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Characterization of gold layers selectively plated by a pulsed current
Thin Solid Films, 169 (1989) 105-115 PREPARATION AND CHARACTERIZATION
105
C H A R A C T E R I Z A T I O N OF G O L D LAYERS SELECTIVELY P L A T E D BY A P U L S E D C U R R E N T GERHARD FRANZ Siemens Research Laboratories, Otto-Hahn-Ring 6, D-8000 Munich 83 (F.R.G.)
(Received May 3, 1988; revised July 8, 1988; accepted August 11, 1988)
It has been shown that pulse-plating galvanization, using a mask of photoresist, can be employed to produce partial deposition of relatively stress-free gold with extremely fine grain sizes and layer thicknesses of up to 7 gm. The "working window" in which a fine grain size deposition is combined with good permanency of size amounts to 6-12 m A c m - 2. Densities of up to p = 19.1 g cm-3 are achieved. These layers exhibit microhardness values of 9 0 + 2 0 (Vickers), and carbon contamination is at the detection limit of the X-ray electron microprobe. The electrical resistivity p has been measured down to 2.8 gf~ cm, and from that value the thermal conductivity ~¢T can be calculated as 2.53 W cm 1 K - 1 at 300 K. The compressive stress after annealing amounts to - 1.5 x 108 dyn cm-2.
1. INTRODUCTION Physical properties of electrodeposits are of obvious importance in engineering applications. Some of the most critical properties of plated metals are density, grain size, hardness and internal stress (ref. 1, pp. 280-282) which can be varied by changing the conditions of deposition over a very wide range. Even the softest of these electrodeposits is harder than the same metal which has been fully annealed/. Internal stress is always created whether the deposition method is a vacumm process or an electrochemical one. For sputtering and evaporating, however, a thickness limit of about 3 gm exists due to the tendency of the layers to peel off the substrate. Therefore, to obtain thicker deposits, one is confined to galvanic processes. Much work has been done over the last 100 years--starting with the first electroplating attempts by W. v. Siemens--resulting in many patents on decorative plating. Baths for this purpose contain additives like polyalcohols to improve the lustre by blocking the screw dislocations from which the dendritic nuclei would otherwise grow. Hence the ions are forced to deposit predominantly on the crystal surfaces. As those additives contaminate the coating they enlarge the built-in stress dramatically 3. Gold layers were required as an integrated heat spreading layer 4 for laser diodes which will be described elsewhere s. For this purpose medium thick (about 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
106
a. FRANZ
5 ~am)layers were desired with high density and small grain size. The internal stress should be as low as possible because it causes the laser properties to deteriorate severely. Moreover, the plating should be "partial" or "selective''6, with a pattern as shown in Fig. 1, which is most easily achieved by a phototechnical process. As positive photoresists are not stable in alkaline media the bath conditions are limited to pH ~< 6. Further restrictions arise from the fact that the width of the area to be plated should be at least twice the thickness ~ of the electrochemical diffusion layer to avoid a depletion of ions, thereby ensuring a homogeneous plating even in the margins. 6 can be estimated from data published by Raub 7 and from the equation \ 2jL ,]
(1)
(where R is the gas constant, T is the absolute temperature, F is Faraday's constant, /~ is the ion mobility, c o is the ion concentration in the solution, andjL is the limiting current density a) to be about 20 gm in solutions to which no conducting salt has been added.
I
300 gm
I
Fig. 1. Pattern of the structure to be plated (light areas) (magnification, 50 x).
As with our structure to be plated, where 3 values are less than 30 gm, it is obvious that simple d.c. plating is at the limit of its ability to produce a homogeneous thickness. One of the most promising methods to overcome these difficulties is pulse plating 9 in which the "breathing" diffusion layer is thinner than with d.c. plating 1°. In particular, detrimental contamination and internal stress should be significantly lower than in d.c. plated layers 11. In this paper, the relationships between selective gold layer preparation conditions and their properties are reported.
D E P O S I T I O N OF
2.
