a negligible amount. The glass-ceramics of the types examined have been shown to have high volume resistivities (greater than 108 fl em at 500°C) and can resist the environmental conditions likely to be encountered in diffusion processes. 1. INTRODUCTION ELECTRONIC engineers are working intensively in the field of micro-miniature solid state electronics. I n many cases technological difficulties arise owing to possible interaction between adjacent circuitry units and this can be particularly troublesome in the case of the manufacture of complementary pair circuits, in which two pieces of silicon (n-type and p - t y p e respectively) have to be arranged together in a p r e - d e t e r m i n e d geometrical relationship. I n order to do this a material is required which can be used to b o n d the pieces o f silicon together and at the same time insulate t h e m from each other. Figure 1 illustrates such a possible arrangement. T h e material could also, where necessary, provide an insulating coating over the resultant unit. I t is essential that the insulating material should wet and adhere strictly to the silicon and also it should be sufficiently refractory to withstand subsequent processing conditions on the silicon. Conventional ceramic materials, whilst they would be sufficiently refractory, would not wet and adhere to the silicon. Glasses could be prepared which would wet and adhere to the silicon but these
///v///= //A/
/I
/i/ FIG. 1. Bonded assembly of p-type and n-type silicon. would not be sufficiently refractory for the required purpose. I t was considered, however, that a glass-ceramic material, prepared b y the controlled crystallization of a special glass, might be suitable for this application. Materials of this type have been prepared which can wet and adhere strongly to various substrates and also materials of this type have been prepared which are sufficiently refractory for the required application. T h e requirements for the glass-ceramic can be set out as follows: (a) T h e linear thermal expansion characteristics of the glass-ceramic should be similar to 113
114
P. W. M c M I L L A N , G. P A R T R I D G E and F. R. WARD
those of silicon in order to ensure that the barrier to the diffusion of the majority of ions silicon is not strained or damaged by the (excepting alkali ions) which could be present in coating and also to ensure satisfactory the glass-ceramic. adhesion to the silicon. The linear thermal Previous work carried out in the Nelson expansion coefficient of silicon is in the Research Laboratories on alkali-free aluminorange 32-39×10 -7 per °C (20-500°C), silicate glass-ceramics indicated that materials of depending on crystallographic direction ~1, 2) the zinc alumino-silicate type might be suitable and it is thought that the linear thermal for the required application. Glass-ceramics of the expansion coefficient of the glass-ceramic ZnO-AI203-SiO 2 type containing proportions of should not differ by more than 5 × 10-7 per other oxides, such as MgO, CaO, BaO, B20 a and °C from this and should preferably be lower GeO 2 were prepared as given in Table 1. The than that of the silicon. This requirement linear thermal expansion characteristics of the that the silicon should not be strained is materials were measured on samples which had important in connexion with semiconductor been formed by compaction of the glasses in technology, since excessive dislocations and powder form and then fired to a temperature at slip, caused in the silicon by the coating, which the compact first started to deform as would render the silicon unsuitable for use shown by rounding of the corners. Devitrification as a semiconductor. to form the glass-ceramic also occurred in this (b) The glass-ceramic must be able to withstand process which simulated fairly closely the firing of temperatures up to about 1000-1200°C with- the glass-ceramic coatings on to silicon. The comout significant deformation occurring. Tem- pact fusion temperatures for the various materials peratures in this region are used in the are given in Table 1. diffusion processes used to implant devices The linear thermal expansion coefficients, 20in the silicon. 500°C, of the various glass-ceramics determined (c) The glass-ceramic must be resistant to on fused compact samples are given in Table 1. chemical attack during the diffusion pro- It was clear from the expansion characteristics of cesses. the materials that the base ZnO-A12Oz-SiO 2 (d) The glass-ceramic must be capable of being materials and those containing MgO and GeOz, applied satisfactorily to silicon and be capable were unsuitable for the present application because of bonding pieces of silicon firmly together of the development of high expansion crystal at a temperature substantially below the phases in the glass-ceramics. Only those glassmelting point of silicon (1410°C). This ceramics could be considered which contained prorequirement may be even more stringent if portions of B203, BaO or CaO as given in Table 1. an intermediate layer of silica is provided Examples of the linear thermal expansion characbetween the glass-ceramic and the silicon teristics of glass-ceramics matched in expansion to since the oxide tends to be unstable and de- silicon are shown in Fig. 2. X-ray diffraction vitrify at temperatures above about 1300°C analysis showed that the glass-ceramics suitable and certainly above 1350°C. Is) for application to silicon contained major proportions of zinc orthosilicate and zinc aluminate. 2. SELECTION OF THE GLASS-CERAMIC TM Other properties have been determined on the The glass-ceramic selected must clearly fulfil glass-ceramics suitable for application to silicon. the above requirements but there are also certain The glass-ceramics have been shown to have high limitations on the chemical composition of the volume resistivities and at a temperature of 500°C material. The glass-ceramic must not contain any have resistivities in excess of 108 l) cm (see Table 1). ions, in particular alkali ions (e.g. Li +, Na ÷ and The glass-ceramics have also been shown to have K +) which could easily diffuse into the silicon high resistances to the environmental conditions during application and subsequent processing which occur during diffusion processes in the since this would clearly influence the devices fabrication of silicon semiconductor devices. For implanted in the silicon. In connexion with this, example, Glass-ceramic No. 18 fired to a temperaan interlayer of silicon dioxide would act as a ture of 1080°C for a period of 16 hr in an a t m o -
GLASS
CERAMICS
FOR COATING
AND BONDING
115
Table 1. Properties of the glass-ceramics and their estimated suitability for coating and bonding silicon
Glassceramic Ref. No.
