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AT0100373 44 RM 9 M. Knuwer et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Injection Moulded and Alloys for Microelectronic Housings

M. Knuwer, H. Meinhardf, K.-H. Wichmanrf

Fraunhofer-lnstitute for and Advanced Materials (IFAM), Bremen, Germany *H.C. Starck GmbH & Co. KG, Laufenburg, Germany +EADS Deutschland GmbH, Ulm, Germany

Summary Molybdenum- and tungsten-copper alloys were developed as promising housing materials for high-frequency- (HF-) microelectronic packaging. Three different powder processing routes (mixing, attritor , compound powder), different particle sizes and copper contents were tested to achieve the best possible result in thermal and mechanical properties. Net shape processing of the parts took place via -injection-moulding, debindering and . It could be seen that the sintering behaviour ( and shrinking rate) was strongly depending on the copper content and tungsten particle size. Lowest sinter activity was detected for a W90Cu10 powder mixture with coarse (d50=3,5um) tungsten powder, highest for the new developed Mo68Cu32 compound powder. Using compound powder to higher density at lower sinter temperatures compared to conventional processed WCu or MoCu powder mixtures. MoCu and WCu housing demonstrators were produced and HF-equipment was integrated. First electrical tests of the integrated demonstrator were performed successfully.

Keywords tungsten, molybdenum, copper, thermal management, high frequency,

1 Introduction The current technical development is distinguished by an extensively growing use of mobile-communication and multi-media systems. The increasing number of these systems results in a likewise increasing demand on high 1. Kniiwer et al. RM9 45

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

frequency (HF-) transmitting wave bands. Number and width of the needed wave-bands are limited, so that it will be necessary to increase the frequency limit - in future up to 300 GHz. These high frequencies require a new generation of advanced HF-circuits integrated on gallium-arsenide (GaAs) chips, a material which is much more sensitive against humidity, mechanical impact and thermal stress than . Furthermore the circuits produce a high amount of heat loss which has to be dissipated by the housing material to avoid overheating and self-destruction of the module. Designated applications are automotive distance- and cruise-control systems, local communication networks or Active Phased Array Radar systems, consisting of more than 1000 of separate radar transmit/receive- (TR-) modules. So the following requirements for proper housing materials can be derived:

• high denseness (protection against moisture and other environmental impacts) • high (dissipation of the accrued heat loss) • coefficient of thermal expansion (a) near to GaAs: 5,8-10"6 K'1 (protection against thermal stress) • sufficient mechanical strength (protection against mechanical damages) • high shielding capability (protection against internal and external interfering signals)

MMIC internal -j bond (GaAs) top cover shielding

housing AI2O3- substrate

adhesive I" cooling duct

Fig. 1: Schematic view into a typical HF-module

It is relatively easy to achieve good results in denseness and mechanical strength by using metal as housing material, but it is difficult to combine high thermal conductivity (X) and low coefficient of thermal expansion cte (a) in one material. A potential material with a low coefficient of thermal expansion 46 RM9 1. Knuwer et al.

15" International Piansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 is Kovar®, but it also has a very low thermal conductivity which limits the performance of the HF-circuit. On the other hand, copper is a material with an excellent thermal conductivity but combined with a very high a-value. There is no single material with suitable high thermal conductivity and low cte. The combination of both is possible in an system without of the two (or more) phases. In this case, the properties of the compound are determined by the properties of the single phases. Such systems are e.g. pseudoalloys of refractory (tungsten, molybdenum) and copper [1,2]. Tungsten and molybdenum show a moderate thermal conductivity and a very low a-value. The addition of copper increases both the thermal conductivity and the cte corresponding to the alloy composition and makes it suitable for microelectronic application. The processing of these alloys is only possible with powder methods. A near net shape mass-production of complex shaped parts is feasible via metal-injection moulding.

Other proper housing materials are SiAI-alloys with an content between 30 and 50 w.% as well as AIN. The SiAl alloys show disadvantages in the net shape processing which is only possible via [3]. Aluminium nitride (AIN) can be net shape produced via injection moulding but shows disadvantages both in metallisation behaviour and brittleness [4]. A comparison of some materials is shown in table 1. An impressive view into a fully integrated high-frequency demonstrator (hand made prototype with metal-injection-moulded housing) gives fig. 2.