Au
LAYERS BY P U L S E P L A T I N G
107
EXPERIMENTAL DETAILS
A Ti/Mo(Pt)/Au contact was deposited on the substrates (wafers of GaAs and glass or InP, 1-4 cm z in area) by sputtering or evaporating. Then the samples were coated with an AZ 4620 photoresist (20 s and 7000 rev m i n - 1 to yield 4.7 ~tm; 20 s and 3000 rev m i n - 1 to yield 7.1 lam) and structured using a mask with a raster size of 200 ~tm x 300 pm, i.e. the area to be plated was 68~o (see Fig. 1). The cyanide bath had an optimum pH value of 6 and was run at 68 °C. For pulse plating it was not possible to measure the peak values of voltage or current; only the r.m.s, values were known. Best results were obtained for a duty cycle of about 70~, with 7 ms "on" time and 3 ms "of[" time using pulse-plating equipment from Techno Instruments Ltd. The layer thicknesses were measured mechanically with a profilometer (s-step) and controlled by SEM. The hardness values were obtained with a force of 50 m N and penetration depths of 1.2-1.6 lam for layer thicknesses between 3.5 and 8 ~tm. These samples were checked for purity. The density, stress and electrical resistance could not be measured directly because of the smallness of the selectively plated gold pads. Therefore, those properties were measured by simultaneous deposition of dummies. For density measurements, wafers with a Ti/Mo/Au contact were weighed before and after galvanic deposition. After removal of surface contamination by a thorough water rinse, the samples were dried in air for 60 rain at 150 °C. The net weight gain was 0 . 3 - 0 . 6 g of gold at a gross weight of 2-3 g. With known wafer area and known layer thicknesses, the densities were calculated. The stress exerted by the deposited layer on a substrate was evaluated using a laser scanner which measures and files the bending data of a wafer. To get numerically high values a two-inch wafer of GaAs with a sputter Ti/Mo/Au contact was used and coated entirely with a 7.5 ~tm thick gold layer. For electrical measurements, glass substrates with a sputtered gold contact and large spots of photoresist were galvanically coated. By means of a fourprobe measurement, and taking into account the gold layers connected in parallel, a value for the resistance of the plated layer was obtained. 3.
RESULTS A N D D I S C U S S I O N
3.1. Influence of the Current Density The current density j strongly influences the layer growth especially where properties like thickness, morphology, permanency of size, and micro throwing power are concerned. The threshold value of the current density was preset by the pulse plater which works reproducibly only with fed-in currents of at least 0.4 A. As the samples to be coated were very small in size (1-4 cm 2) and it was desired to investigate a larger interval of current density, three-inch silicon wafers were used which were coated with sputtered or evaporated gold contacts. Thus the range 4-150 mA cm-2 was examined.
3.2. Morphology Below the threshold value of 5 mA cm-2 reproducible growth is not possible: the photoresist is infiltrated, resulting in bad permanency of size. At values above
108
G. FRANZ
Fig. 2. SEM picture of a gold layer deposited at 10 mA cm- 2 (layer thickness, 3.8 gm; magnification, 1000 x). Fig. 3. Detail of Fig. 2 (magnification, 10000 x).
15 mA cm 2, however, the roughness height increases significantly and there is considerable grain coarsening. Above 20 m A c m - 2 the dendritic growth begins--a distinct "sand rose" structure is observed. Therefore, the window in which good permanency of size is combined with fine grain size deposition has a width from 6 to 12 mA c m - 2 ; see Figs. 2 and 3. T E M analyses of those layers show that they are nearly amorphous, exhibiting a grain size of 5 nm or less, as shown in Fig. 4. The dimensions of the mask are retained up to the top of the photoresist. When the gold layer becomes thicker than the resist, typical "balconies" are created because a horizontal growth component is now added to the vertical one. The grain size, however, is not changed; see Figs. 5 and 6. The infiltration of the photoresist at very low current densities can be explained by the fact that relatively few nuclei are formed in this range. Thus the layer growth is mainly determined by the enlargement of grains which can remove the margins of the photoresist. At larger current densities the rate of formation of nuclei increases, causing smaller grains. However, this is limited by the simultaneous increase in hydrogen formation, by which most nuclei are blocked and as a result of which there is a dendritic growth. Moreover, a reproducible growth at low current densities is not possible unless the sputtered or evaporated surface is wet etched. SEM pictures of cross sections show that the gearing of the plated gold with an extremely smooth surface is relatively poor, but is significantly improved by a wet etch step before plating. These are the obstacles that hamper selective fine grain plating in contrast to complete precipitation.
DEPOSITION OF
Au
LAYERS BY PULSE PLATING
109
Fig. 4. T E M picture of an annealed layer plated at 8 m A c r o -2 (magnification, 300000x), with diffraction pattern showing that the microcrystalline areas are in the 5 n m range.