ZnO/SiOz ratio
Weight % addition
Compact fusion temp. (°C)
Linear thermal expansion coeff, x 107 20-500°C
Volume resistivity at 500°C (t~ cm)
Firing temp. on silicon (°C)
Suitability for coating silicon based on expansion characteristics
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
1"08 1"43 1"70 1"11 0.96 0.84 0"72 0'96 0'84 0"72 0'60 1-03 0'95 0"76 0"96 1"35 1'50 0"75 1"18 1"17 1.33 0"82 0"88 0'75 1"43 0"96 0"84 0"72 0'60 1"55 1"22 1'70
-10% B~O3 15% B~Os 1% CaO 5% MgO 10% MgO 15% MgO 5% CaO 10% CaO 15% CaO 20% CaO 4% BaO 10% BaO ---30% B 2 0 3 10% BsO s 10% B,Os 15% B20, 16% B2Os 7% B208 5% B~Os 10% GeO~ 10% GeO~ 5% BaO 10% BaO 15% BaO 20% BaO 5% BaO 5% BaO --
1320 --1320 1280 1280 1260 1220 1220 1200 1160 1280 1240 1320 1320 1320
90"6 40'8 44'3 84.1 76-6 91.1 103.0 67.4 31"5 38'5 43"2 66"1 32-5 112.1 88'0 60"4
------------5"4 × 10 s ----
-1080 ------1170 1140 --1230 ----
No Yes No No No No No No Yes Yes No No Yes No No No
--
41"1
1220 ---1280 1320 1360 1320 1300 1260 1240 1280 1300 1310 1320
43"4 36'6 31"7 36-9 41"6 33"2 101.6 63.5 48.4 41"3 38'7 37"7 29"2 36.4 67'4
s p h e r e o f d r i e d h y d r o g e n c h l o r i d e gas lost o n l y 0-29 p e r c e n t o f its original w e i g h t ; t h e s u r f a c e lost s o m e o f its original lustre b u t n o d e f o r m a t i o n occurred. 3. APPLICATION
OF TO
THE
SILICON
GLASS-CERAMICS ~4~
P r e l i m i n a r y trials s h o w e d t h a t t h e c o m p a c t f u s i o n t e m p e r a t u r e , as d e t e r m i n e d above, a n d t h e f u s i o n t e m p e r a t u r e for a p a r t i c u l a r g l a s s - c e r a m i c o n silicon w e r e g e n e r a l l y similar as g i v e n in T a b l e 1. T h e c o m p a c t f u s i o n t e m p e r a t u r e s w e r e a c c o r d i n g l y u s e d to o b t a i n an initial i d e a o f t h e
--
-1'3 X 108 -6'3 x 10 s --------1'4 × 108 ---
--
No
1220 1180 1180 1170 1260 1260 ---1240 1200 1220 1300 1280 --
Yes Yes Yes Yes Yes Yes No No No Yes Yes Yes Yes Yes No
f u s i o n t e m p e r a t u r e s for t h e m a t e r i a l s t h o u g h t t o b e s u i t a b l e for a p p l i c a t i o n to p r e - o x i d i z e d silicon. N - t y p e silicon slices, 1 in. dia. × 0 . 0 1 0 i n . thick, w e r e d e g r e a s e d a n d p r e - o x i d i z e d b y h e a t i n g at 1100°C for 3 h r in an a t m o s p h e r e o f w e t argon, u s i n g t h e m e t h o d d e s c r i b e d b y A i n g e r . TM S u s p e n s i o n s o f t h e p o w d e r glasses w e r e a p p l i e d to t h e p r e - o x i d i z e d silicon, u s i n g a flow c o a t i n g t e c h n i q u e . T h e p o w d e r c o a t i n g o n t h e silicon w a s a l l o w e d to d r y a n d t h e p o w d e r c o a t e d slices w e r e fired to t h e a p p r o p r i a t e f u s i o n t e m p e r a t u r e s in an a t m o s p h e r e o f d r i e d , h i g h p u r i t y a r g o n (see T a b l e 1), for a p e r i o d o f 5 m i n to fuse t h e
116
P . W . M c M I L L A N , G. P A R T R I D G E and F. R. WARD
0.4
,39 Z 'si
73o
g
"~ 0.3 o
g 2 i 0.2 o.