Table 1: Different potential housing materials for integrated HF circuits on GaAs chips. W/MoCu-values are theoretical values, based on the use of different mixing rules P a E 3 Material [gem ] [W m-1 K-1] [10-6K-1] [GPa] Si70AI30 2,4 130 6,8 44 Kovar(Fe54Ni31Co15) 17 4,9 138 Copper 9,0 397 17,0 128 Tungsten 19,3 174 4,5 407 Molybdenum 10,2 137 5,1 329 W80Cu20 15,66 216-252 6,1-8,8 242-309 Mo68Cu32 9,76 178-228 6,75-9,27 212-259 AIN 3,3 200 320 M. Knüwer et al. RM 9 47

15lh International Plansee Seminar, Eds. G. Kneringer, P. Ródhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

HF-input

HF-output

MoCu ceramic housing, substrate metal- with injection- integrated moulded HF-circuit

Fig. 2: Fully integrated HF-demonstrator with a metal-injection-moulded Mo68Cu32-housing. The housing will be closed for usage and tests by a laser welded top cover

2 Experimental details The experimental work was focused on the sintering behaviour. The influence of following parameters was investigated:

• copper content • particle size • sinter temperature • sinter time • attritor milling • compound powder

Copper content First important parameter is the copper content of the alloy. During sintering it determines the amount of liquid and therewith the sintering speed as well as the gravity induced separation of liquid and phase. In the sintered part it defines the cte and the thermal conductivity. In most 48 RM9 M. Kniiwer et al.

15:r International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 commercial alloys the copper content is set between 5 and 50 w.%. In this work following copper contents were investigated:

• MoCu: 32 w.%, 40 w.%, 60 w.% . WCu: 10 w.%, 20 w.%, 30 w.%

Particle size The particle size is the second important parameter. It was varied both for the tungsten and for the copper particles. The molybdenum particle size was constant. The particle sizes of the used powders (d50-values, laser diffraction measurement) are shown in table 2:

Table 2: Particle sizes and internal code of the used metal powders

powder internal code d5o [urn] Mo P89-II 6,58 Wfine P71 1,54 W coarse P72 3,55 Cu fine P73 6,04 Cu coarse P74 12,00

Sintering temperature The sintering temperature was set above the melting temperature of copper, so that liquid phase sintering occurred. On the one hand, an increasing sintering temperature decreases the viscosity of the copper melt and increases the interactivity of copper and refractory metal. On the other hand, caused by the low of copper, there is a danger of copper and contamination of the sinter at higher temperatures. So the samples were sintered at temperatures between 1200°C and 1350°C [5-7].

Sintering time The densification during liquid phase sintering is mainly caused by particle rearrangement [8]. Particle rearrangement is much faster than other sintering processes, so that above the of copper at first an intensive shrinkage should appear with is followed by a slight shrinkage caused by diffusion and/or solution-precipitation processes. M. Knuwer et al. RM9 49

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Attritor milling In the case of MoCu-powders the Mo60Cu40-powder was additionally 9h attritor milled after mixing. It was expected to reach a higher sinter activity with attritor milling because of the high amount of inducted mechanical energy. Furthermore it was anticipated that the of agglomerates leads to a much better microstructural homogeneity of the sintered samples.

Compound powder A further step in optimisation of both sinter activity and microstructural homogeneity was the development of a so called "compound powder" by H.C. Starck. It consists of a not demixable and extremely homogeneous distribution of copper and refractory metal particles.

The different processed and investigated W/MoCu powders are shown in table 3. Because of the high number of investigated powders the results shown further on are a selection of the whole experimental work.

Table 3: Processed and investigated W/MoCu powders powder alloy composition (w.%) powder composition P75 W90Cu10 W-P71 CU-P73 P76 W90Cu10 W-P72 Cu-P74 P125 W80Cu20 W-P71 Cu-P73 P78 W70Cu30 W-P71 CU-P74 P77 W70Cu30 W-P72 CU-P73 P84 Mo40Cu60 MO-P89-II Cu-P74 P90 Mo60Cu40 Mo-P89-ll Cu-P73 P90AT Mo60Cu40 P90 attrited P116 Mo68Cu32 compound powder P137 W80Cu20 compound powder

The powders were mixed in a double sigma kneader with a polymer/wax binder system resulting in a homogeneous feedstock. Due to the different particle sizes and shapes it was necessary to vary the powder loading of the feedstock between 50 vol.% and 63 vol.%. The injection moulding took place either on a laboratory or on a Klockner-Ferromatik FM20/40 injection 50 RM 9 M. Knuwer et al.

151" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 moulding machine and was followed by the solvent (hexane) and thermal debindering steps. Processing was completed by liquid phase sintering.