3.3. Layer Thickness From Fig. 7 and the derived Fig. 8 it can be seen that the growth rate is proportional to the current densityj in the range 5-20 m A c m - 2 and drops to zero forj ~< 3 m A c m - 2. At very high current densities the growth rate is larger and there is a simultaneous enlargement of hydrogen development (the consequences of this
110
G. FRANZ
Fig. 5. SEM picture of a gold layer deposited at 10 mA cm-2 (layer thickness, 6.8 gm; magnification, 1000x). Fig. 6. Detail of Fig. 5 (magnification,12000 x).
are discussed in Section 3.4 below). After a I min anneal at 400 °C in argon, the thickness is constant within the margin of error.
3.4. Density Measurements of the precipitated mass show that the density p (or, more exactly, the inverse porosity (ref. 1, pp. 237-241) decreases with increasing current density to assume a constant value a t j / > 20 mA cm -2 (about 5 g cm -3, as shown in Fig. 9). Truely dense layers with densities greater than 17.5 g cm -3 are attainable only in the range 6-12 mA cm -2. Thus no more gold is deposited at all but only a higher growth rate is pretended at larger current densities. This is reflected in the cathode current efficiency which drops from 95~o to 35.6~ when the densities decrease from 19.1 g c m - 3 (which is the highest density achieved) to 4.55 g cm-3. The roughness height is correlated with the decreasing density and is measured simultaneously with the thickness; it is plotted in Fig. 9 together with the density p. 3.5. Hardness Hardness measurements show values of 90_+20 (Vickers hardness; force, 50 mN; penetration depth, less than 30~o of the layer thickness) for densely packed layers plated with 10mAcm z. This agrees well with data in the literature 2, whereas "sand rose" layers cannot be measured because of the shift of the single grains during measuring. In view of the layer thicknesses of several microns, those layers can well serve as bonding pads for ballbonding. The force necessary for good contacts does not exceed 15 raN.
DEPOSITION OF A u LAYERS BY PULSE PLATING
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A f-
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--Z "o0. 5
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Fig. 8. Growth rate as a function of current density.
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Fig. 9. Density p and roughness height T as a function of current density.
Scale Varp Bow
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contour
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DEPOSITION OF Au LAYERS BY PULSE PLATING
113
3.6 Measurement o f Stress
We used an interferometric technique, that has only recently become commercially available, to measure the bending of a wafer caused by the plated layer (10 mA cm -2) atop a two-inch GaAs wafer (300 ~tm thick) with a sputtered contact (0.7 ~tm thick), using a laser scanner to obtain surface pictures as in Figs. 10 and 11. They are corrected for the bending caused by the sputtered contact layer. The bending of the wafer is paraboloidally fitted and the stress is determined using data such as elastic coefficients and layer and wafer thicknesses. The stress values amount to - 3.1 x 108 dyn cm -2 for the as-deposited and - 1.5 × 108 dyn cm -2 for the annealed wafer, and are equal to the values obtained by Herzog etal. 12 who measured the stress by means of X-ray curvature analysis. In particular, both experiments showed that plated gold layers exert a compressive stress which is one order of magnitude lower and has the opposite sign compared with evaporated or sputtered layers. Using the formula (ref. 1, p. 281)
EGaAsdGaAsA ff-
(2)
3dAur2GaAs
(where E is the elasticity coefficient, d is the thickness, A is the amount of bending in the middle of the sample, and r is the radius of the coated sample) the stress would
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contour
Fig. ll. Thewa~rshowninFig. 1 0 a ~ e r a n n e a l i n g t r e a t m e n t o f l m i n a t 4 ~ ° C i n a r g o n .