Ol
200
400
600 Tsmperoture, "C
800
IO00
FIO. 2. Linear thermal expansion characteristics of glass-ceramics Nos. 9, 13 and 30 fired to compact fusion temperatures. coatings. The best coatings were obtained when the firing temperature was a little lower than the compact fusion temperature. Too high a firing temperature resulted in blistering of the coatings and this was believed to result from the glassceramic penetrating the silicon dioxide interlayer and reacting with the underlying silicon. The glassceramic coatings on the pre-oxidized silicon were white, smooth, free from cracks and were adherent to the silicon. Further coating trials with the various glass-ceramics showed that the firing temperatures could be reduced significantly (by up to 50°C) although it was necessary to increase the time held at this lower temperature to half an hour in order to obtain satisfactory coatings on the preoxidized silicon. Examples of silicon slices coated with glass-ceramic are shown in Fig. 3. The coating thicknesses were generally 0.002-0.003 in. but this could be reduced to 0.001 in. or less by control of the powder glass particle size. Concurrently with the coating work described above, bonding trials were also carried out. Powder coated silicon slices were stacked in a suitable jig and fired under a suitable load to the conditions determined in the coating trials. The silicon slices were found to be firmly bonded together and the bonding na~teri~l was almost completely free of
bubbles. Examples of silicon slices bonded with glass-ceramic are shown in Fig. 4. 4. E X A M I N A T I O N OF THE COATED A N D B O N D E D SILICON
4.1 Techniques used in the examinations N-type silicon slices were thoroughly cleaned and pre-oxidized in wet argon, as described above, to give oxide thicknesses on the slices ranging from 0.5 to 1.7 t~. The oxide-layers on one half of each of the silicon slices were removed by etching in 40 per cent hydrofluoric acid. The slices were then coated over half their surfaces with each glass in powder form, the half of the disc coated being at right angles to the half of the disc pre-coated with oxide. This gave each disc the appearance shown in Fig. 5, each disc being divided into quadrants. The discs were then fired to the appropriate fusion temperature given in Table 1. The whole slice was etched in 40 per cent hydrofluoric acid to remove the oxide and glass-ceramic coatings and expose the silicon beneath, thoroughly washed in distilled water, acetone and dried. The four regions on each slice were examined for damage and the presence of a p - n junction. A preliminary examination for a p - n junction was carried out using a localized heating technique
FIG. 3. Silicon slices coated with Glass-ceramic
No. 18.
FIG. 4. Silicon slices bonded together with Glass-ceramic
No. 18.
!b)
FIG. 6.
Glass-ceramic
(a) After
removal
No.
13 on silicon
of glass-ceramic
(coated
at llSO’C!& surface. with hydrofluoric acid.
hr).
Photomicrographs
(h) Exposed
silicon
of the surface
underlying
copper
stained.
silicon
FIG. 7. Scanning by Glass-ceramic
electron micrograph of section through p-n junction produced No. 30 fired on at 128OT/5 min with an oxide barrier 1.3 IL thick.
t,
Frc.
8.
