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Direct bonding of copper to aluminum nitride
MATERIALS SCIENCE & ENGINEERING ELSEVIER
A
Materials Scienceand EngineeringA212 (1996) 206-212
Direct bonding of copper to aluminum
nitride
M. Entezarian, R.A.L. Drew Depnrtmetlt
of Mirlirzg
and Metallurgical
Engitzeering,
McGill
Unicersit)!,
Motweal,
Quebec
I-13.4 24 7 Cmtdr
Received 1.5August 1995;in revised form 4 January 1996 Abstract
Direct bonding (DB) of copper to aluminum nitride was studied. The process parameters of DB were optimized based on time, temperature and thickness of the Cu-foil for Cu-A&O, system in an N, atmosphere containing 500 ppm 0, in a temperature range of 1065 to 1075 “C. These conditions were then applied to the Cu-AlN system. Wettability of AIN by Cu was studied and improved through oxidation of AlN and modification of Cu by adding 1 at.% 0,. The interface of AIN and Cu containing 0, was then simulated using powder mixtures. The oxidation of AlN was found to be the driving force for improving the wettability of the AlN by copper. The activation energy for oxidation of AIN was found to be 94 kJ mol-‘. It was demonstrated that direct bonding of Cu to AlN can be performed without any intermediate layer. The average peel strength of AIN-Cu, A120,-Cu and
AlN-Al,O,-Cu
systems were 42, 49 and 14.7 MPa, respectively.
Kq~ords: Direct bonding; Copper; Aluminium nitride
1. Introduction
Aluminum nitride has recently been studied for electronic ceramic packaging applications because of its high thermal conductivity and non-toxic nature [l]. The estimated theoretical thermal conductivity at room temperature is 320 Wm-’ K-’ [2]. However, measured values vary from 30 to 260 Wm-’ K-’ [3,4]. AlN has a coefficient of thermal expansion (CTE) between 4 and 4.5 x low6 K-l [5], and is close to that of the silicon (2.7 x lO-‘j) and thus suitable for direct attachment to very large scale integration (VLSI) dies. The combination of high thermal conductivity and low CTE gives AlN good thermal shock resistance [6]. Moreover AlN possesses higher flexural strength than alumina or berylia. The lower hardness of AlN compared with AlsO3 also facilitates machining processes. Heat dissipation from silicon chips is becoming critical as a result of increasing circuit density and the higher power applied to these chips. In addition, metal-ceramic interfaces determine the heat dissipation through a circuit and, in order to minimize the effect of interfaces,
direct bonding
(DB)
of the substrate
to the
conductor has been suggested. Historically, copper has been the most common thin-film conductor material and is comparable in performance with Al and Au. Copper also has lower bulk resistivity than Al but is
cheaper than Au, however, corrosion protection is required in the form of a thin surface coating such as a gold flash. The direct bonding process was essentially developed for bonding copper to alumina. In this process, the metal is joined directly to the ceramic and only a few monolayers of transition layer occur between the copper and ceramic. Therefore, the thermal contact resistance between copper and ceramic is low and, as a result, high power and high frequency operation is possible. Another advantage of direct bonding copper is the elimination of low thermal conductivity solder (usually glass based) between the Cu-metallization and the ceramic substrates. It is also reported that the directly bonded substrates are almost free of thermal fatigue, since no plastic deformation occurs on temperature cycling below 100 “C [7]. The basic idea of the direct bonding is to form a liquid shell around the metal to be bonded. This is done by either rapid heating of the metal-ceramic assembly for short times at the desired temperature and subsequent cooling, or forming a liquid at the metal/ceramic interface with a lower melting point than the bulk metal. Fortunately, these requirements can be achieved by utilizing the oxygen-copper eutectic melt which can wet and bond to the ceramic [8]. Fig. 1 indicates that the melting temperature decreases with oxygen content 0921-5093/96/$15.000 1996- Elsevier ScienceS.A. All rights reserved
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of refractory metals to AlN has also been reported. The basic attraction of the latter is the low residual thermal stresses owing to their close thermal expansion coefficients to that of AlN. However, only mechanical interlocking has been found at the interface and no chemical reaction has been observed for these systems
POI.
1065 -a
C
‘C,
1050
-4-a
1000
l_G
a t cu20
400
0 -
EH 350
, 0
o.c.24@x08 c.O*
315
3
atcuo
, , , I , , , I , , , , , , 1 , , , 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Weight percentage oxygen Fig. 1. 01-0
binary phase diagram.
up to the eutectic point (1065 “C). Therefore, by heating copper containing dissolved oxygen to above 1065 “C, a Cu rich eutectic liquid will be formed. Two procedures have been suggested for controlling the eutectic layer [7]. These are (a) preoxidation of copper and (b) heating the assembly, copper foil and ceramic, in an oxidizing atmosphere with a given partial pressure of oxygen. Previous studies on the bonding of AlN to Cu have mainly concentrated on the surface oxidation of AlN by the formation of A&O3 [7,9], after which standard oxide-metal joining processes can be applied. Joining 170
160
150
140
130 1080
1065
1090
1100
1120
1140
1160
1160
1200
1220
Temperature (3) Fig. 2. Contact angles for AIN-Cu
system under flowing N,.