114
O. FRANZ
amount to - 1 . 8 x 108 d y n c m -2 for the as-desposited and - 1 × 108 d y n c m -2 for the annealed wafer, i.e. the values obtained using the method of bending a rigid strip would be about 50~o lower. Nevertheless, they are very low compared with stress data for other metals e.g. for pulse-plated nickel ( - 10 9 dyn cm- 2 (ref. 13)) or copper ( - - 7 × 10 9 d y n c m -2 (ref. 14)). The elastic coefficients of InP are almost equal to those of GaAs. Hence we can transfer this result to InP and expect a bending of about 100 nm for a laser diode with a length of 200 ~tm. 3.7 Purity As mentioned above, galvanically deposited layers are by no means pure; in particular, cyanidic contaminations such as insoluble AuCN are expected. We carried out an electron microprobe analysis with densely and loosely packed layers plated with 10 and 25 mA cm-2 respectively. As a suitable standard does not exist, only comparisons of the C K s radiation can be carried out. Carbon is at the detection limit for dense layers and rises to twice the value of the background count rate for so called "sand rose" structures, giving an estimated maximum value for carbon of about 0.1 wt.~o. According to Raub 7 this low incorporation of contaminants is mainly due to the nearly neutral bath conditions and should be responsible for the low stress 13
3.8 Electrical Resistivity p and Thermal Conductivity ~cv A value for the resistance R of the plated layers is obtained by a four-probe measurement. Using the conformal map method, a calculation of the resistivity is possible. The formula p = Rdn/ln2
(3)
where d is the layer thickness, yields values for the densely packed layers down to 2.8_+0.3 ~t~ cm (i.e. 130~o of the bulk value (ref. 15)). This is similar to the results obtained by Reid 16. Using the Wiedemann-Franz law a value of 2.53 W cm - 1 K - 1 for the thermal conductivity (i.e. 80~ of the bulk value (ref.17)) is calculated with a Lorenz number of 2.36 for 300 K (ref. 18). 4. CONCLUSION In conclusion it has been shown that gold layers with very fine grain sizes can be selectively deposited by pulse plating. The densities can reach 19.1 g cm-3, and the compressive stress after annealing amounts to - 1 . 5 × 108 dyn cm-2. The thermal conductivity ~ was calculated from electrical measurements. For the densest layers ~:v reaches 80~o of the bulk value. ACKNOWLEDGMENTS
The author wishes to thank J. Heinen for his encouragement. Thanks are due to W. Krause and J. Vanhumbeeck for valuable discussions. The hardness measurements were carried out by R. Dittmann, the X-ray microprobe analysis by R.
DEPOSITION OF AU LAYERS BY PULSE PLATING
1 15
Kutzner, the TEM studies by A. Mitwalsky, and the stress measurements by N. Arnold; these are gratefully acknowledged. This work was supported under the technological program of the Federal Department of Research and Technology of the F.R.G. The author alone is responsible for the content. REFERENCES 1 F.H. Reid and W. Goldie, Gold als Oberfldche, E. G. Leutze, Saulgau, 1982. 2 Landolt and B6rnstein, Zahlenwerte und Funktionen, Vol. IV, Part 2b, Springer, Berlin and New York, 1964, p. 583. 3 H. Binder, Metalloberfliiehe, 17(1963) 263. 4 B. Stegmfiller, M. Honsberg, J. F. Luy and W. Harth, Arch. Elektronik Ubertragungstechnik, 39 (1985) 63. 5 G. Franz and B. Stegm/iller, to be published. 6 K.H. Mficke, Metalloberfliiche, 31 (1977) 145. 7 E. Raub, Metalloberfldche, 21 (1967) 709. 8 G. Kort/im, Lehrbuch der Elektrochemie, Verlag Chemic, Weinheim, 5th edn., 1972, pp. 450 455. 9 H.Y. Cheh, J. Electrochem. Soc., 118 (1971 ) 551. 10 L.G. Holmbom and B. E. Jacobson, Plating and Surface Finishing, 74 (1987) 74. 11 D.L. Rehrig, Plating, 61 (1974) 43. 12 H.J. Herzog, J. Hersener and K. M. Strohm, in Proc. 84th Int. Conf. on Microcircuit Engineering, London, 1985, Academic Press, London, 1985, pp. 317 324. 13 W. Kleinekath6fer, C. J. Raub and E. Raub, Metalloberfliiche, 36 (1982) 41 I. 14 M. Rubinstein, Transact. Metal Finishing, 33 (1956) 52. 15 Landolt and B6rnstein, Zahlenwerte und Funktionen, Vol. II, Part 6, Springer, Berlin and New York, 1959, p. 8. 16 F.H. Reid, Metalloberfliiche, 30 (1976) 453. 17 Landolt and B6rnstein: Zahlenwerte und Funktionen, Vol. II, Part 5b, Springer, Berlin and New York, 1968, p. 134. 18 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 4th edn., pp. 260,264.
105
C H A R A C T E R I Z A T I O N OF G O L D LAYERS SELECTIVELY P L A T E D BY A P U L S E D C U R R E N T GERHARD FRANZ Siemens Research Laboratories, Otto-Hahn-Ring 6, D-8000 Munich 83 (F.R.G.)