Glass-ceramic
18 on oxide
No. (a)
Surface
on silicon
copper
(coated at 1180 silicon surface. stained. (h) Surface
C, $ hr). after
Photomicrographs
“Dash”
etch.
of
the
undtxlyiny
G L A S S C E R A M I C S FOR C O A T I N G AND B O N D I N G (a) Planvlew
oxide layers had been removed were etched in fast "Dash" etch (1 pt 48 per cent hydrofluoric acid: 3 pt concentrated nitric acid, 6 pt glacial acetic acid) for periods of up to ½ hr, after which time the slices were thoroughly washed in distilled water, acetone and allowed to dry. The various regions were then examined microscopically for dislocations and the presence of slip damage. In the above microscopical examinations, both optical and electron microscopy were employed. Several slices were examined using an electron probe microanalyser to see if any of the glassceramic constituents were present in the silicon surface layer. In all cases nothing was detected, so that the concentration of any diffusive elements was less than the sensitivity of the instrument (30 pts/million).
(b) Sectionalviews
jGlass
l///////q~SUicon A
Glass_
Silicon.-"V/ / / / / / I S
.~Oxide
V / / / / / A-"silicon D
117
silicon~L// / / / / A C
FIG. 5. Plan and sectional view of oxidized and coated silicon disc in quadrants. involving a hot soldering iron as one probe and an auxiliary probe close to the hot iron, the two being connected to a sensitive galvanometer. The hot soldering iron started the diffusion process and the direction of throw of the galvanometer indicated the type of silicon in the surface layer. More detailed examination was carried out using a grooving and staining technique. Grooves were cut in the surface of the slices and the grooves and surrounding areas stained with a copper sulphate/ hydrofluoric acid stain. The stain is preferential and stains only n-type silicon so any p-type silicon is unstained and easily visible under a microscope. The depth of any p-type layer was determined by microscopical examination. Resistivity measurements were also carried out on the silicon slices. The slices used in the experiments were n-type and had resistivities of 0-5-1,5 ~ cm. If a p-type layer were present the resistivities were appreciably greater than 1.0 fl cm. The slices from which the glass-ceramic and
4.2 Results of the examinations The results of the examinations of the silicon slices coated with selected glass-ceramics are summarized in Appendix 1. In all cases the section of the silicon slice on which the glass-ceramic had been applied without the protection of an intermediate oxide layer, showed the development of a p-type surface layer. Damage to the silicon was also heavy in this region (Fig. 6). On the sections of the slices where an intermediate oxide layer had been present and attack of the underlying silicon had occurred it was found, in certain cases, that the p-type layer in the silicon was non-uniform having several deep troughs as shown in Fig. 7. It is believed that this result indicates that the oxide layer was pinholed and attack on the underlying silicon by the glassceramic was heavier through these pinholes than over the remainder of the surface. The various glass-ceramics attacked the silicon underlying the oxide layer to different extents, depending on composition of the glass-ceramic, fusion temperature of the glass-ceramic on to the pre-oxidized silicon and the thickness of the intermediate oxide layer. It was clear that reduction in the fusion temperature of a particular glass-ceramic on pre-oxidized silicon decreased the extent of the attack by the glass-ceramic on the underlying silicon even though the fusion time had to be increased at the lower temperatures. Thinner oxide layers were also more effective as barriers at the lower fusion temperatures than at the higher fusion
118
P. W. M c M I L L A N , G. P A R T R I D G E and F. 1t. W A R D
temperatures. For example, at the temperatures where fusion was satisfactorily completed in 5 m i n an oxide thickness of 1 Ez was necessary and in m a n y cases a thickness of 1.3 Ezwas required, but with the lower firing temperatures for which a 30 m i n fusion period was necessary an oxide thickness of 0.5 ~ was adequate for some glass-ceramics. This was the case for Glass-ceramics Nos. 13 and 18 which were fired on to pre-oxidized silicon at a temperature of 1180°C maintained for 3 0 m i n . For these glass-ceramics, when a suitable oxide barrier was present, the extent of damage of the underlying silicon was very light. I n general it appears that for all the range of glass-ceramics applied to silicon it is advantageous to employ as low a firing temperature as possible since this leads to less attack on the silicon and enables t h i n n e r intermediate oxide layers to be used. Although firing temperatures below the 1180°C used for Glass-ceramics Nos. 13 and 18 might be better from the above viewpoint such fusion temperatures would mean that the glassceramic would be less refractory and less able, therefore, to withstand the temperatures involved in subsequent diffusion processes.
convert n-type silicon to p-type) and damage to the silicon is at an acceptable level. T h e glass-ceramics have high volume resistivities (greater than l0 s f~ cm at 500°C) and can resist the environmental conditions likely to be encountered in diffusion processes.
Acknowledgements--The authors thank Dr. E. Eastwood, Director of Research, the English Electric Company Limited, for permission to publish this paper. They would also like to thank the Ministry of Technology for their financial support of the work described in the present paper and Dr. S. Nielsen and colleagues of the Royal Radar Establishment for carrying out some of the examinations of the coated and bonded silicon slices.