3 Expected properties of the processed alloys The important physical properties of molybdenum- and tungsten-copper pseudoalloys are determined by the volume content of copper, eq. (1). A quantitative prediction of the physical properties influenced by the copper content is -in contrast to conventional alloys- possible by the use of relatively simple mixing rules from the pure metals properties [9,10]. The comparison of different mixing rules (linear, inverted, exponential) with experimental results shows that the linear mixing rule gives the best results for a calculation of the cte [11], see eq. (2) and table 4. The calculated cte-values are matched well by the experimental data, the mismatch is mostly <10%.

vol%,=—^ 100% (1)

P^ A>

PM=^-P,+V2-P2 (2) m-,, m2 = mass fraction of each component;

Pi, p2 = density of each component PM = property of the mixture Pi. P2 = property each component V-i, V2 = volume fraction of each component

The calculated values shown in table 4 are only valid for the ideal case of full theoretical density and pure starting substances. However, in most real samples residual porosity and added alloying elements are responsible for a decreased property profile. Frequently Ni, Co or Fe are added as an effective sinter- [12-14]. But especially the use of decreases the thermal and electrical conductivity of copper significantly. An addition of 2 w.% Ni to a W80Cu20-alloy corresponds to a Ni-concentration of 10 w.% in the copper matrix and reduces their thermal conductivity from 400 W/m-K down to 45 W/m-K [15]. The resulting thermal conductivity of the tungsten copper alloy is therewith decreased to 130 W/m-K, compared with more than 200 W/m-K for the use of pure copper. M. Kniiwer et al. RM9 51

15ih International Plansee Seminar, Eds. G. Kneringer, P. Rddhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Table 4: Thermal expansion coefficient of different W/MoCu-alloys, theoretical values calculated with equation (2) compared with measured values

composition composition a[10"6K-1] (X[10-6K-1] (w.%) (vol.%) (eq. 2) (meas.) W90Cu10 W81Cu19 6,88 6,80 - 7,50 W80Cu20 W65Cu35 8,88 7,30 - 8,00 W70Cu30 W52Cu48 10,50 9,30-10,50 Mo68Cu32 Mo65Cu35 9,27 8,80- 9,70 Mo60Cu40 Mo57Cu43 10,22 9,50 - 9,74

4 Sintering and microstructure of molybdenum-copper alloys

The sintering behaviour of both molybdenum-copper and tungsten-copper alloys were studied with a Bahr TMA 801 S thermomechanical analyser used as a sinter dilatometer. The dilatometric plots of the MoCu alloys P84 (Mo40Cu60), P 90 (Mo60Cu40) and P90 attrited are shown in fig. 3. The alloys were tested as feedstock samples, so for an exact interpretation of the shrinkage the powder loading of the feedstock has to be taken in account, see table 5. A high copper content guarantees obviously not a high shrinkage. Independent from copper content and powder loading the max. measured shrinkage amounts 86,4% to 90,6% of the max. theoretical shrinkage. The measured density of the samples after sintering reaches more than 95% of the theoretical density, depending on dwell-time and - temperature (up to 1300°C).

Table 5: Powder loading and shrinkage of different MoCu-feedstocks Powder powder theoretical measured percentage loading shrinkage shrinkage of th. shrink. P84 53 vol.% 19,1 % 16,5% 86,4 % P90 50 vol.% 20,6 % 18,3 % 88,8 % P 90 attrited 55 vol.% 18,1 % 16,4% 90,6 % 52 RM9 1. Knuwer et al.

15'h International Plansee Seminar, Eds. G. Kneringer, P. Rddhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

For a better comparison of the different powders the shrinkage plots are normalised to the percentage of theoretical shrinkage. Sintering starts at a temperature of about 700°C as solid state sintering. As expected, after exceeding the melting point of copper a very fast shrinkage appears with a clearly transition to a slower shrinkage after a few minutes. Dwell time at sinter temperature of 1200°C was 1h. For a longer sinter time a further slight shrinkage can be expected.

0- copper melting point (1080°C) -10- shrinkage starts (700°C) -20- \

-30- end temperature •s (1200°C) -40- -50- Mo60Cu40 -60- Mo40Cu60 03 -70- -80- 0 Mo60Cu40 \\,_ -90- P90 attrited -"^==r:

100- i • i • 50 100 150 200 250 300 process time [min] Fig. 3: Sinter dilatometric investigation of MoCu alloys

Comparing the mixed and attrited powders, it can be seen that the sinter activity of the attrited P90 powder is significantly higher both in the solid state and in the liquid phase and matches nearly the P84 powder with a much higher copper content of 63 vol.%. Both powders achieve nearly 40% of theoretical shrinkage before melting point of copper is reached. Only at the last stage of sintering the attrited powder needs a bit more time to come to the final density. The sinter rate of the mixed P90 powder is much slower, it achieves only 25% shrinkage in the solid state. From this point of view the attrited powder is the best choice. However, with a high sinter rate the danger of distortion during sintering rises so that advantages and disadvantages of the attrited powder have to be balanced carefully. M. Kniiwer et al. RM9 53