(Received May 3, 1988; revised July 8, 1988; accepted August 11, 1988)
It has been shown that pulse-plating galvanization, using a mask of photoresist, can be employed to produce partial deposition of relatively stress-free gold with extremely fine grain sizes and layer thicknesses of up to 7 gm. The "working window" in which a fine grain size deposition is combined with good permanency of size amounts to 6-12 m A c m - 2. Densities of up to p = 19.1 g cm-3 are achieved. These layers exhibit microhardness values of 9 0 + 2 0 (Vickers), and carbon contamination is at the detection limit of the X-ray electron microprobe. The electrical resistivity p has been measured down to 2.8 gf~ cm, and from that value the thermal conductivity ~¢T can be calculated as 2.53 W cm 1 K - 1 at 300 K. The compressive stress after annealing amounts to - 1.5 x 108 dyn cm-2.
1. INTRODUCTION Physical properties of electrodeposits are of obvious importance in engineering applications. Some of the most critical properties of plated metals are density, grain size, hardness and internal stress (ref. 1, pp. 280-282) which can be varied by changing the conditions of deposition over a very wide range. Even the softest of these electrodeposits is harder than the same metal which has been fully annealed/. Internal stress is always created whether the deposition method is a vacumm process or an electrochemical one. For sputtering and evaporating, however, a thickness limit of about 3 gm exists due to the tendency of the layers to peel off the substrate. Therefore, to obtain thicker deposits, one is confined to galvanic processes. Much work has been done over the last 100 years--starting with the first electroplating attempts by W. v. Siemens--resulting in many patents on decorative plating. Baths for this purpose contain additives like polyalcohols to improve the lustre by blocking the screw dislocations from which the dendritic nuclei would otherwise grow. Hence the ions are forced to deposit predominantly on the crystal surfaces. As those additives contaminate the coating they enlarge the built-in stress dramatically 3. Gold layers were required as an integrated heat spreading layer 4 for laser diodes which will be described elsewhere s. For this purpose medium thick (about 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
106
a. FRANZ
5 ~am)layers were desired with high density and small grain size. The internal stress should be as low as possible because it causes the laser properties to deteriorate severely. Moreover, the plating should be "partial" or "selective''6, with a pattern as shown in Fig. 1, which is most easily achieved by a phototechnical process. As positive photoresists are not stable in alkaline media the bath conditions are limited to pH ~< 6. Further restrictions arise from the fact that the width of the area to be plated should be at least twice the thickness ~ of the electrochemical diffusion layer to avoid a depletion of ions, thereby ensuring a homogeneous plating even in the margins. 6 can be estimated from data published by Raub 7 and from the equation \ 2jL ,]
(1)
(where R is the gas constant, T is the absolute temperature, F is Faraday's constant, /~ is the ion mobility, c o is the ion concentration in the solution, andjL is the limiting current density a) to be about 20 gm in solutions to which no conducting salt has been added.
I
300 gm
I
Fig. 1. Pattern of the structure to be plated (light areas) (magnification, 50 x).
As with our structure to be plated, where 3 values are less than 30 gm, it is obvious that simple d.c. plating is at the limit of its ability to produce a homogeneous thickness. One of the most promising methods to overcome these difficulties is pulse plating 9 in which the "breathing" diffusion layer is thinner than with d.c. plating 1°. In particular, detrimental contamination and internal stress should be significantly lower than in d.c. plated layers 11. In this paper, the relationships between selective gold layer preparation conditions and their properties are reported.
D E P O S I T I O N OF
2.