REFERENCES
1. Handbook of Thermophysical Properties of Solid Materials, Vol. 1. Macmillan (1961). 2. L. MAISSEL,J. Appl. Phys. 31, 211 (1960). 3. F. W. AINGEa, The formation and devitrification of oxides on silicon..7. Matl. Sci. 1, 1-13 (1966). 4. Detailed account submitted for publication in the ft. Marl. Sci.
5. C O N C L U S I O N S
It has been shown that glass-ceramics can be prepared, basically of the zinc alumino-silicate type with additions of calcium oxide, barium oxide or boric oxide which are matched in linear thermal expansion characteristics to silicon and which can be applied satisfactorily to pre-oxidized silicon at temperatures in the region of 1200°C to give smooth, adherent coatings on the silicon, the coatings being refractory at temperatures of at least ll00°C. T h e glass-ceramics can also be used to bond together silicon pieces using similar firing schedules to those used to provide the coatings on preoxidized silicon. I t has been shown that by suitable choice of the firing conditions and the thickness of the silicon dioxide interlayer the attack b y the glass-ceramic on the underlying silicon can be reduced to a negligible quantity. Where o p t i m u m conditions are employed there is no evidence of any diffusion of any of the glass-ceramic constituents, including boron, into the underlying silicon (which would
APPENDIX 1
Detailed results of the examination of the underlying silicon which has been pre-oxidised and coated with glass-ceramic Glass-ceramic No. 13 The effectiveness of the oxide layer as a barrier was determined both by its thickness and the temperature of the glass-ceramic fusion process. When coated at 1180°C for 30 min, the surface of the silicon remained n-type for a 0"5 ~ thickness of oxide, whereas 0.9 ~t of oxide did not prevent a p-type diffusion when fired at 1230°C for 5 min, a p-type layer 0.9 ~ deep being formed. Oxides of greater than 1.3 ~ thick were effective barriers against attack on the silicon when fired to 1230°C for 5 rain. The damage to the silicon was light and no pitting was evident. No slip damage was revealed when dislocation etched with fast "Dash" etch for periods of up to 30 min. Glass-ceramic No. 18 An oxide layer of 0.5 ~ thickness gave adequate protection when the coating was applied at 1180°C for 30 min, but 0.9 ~ of oxide was ineffective when the coating was applied at 1220°C for 5 min, a p-type layer
GLASS
CERAMICS
FOR COATING
AND BONDING
119
1'1 g thick being formed in the surface of the silicon. Oxide thicknesses greater than 1"3 g were effective barriers against attack on the silicon when fired to 1220°C for 5 rain. On surfaces which had no junction, scattered "star-shaped" pitted areas were evident (Fig. 8), although these were less pronounced on samples fired to the lower temperatures, and were thought to be due to pinholes in the oxide barrier. " D a s h " etching revealed a high density of dislocations in all cases. Slip damage was seen in only one case on a slice with an oxide layer of 0'9 g and coated at 1220°C for 5 rain, but this was very faint and disappeared on dislocation etching.
locations. Slip damage was also observed in the boundary area between the quadrants silicon and silicon/ceramic removed on a quartered slice and there was a possibility of slip in the silicon/silica/glass-ceramic region.
Glass-ceramic No. 27 When the coating was applied on to an oxide coating of 0-9 g thick at a temperature of 1240°C for 5 rain a shallow junction 0'4 g deep was formed. Oxides greater than 1.3 g thick were effective barriers under these firing conditions. The silicon surface had a very low level of damage, consisting of scattered clusters of small pits which, it was thought, were due to pinholes in the oxide layer. "Dash" etching revealed a high density of dis-
Glass-ceramic No. 30 An oxide thickness of 1.7 tt did not prevent the formation of a p-type layer in the silicon when coated at 1280°C for 5 rain or 1260°C for 30 rain. With the former schedule a junction 1.4 tt deep was formed. A large number of dislocations was revealed by "Dash" etching for periods up to 30 min. Line defects were also seen which seemed to originate from a centre point, indicating the possibility that the oxide layer contained pinholes.
Glass-ceramic No. 28 An oxide thickness of 0.9 ~ was ineffective as a barrier when coating with this glass-ceramic was carried out at 1240°C for 5 rain but oxide thicknesses greater than 1.3 g appeared effective with this coating schedule. " D a s h " etching, however, revealed a large number of dislocations and slip damage in the silicon.