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

One problem was the intensively agglomerated molybdenum powder. Because of the low interaction of molybdenum with copper melt the agglomerates were not cracked and infiltrated by the copper melt. The use of highly agglomerated Mo powder resulted in a contiguous Mo phase in the microstructure (fig. 4) and consequently in a low mechanical and physical property profile. Mo agglomerates in the microstructure with a diameter of 30- 40 urn were found. Copper content and porosity differed between the centre and the border of the sample. These problems could be solved mostly by attritor milling of copper and molybdenum. Most Mo-agglomerates were cracked and subsequent infiltrated by copper melt, so that the contiguity of the Mo-phase was reduced significantly. With this material a tensile strength of more than 480MPa and an elongation of max. 9,8% was reached. A welcome side-effect of attritor-milling was the input of mechanical energy in the alloy system aligned with a higher sinter activity as visible in the dilatometric plots. However, the best progress in producing MoCu-alloy powders was with the development of a so called compound powder. The Mo-particle size was reduced down to 2-4um with an absolute homogeneous distribution in the copper matrix. The influence of the powder processing on the microstructure of sintered parts is shown in the following micrographs, fig. 4-6.

ffj

V 4 'I I

40 iJm :*.#: ••••

Fig. 4: Microstructure of sintered part, powder P90 mixed. Visible is the relatively inhomogeneous microstructure with Mo-agglomerates up to a particle size of 40um and some pores. For the designated application as microelectronic housing this material seems not to be satisfactory. 54 RM 9 M. Knüwer et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rôdhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Fig. 5: Microstructure of sintered part, powder P90 attrited. The homogeneity of the microstructure is, compared to the mixed powder, significantly higher. Mo-agglomerates are mostly cracked. This powder is suitable but not yet ideal for microelectronic housing production.

Fig. 6: Microstructure of sintered part, powder P116 compound. Visible is the outstanding homogeneous microstructure with an extremely low molybdenum particle size. This powder is the best choice for the use in microelectronic housing production. M. Knuwer et al. RM9 55

15lh International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

5 Sintering and microstructure of tungsten-copper alloys

Sintering behaviour of the WCu alloys was investigated in the same way as for the MoCu alloys. Because of the lower agglomeration tendency of the tungsten powders an attritor-milling was not applied. The values for powder loading, theoretical and measured shrinkage are summarised in table 6, the sinter-dilatometric plots are shown in fig. 7.

Table 6: Powder loading and shrinkage of different WCu-feedstocks Powder powder theoretical measured percentage loading shrinkage shrinkage of th. shrink. P75 53 vol.% 19,1 % 16,9% 86,4 % P76 62 vol.% 14,7% 6,0 % 40,8 % P77 55 vol.% 18,1 % 21,2% 117,1 % P78 53 vol.% 19,1 % 20,4 % 106,8% P125 55 vol.% A Q A 0/ 20,3 % 112,2% I o, I /o

end temperature (1350°C)

W90Cu10F76

W80Cu20 PI 25 W70Cu30 F77

300 400 500 600 700 process time [min] Fig. 7: Sinter dilatometric investigation of WCu alloys 56 RM 9 M. Kniiwer et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Compared to molybdenum copper alloys the sinter activity of tungsten copper alloys is once more reduced, caused by a smaller interactivity of tungsten and copper. So for a proper densification a significantly higher temperature (1350°C) has to be applied. Generally, sintering behaviour is comparable to the MoCu alloys. Concerning the different particle sizes and copper contents it is obvious that the sinter rate and final density are influenced both by the tungsten particle size and the copper content. At a low copper content of 10 mass% (19 vol.%) the use of a coarser tungsten powder results in a very low shrinkage. At a higher copper content of 30 w.% (48 vol.%) this effect disappears. All alloys with a copper content of 20 w.% (35 vol.%) and more show nearly the same high sinter rate. This corresponds to the results of Kingery [8]. He found out that above 35 vol.% liquid phase densification is mainly determined by particle rearrangement. Furthermore these alloys show a higher shrinkage than calculated from powder loading although a residual porosity was visible. Reason for the sample deformation is probably the low force caused by the sensing device combined with the reduced viscosity of the melted copper at higher temperatures. This is a hint for a low stability of these materials and an increased risk of warping during the sinter process. The microstructures of the investigated samples are shown in fig. 8-10.