Au
LAYERS BY P U L S E P L A T I N G
107
EXPERIMENTAL DETAILS
A Ti/Mo(Pt)/Au contact was deposited on the substrates (wafers of GaAs and glass or InP, 1-4 cm z in area) by sputtering or evaporating. Then the samples were coated with an AZ 4620 photoresist (20 s and 7000 rev m i n - 1 to yield 4.7 ~tm; 20 s and 3000 rev m i n - 1 to yield 7.1 lam) and structured using a mask with a raster size of 200 ~tm x 300 pm, i.e. the area to be plated was 68~o (see Fig. 1). The cyanide bath had an optimum pH value of 6 and was run at 68 °C. For pulse plating it was not possible to measure the peak values of voltage or current; only the r.m.s, values were known. Best results were obtained for a duty cycle of about 70~, with 7 ms "on" time and 3 ms "of[" time using pulse-plating equipment from Techno Instruments Ltd. The layer thicknesses were measured mechanically with a profilometer (s-step) and controlled by SEM. The hardness values were obtained with a force of 50 m N and penetration depths of 1.2-1.6 lam for layer thicknesses between 3.5 and 8 ~tm. These samples were checked for purity. The density, stress and electrical resistance could not be measured directly because of the smallness of the selectively plated gold pads. Therefore, those properties were measured by simultaneous deposition of dummies. For density measurements, wafers with a Ti/Mo/Au contact were weighed before and after galvanic deposition. After removal of surface contamination by a thorough water rinse, the samples were dried in air for 60 rain at 150 °C. The net weight gain was 0 . 3 - 0 . 6 g of gold at a gross weight of 2-3 g. With known wafer area and known layer thicknesses, the densities were calculated. The stress exerted by the deposited layer on a substrate was evaluated using a laser scanner which measures and files the bending data of a wafer. To get numerically high values a two-inch wafer of GaAs with a sputter Ti/Mo/Au contact was used and coated entirely with a 7.5 ~tm thick gold layer. For electrical measurements, glass substrates with a sputtered gold contact and large spots of photoresist were galvanically coated. By means of a fourprobe measurement, and taking into account the gold layers connected in parallel, a value for the resistance of the plated layer was obtained. 3.
RESULTS A N D D I S C U S S I O N
3.1. Influence of the Current Density The current density j strongly influences the layer growth especially where properties like thickness, morphology, permanency of size, and micro throwing power are concerned. The threshold value of the current density was preset by the pulse plater which works reproducibly only with fed-in currents of at least 0.4 A. As the samples to be coated were very small in size (1-4 cm 2) and it was desired to investigate a larger interval of current density, three-inch silicon wafers were used which were coated with sputtered or evaporated gold contacts. Thus the range 4-150 mA cm-2 was examined.
3.2. Morphology Below the threshold value of 5 mA cm-2 reproducible growth is not possible: the photoresist is infiltrated, resulting in bad permanency of size. At values above
108
G. FRANZ
Fig. 2. SEM picture of a gold layer deposited at 10 mA cm- 2 (layer thickness, 3.8 gm; magnification, 1000 x). Fig. 3. Detail of Fig. 2 (magnification, 10000 x).
15 mA cm 2, however, the roughness height increases significantly and there is considerable grain coarsening. Above 20 m A c m - 2 the dendritic growth begins--a distinct "sand rose" structure is observed. Therefore, the window in which good permanency of size is combined with fine grain size deposition has a width from 6 to 12 mA c m - 2 ; see Figs. 2 and 3. T E M analyses of those layers show that they are nearly amorphous, exhibiting a grain size of 5 nm or less, as shown in Fig. 4. The dimensions of the mask are retained up to the top of the photoresist. When the gold layer becomes thicker than the resist, typical "balconies" are created because a horizontal growth component is now added to the vertical one. The grain size, however, is not changed; see Figs. 5 and 6. The infiltration of the photoresist at very low current densities can be explained by the fact that relatively few nuclei are formed in this range. Thus the layer growth is mainly determined by the enlargement of grains which can remove the margins of the photoresist. At larger current densities the rate of formation of nuclei increases, causing smaller grains. However, this is limited by the simultaneous increase in hydrogen formation, by which most nuclei are blocked and as a result of which there is a dendritic growth. Moreover, a reproducible growth at low current densities is not possible unless the sputtered or evaporated surface is wet etched. SEM pictures of cross sections show that the gearing of the plated gold with an extremely smooth surface is relatively poor, but is significantly improved by a wet etch step before plating. These are the obstacles that hamper selective fine grain plating in contrast to complete precipitation.
DEPOSITION OF
Au
LAYERS BY PULSE PLATING
109
Fig. 4. T E M picture of an annealed layer plated at 8 m A c r o -2 (magnification, 300000x), with diffraction pattern showing that the microcrystalline areas are in the 5 n m range.
3.3. Layer Thickness From Fig. 7 and the derived Fig. 8 it can be seen that the growth rate is proportional to the current densityj in the range 5-20 m A c m - 2 and drops to zero forj ~< 3 m A c m - 2. At very high current densities the growth rate is larger and there is a simultaneous enlargement of hydrogen development (the consequences of this
110
G. FRANZ
Fig. 5. SEM picture of a gold layer deposited at 10 mA cm-2 (layer thickness, 6.8 gm; magnification, 1000x). Fig. 6. Detail of Fig. 5 (magnification,12000 x).
are discussed in Section 3.4 below). After a I min anneal at 400 °C in argon, the thickness is constant within the margin of error.