The nature of the bonding of AlN to Cu was studied by Ohuchi [l I] who investigated the electronic structure and adhesion of Cu-AlN interface. It was concluded that the adhesion strength of Cu to AlN is strongly influenced by the surface crystallographic orientations. In contrast, in the Cu-Al,O, system, Courbiere et al. [12,13] reported that CuAIO, forms at the beginning of joining and then is stabilized with time in favour of Al,O, and Cu,O. Active metals are available (e.g. titanium) that possess a higher affinity for aluminum than nitrogen. Addition of these metals to copper can improve both wettability and bonding in the Cu-AlN system. However, for electronic applications the conductivity of copper should also be considered when these elements are added to copper in order to improve its bonding to ceramics. Unfortunately, active metals affect the electrical conductivity of copper when they are added in small quantities and in solid solution [14]. On the contrary, oxygen has a less harmful effect on the conductivity of copper [8]. It can also be directly introduced at the interface and forms stable compounds such as oxides and oxynitrides. It would be very beneficial to the electronics industry to join AlN to Cu without diminishing the excellent physical properties of both materials. Therefore, the goal of the current study was to investigate the feasibility of direct bonding of Cu to AlN.
2. Experimental
procedure
2.1, Sample preparatio~z/t~eatment
AlN was used in the form of both substrates and powder (obtained from Tokuyama Soda Co., Ltd.). The AlN powder was Grade F powder with nominal particle size of 3 pm, a surface area of 3.4 m2 g-l and 0.8 wt.% oxygen. The hot pressed substrates, with a surface roughness (Ra) 0.3-0.8 pm, a thermal expansion coefficient 4.4 lop6 ‘C-l, density of 3.3 g cmm3, and thermal conductivity of 180 W m-l K-’ were used for copper bonding experiments whereas the powder was used to investigate the reaction between AlN and Cu,O. Oxygen free high conductivity copper (OFHC) foil, supplied by Aldrich, was used for direct bonding of copper to AlN. The Cu,O powder was supplied by Aldrich (Aldrich) with an average particle size of 7 ym and a surface area of 0.3 m2 g-‘. Conventional thick
208
hf. Entezarian, R.A.L. Drew / Materials Science and Engineering A212 (1996) 2OG-212
film copper powder was supplied by Sherritt Gordon Inc. (Grade: 2M, Organic Treated). Two different sources of nitrogen gas were used, commercial and pre-purified nitrogen (Lindegas products UN No.: UN 1066) The former contains 500 ppm oxygen and 25 ppm moisture and was used for direct bonding. The latter contains only I ppm oxygen and was used as an inert gas for simulation of the reaction taking place at the interface. A controlled atmosphere tube furnace was used for both wettability and bonding experiments. The furnace was operated at 10m2 mbar or less. Photographs of sessile drops were then taken at the experimental temperature. Oxidation experiments were performed on substrate coupons (12 x 12 x 2 mm) in a tube furnace with flowing air at a rate of 15 ml min-‘. Resultant weight gains were then used to both study the oxidation kinetics of AlN and for calculating the thickness of the oxide layer. Substrates for wetting experiments were degreased ultrasonically with acetone. Both copper powder and mixtures of Cu/Cu,O were used to measure the wettability of each substrate by copper. The drop size was kept as small as possible (5 mg) to minimize the effect of gravity on the sessile drop. These experiments were performed under pre-purified N, for different times at the desired temperature. The oxygen content of the powder mixtures was determined by chemical analysis (ON-MAT 822, supplied by Strohlein Instrument Co., West Germany). The dimensions of the metal drop and contact angle were recorded as a function of time using a camera. In some cases, the samples were cut through the centre of the drop and the contact angles were measured using a scanning electron microscope (Jeol, JSM-840).
.;.~..............,,,.,,
120
,..,.(.:,
..,
I
/
,
I
1
/
I
/
0
1
5
IO
15
20
25
30
35
Time (min.)
Fig. 3. Contact angles for AIN-Cu flowing nitrogen at 1100 “C.
system in vacuum, nitrogen, and
using a mortar and pestle and X-ray powder diffraction (XRD) analysis was performed to identify the reaction products. In addition, reaction kinetics of hlN/Cu,O powders were studied by thermogravimetry. Microstructural and chemical analysis were performed by SEM and a Noran EDS analysis. A peel test was used to evaluate the adhesion of the copper to the base materials after bonding (ASTM, B 571-91). Three strips of copper measuring 5 x 5 x 0.1 mm3 were placed parallel and equally spaced on the top of the substrates and bonded. The peel tests were then made by pulling the copper perpendicular to the surface at a rate of 0.5 mm min-‘, using a tensile testing machine (Instron Model 1362).