Fig. 8: Microstructure of W90Cu10 P76. The copper content is obviously too low for a sufficient tungsten particle rearrangement. A rigid tungsten skeleton has been built and is only partially infiltrated by copper melt. The sample shows a linear shrinkage of only 6 %. M. Kniiwer et al. RM9 57

15lh International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Fig. 9: Microstructure of W70Cu30 P77. The higher copper content prevents the formation of a rigid tungsten skeleton and separates the particles from each other. At higher sinter temperatures leads this microstructure to a mechanical unstable sample and an increasing risk of warping.

*. •

20 yum

Fig. 10: Microstructure of W80Cu20 P137 compound powder. Visible formation of a tungsten skeleton, but mostly infiltrated by copper melt. A good compromise between porosity and instability with a homogeneous microstructure. 58 RM9 M. Knuwer et al,

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

6 Conclusion

Processing MoCu and WCu-alloys via Metal Injection Moulding is a promising and economic way to produce high numbers of accurate shaped microelectronic components. The obtained mechanical and physical properties were satisfying. The properties can be adjusted and the accuracy can be optimised with a variation of the copper content. At higher copper contents (>35 vol.%) and process temperatures (1350°C) a better densification could be reached but at the same time the risk of v/arping increases. With a new for patent applied powder production route, leading to microhomogeneous MoCu- and WCu-compound powders, a further successful step in optimisation of MoCu and WCu pseudoalloys was made. Additional alloying elements were not necessary to achieve nearly full density.

ACKNOWLEDGEMENT

The work was performed in the project "New Metallic Material Concepts for Functional MM-Wave Packaging", financed by the German Federal Ministry for and Research (BMBF) in the research programme "New Materials for Key of the 21st Century". We gratefully acknowledge the financial support.

REFERENCES

[1] Ludvik, S, Chai, S., Kirschmann, R., Clark, I.: Metal Injection Molding (MIM) for Advanced Electronic Packaging; Advances in & Particulate Materials, Vol. 2; , 1991 [2] Kny, E.: Properties and Uses of the Pseudobinary Alloys of Cu with ; Proceedings of the 12th International Plansee Seminar, pp. 763-772, Plansee AG, Reutte 1989 [3] Osprey Metals Ltd.: Low Expansion Alloys; product information sheet [4] Fandel, M., Hager, W.; Schafer, W., Wichmann, K.-H.: Powder Injection Moulded Aluminium Nitride for Packaging of MM-Wave Circuits; MicroMat 2000: 3rd International Conference and Exhibition, Berlin, 17.- 19.4.2000 M. Knuwer et al. RM 9 59^

15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

[5] Kirk, T.W., Caldwell, S.G., Oakes, J.J.: Mo-Cu-Composites for Elektronic Packaging Applications, Advances in Powder Metallurgy & Particulate Materials, Vol. 9; San Francisco 1993 [6] , I. H., Kim, E.P., Petzow, G.: Full Densification of loosely packed W-Cu composite powders, Powder Metallurgy, Vol. 41 (1998), No. 1, pp.51-51 [7] Upadhyaya, A., German, R.M.: Densification and Dilatation of Sintered W-Cu-Alloys, The International Journal of Powder Metallurgy, Vol. 34 (1998), No2, pp. 43-55 [8] Kingery, W.D.: Densification during Sintering in the Presence of a Liquid Phase, I. Theory; Journal of Applied Physics Vol. 30 (1959), No. 3, pp. 301-306 [9] Johnson, J.L., German, R.M.: Factors Affending the Thermal Conductivity of W-Cu Composites; Advances in Powder Metallurgy & Particulate Materials, Vol. 4; Nashville, Tennessee, 1993 [10] WeiUmantel, Ch.: - und Kernphysik; Verlag Harri Deutsch; Thun 1983 [11] Knuwer, Matthias; Wichmann, Karl-Heinz; Meinhardt, Helmut: Tungsten- and Molybdenum-Copper Alloys for Microelectronic Packaging - Processing via Metal-lnjection-Molding (MIM); Proceedings of the 5th International Conference on Tungsten, Hard Metals and Refractory Alloys, pp.45-52, September 25-27,2000, Annapolis, Maryland [12] N. Akiyoshi: Production of copper-tungsten alloy, Japanese Patent JP7216477, 1995 [13] N. Akiyoshi: Copper-molybdenum alloy and its production; Japanese Patent JP8253833, 1996 [14] N. Akiyoshi et. al: Manufacture of alloy of Tungsten and/or Molybdenum and Copper; JP 10280064 1998 [15] ASM-lntemational; Metals Handbook Vol. 2 Properties and Selection: Nonferrous Alloys and Special Purpose Materials, 2nd Printing 1990