3.4. Density Measurements of the precipitated mass show that the density p (or, more exactly, the inverse porosity (ref. 1, pp. 237-241) decreases with increasing current density to assume a constant value a t j / > 20 mA cm -2 (about 5 g cm -3, as shown in Fig. 9). Truely dense layers with densities greater than 17.5 g cm -3 are attainable only in the range 6-12 mA cm -2. Thus no more gold is deposited at all but only a higher growth rate is pretended at larger current densities. This is reflected in the cathode current efficiency which drops from 95~o to 35.6~ when the densities decrease from 19.1 g c m - 3 (which is the highest density achieved) to 4.55 g cm-3. The roughness height is correlated with the decreasing density and is measured simultaneously with the thickness; it is plotted in Fig. 9 together with the density p. 3.5. Hardness Hardness measurements show values of 90_+20 (Vickers hardness; force, 50 mN; penetration depth, less than 30~o of the layer thickness) for densely packed layers plated with 10mAcm z. This agrees well with data in the literature 2, whereas "sand rose" layers cannot be measured because of the shift of the single grains during measuring. In view of the layer thicknesses of several microns, those layers can well serve as bonding pads for ballbonding. The force necessary for good contacts does not exceed 15 raN.
DEPOSITION OF A u LAYERS BY PULSE PLATING
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!
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./ 0
~ 10
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,
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, ,
a 30 t
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Fig. 7. Layer thickness as a function of time (with current density given in m A c m - 2).
A f-
1.0
--Z "o0. 5
1 m
0
10
20 j ( m A / c m 2)
Fig. 8. Growth rate as a function of current density.
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Fig. 9. Density p and roughness height T as a function of current density.
Scale Varp Bow
1.00 l~m / 25.4 ~m -20.8 ~m
contour
Fig.~Laserscannerpicture~atw~-inc~wa~r~3~pmthick~c~atedwitha~5~mg~d~ayera~er dep•siti•n(••werpart•••ntsightang•e•f45°;upperpart•heightpr•••emapwithti•tang•e•f•°) •
DEPOSITION OF Au LAYERS BY PULSE PLATING
113
3.6 Measurement o f Stress
We used an interferometric technique, that has only recently become commercially available, to measure the bending of a wafer caused by the plated layer (10 mA cm -2) atop a two-inch GaAs wafer (300 ~tm thick) with a sputtered contact (0.7 ~tm thick), using a laser scanner to obtain surface pictures as in Figs. 10 and 11. They are corrected for the bending caused by the sputtered contact layer. The bending of the wafer is paraboloidally fitted and the stress is determined using data such as elastic coefficients and layer and wafer thicknesses. The stress values amount to - 3.1 x 108 dyn cm -2 for the as-deposited and - 1.5 × 108 dyn cm -2 for the annealed wafer, and are equal to the values obtained by Herzog etal. 12 who measured the stress by means of X-ray curvature analysis. In particular, both experiments showed that plated gold layers exert a compressive stress which is one order of magnitude lower and has the opposite sign compared with evaporated or sputtered layers. Using the formula (ref. 1, p. 281)
EGaAsdGaAsA ff-
(2)
3dAur2GaAs
(where E is the elasticity coefficient, d is the thickness, A is the amount of bending in the middle of the sample, and r is the radius of the coated sample) the stress would
Scale Warp Bow
0
50 ~m / 14.5 ~m -10.2 um
contour
Fig. ll. Thewa~rshowninFig. 1 0 a ~ e r a n n e a l i n g t r e a t m e n t o f l m i n a t 4 ~ ° C i n a r g o n .
114
O. FRANZ
amount to - 1 . 8 x 108 d y n c m -2 for the as-desposited and - 1 × 108 d y n c m -2 for the annealed wafer, i.e. the values obtained using the method of bending a rigid strip would be about 50~o lower. Nevertheless, they are very low compared with stress data for other metals e.g. for pulse-plated nickel ( - 10 9 dyn cm- 2 (ref. 13)) or copper ( - - 7 × 10 9 d y n c m -2 (ref. 14)). The elastic coefficients of InP are almost equal to those of GaAs. Hence we can transfer this result to InP and expect a bending of about 100 nm for a laser diode with a length of 200 ~tm. 3.7 Purity As mentioned above, galvanically deposited layers are by no means pure; in particular, cyanidic contaminations such as insoluble AuCN are expected. We carried out an electron microprobe analysis with densely and loosely packed layers plated with 10 and 25 mA cm-2 respectively. As a suitable standard does not exist, only comparisons of the C K s radiation can be carried out. Carbon is at the detection limit for dense layers and rises to twice the value of the background count rate for so called "sand rose" structures, giving an estimated maximum value for carbon of about 0.1 wt.~o. According to Raub 7 this low incorporation of contaminants is mainly due to the nearly neutral bath conditions and should be responsible for the low stress 13
3.8 Electrical Resistivity p and Thermal Conductivity ~cv A value for the resistance R of the plated layers is obtained by a four-probe measurement. Using the conformal map method, a calculation of the resistivity is possible. The formula p = Rdn/ln2
(3)
where d is the layer thickness, yields values for the densely packed layers down to 2.8_+0.3 ~t~ cm (i.e. 130~o of the bulk value (ref. 15)). This is similar to the results obtained by Reid 16. Using the Wiedemann-Franz law a value of 2.53 W cm - 1 K - 1 for the thermal conductivity (i.e. 80~ of the bulk value (ref.17)) is calculated with a Lorenz number of 2.36 for 300 K (ref. 18). 4. CONCLUSION In conclusion it has been shown that gold layers with very fine grain sizes can be selectively deposited by pulse plating. The densities can reach 19.1 g cm-3, and the compressive stress after annealing amounts to - 1 . 5 × 108 dyn cm-2. The thermal conductivity ~ was calculated from electrical measurements. For the densest layers ~:v reaches 80~o of the bulk value. ACKNOWLEDGMENTS
The author wishes to thank J. Heinen for his encouragement. Thanks are due to W. Krause and J. Vanhumbeeck for valuable discussions. The hardness measurements were carried out by R. Dittmann, the X-ray microprobe analysis by R.
DEPOSITION OF AU LAYERS BY PULSE PLATING
1 15
Kutzner, the TEM studies by A. Mitwalsky, and the stress measurements by N. Arnold; these are gratefully acknowledged. This work was supported under the technological program of the Federal Department of Research and Technology of the F.R.G. The author alone is responsible for the content. REFERENCES 1 F.H. Reid and W. Goldie, Gold als Oberfldche, E. G. Leutze, Saulgau, 1982. 2 Landolt and B6rnstein, Zahlenwerte und Funktionen, Vol. IV, Part 2b, Springer, Berlin and New York, 1964, p. 583. 3 H. Binder, Metalloberfliiehe, 17(1963) 263. 4 B. Stegmfiller, M. Honsberg, J. F. Luy and W. Harth, Arch. Elektronik Ubertragungstechnik, 39 (1985) 63. 5 G. Franz and B. Stegm/iller, to be published. 6 K.H. Mficke, Metalloberfliiche, 31 (1977) 145. 7 E. Raub, Metalloberfldche, 21 (1967) 709. 8 G. Kort/im, Lehrbuch der Elektrochemie, Verlag Chemic, Weinheim, 5th edn., 1972, pp. 450 455. 9 H.Y. Cheh, J. Electrochem. Soc., 118 (1971 ) 551. 10 L.G. Holmbom and B. E. Jacobson, Plating and Surface Finishing, 74 (1987) 74. 11 D.L. Rehrig, Plating, 61 (1974) 43. 12 H.J. Herzog, J. Hersener and K. M. Strohm, in Proc. 84th Int. Conf. on Microcircuit Engineering, London, 1985, Academic Press, London, 1985, pp. 317 324. 13 W. Kleinekath6fer, C. J. Raub and E. Raub, Metalloberfliiche, 36 (1982) 41 I. 14 M. Rubinstein, Transact. Metal Finishing, 33 (1956) 52. 15 Landolt and B6rnstein, Zahlenwerte und Funktionen, Vol. II, Part 6, Springer, Berlin and New York, 1959, p. 8. 16 F.H. Reid, Metalloberfliiche, 30 (1976) 453. 17 Landolt and B6rnstein: Zahlenwerte und Funktionen, Vol. II, Part 5b, Springer, Berlin and New York, 1968, p. 134. 18 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 4th edn., pp. 260,264.