3. Results and discussion 2.2. Bondhg procedure 3.1. Wettability of AlN by copper,
OFHC copper foils were cut into shape and degreased ultrasonically followed by etching in a 15% nitric acid solution. Substrates were then bonded to Cu by heating the assembly at 30 “C min-’ to 10651075 “C for various times (5-60 min.) in a chamber evacuated to 10m2 mbar and re-filled with commercial N, gas. A flow rate of a 15 ml min-‘, was maintained throughout the experiment. 2.3. Simulation of interfacial reaction
In order to determine the effect of oxygen content on the interaction of Cu and AlN, mixtures of AlN and Cu,O power were prepared. Samples of the mixture were formed by die-pressing approximately 2 g of the powder mixture at a pressure of about 4 MPa. These samples were fired at the bonding temperature (1065 “C) followed by pulverizing to a fine powder
The instantanous contact angles between Cu/AlN upon melting at various temperatures under nitrogen are shown in Fig. 2. Fig. 3 indicates a decreasing contact angle with time, particularly for still and flowing nitrogen, wit’h the lowest contact angle being under Table 1 Effect of time on wetting behavior Time (min) 1 5 10 15 20 25 30
0 o\r,)
y,, (N cm-‘)
It:, (N cm-‘)
160
0.02084 0.02062
0.00702 0.00926 0.00127 0.00156 0.00199 0.00246 0.00327
157 153 150 146 142 136
0.01928 0.01998 0.0195G 0.01902 0.01828
hf. Enierasian,
R.A.L.
Dyelv
1 Materials
Science
and Engineering
A212
(1996)
xAl,O,
x IO4 El
(
30.0
209
206-212
32.0
/I,
A;ec"i
34.0
36.0
,
38.0
40.0
,
42.0
K,
,
44.0
Fig. 5. X-ray diffraction pattern of mixed powders heated at 900 “C.
Fig. 4. Microstructure cu,o.
obtained from wettability study of AlN by
flowing nitrogen. This difference can be attributed to the impurity levels of the gas. In comparison with static gas, flowing gas (with high impurities) would provide more 0, and H20 absorption by the melt. No significant change in contact angle with time was exhibited for the experiments performed under vacuum. In all cases, contact angle between Cu and AlN was < 135”, i.e. non-wetting. Fig. 2 also shows that increasing the temperature does not significantly affect the contact angle. The work of adhesion (W,) and wettability in the Cu-AlN system were calculated using Young’s equations [15] (Table l), (1)
79” ~0s e = 75” - hr and W,=y,,*(1+cos8)
(2)
yS,, ySl, and ylv the interfacial surface energies are for solid-gas, solid-liquid and liquid-gas, respectively. The surface energy ylv, of Cu was estimated by Rhee [16], who found the relation between yrv and temperature to be: yrV(Cu)(dynes
cm-‘) = 1462-0.27T
(“C)
stable than AlN in air. Furthermore, copper can dissolve oxygen and form copper oxide as demonstrated by the Cu-0 phase diagram Fig. 1 [17]. It is postulated that AlN may react with copper containing oxygen. In order to confirm this assumption, the reaction between AlN and Cu,O was investigated and thermodynamic calculations suggest that the reaction occurs as follows: 0.66AlN + Cu,O = 0.33A1,03 + 2Cu + 0.33N2
(5)
AG” = - 178198-37.1T
(6)
This indicates that AlN will reduce Cu,O to elemental cu. It was found that AlN is well wetted by Cu,O with a 30” contact angle and microstructural examination also confirmed that a reaction takes place at the interface and Fig. 4, shows the microstructure of the Cu,O/AlN interface. The formation of elemental copper, where Cu,O is in intimate contact with AlN, is clearly evident and corroborates the thermodynamic prediction given in Eq. (5). These results were also confirmed when blended powders of AlN and Cu,O were pressed and heated in vacuum in the range of 700-1060 “C for 10 min. The presence of Cu was confirmed by X-ray analysis, as shown in Fig. 5. The X-ray showed that this reaction is complete at about 900 “C. Both Fig. 6 and 7, show the amount of copper formation and In rate of copper formation vs. l/r, respectively. The activation energy was found to be
(3)
The decrease of the contact angle as a result of increasing temperature in Fig. 2 can be attributed to the decrease in yIU, The decrease in Fig. 3 can be attributed to 0, and other impurities and improved bonding of Cu to the surface of AlN which results in lower values of ylS. The values of ySufor AlN were taken from Rhee [16] in order to calculate the liquid/solid surface tension yIS, particularly under flowing nitrogen where oxygen pick-up from the gas appears to be important. Thermodynamic calculations suggest that oxygen can react with AlN forming A&O, and nitrogen. The overall reaction can be written as: 2AlN + ;O, = A1,03 + N,
(4)
By comparing the free energy of formation of AlN and Al,O,, it can be easily concluded that A1,03 is more
8.5
9.0 Temperature
8.5
10.0
10.5
11.0
( 1/Tx104K)
Fig. 6. Amount of the elemental copper formed vs. temperature.
Fig. 9. The SE1 of AIN-Cu
8
8.5
8.9 Temperature
9.3 ( I/T
9.8
10
12
xl 04K)
Fig. 7. The Arrhenius plot for the rate of copper formation vs. l/T.
approximately 117kJ mol-’ with a reaction constant of about 3.92, accordingly the following equation can be derived: K = 3.92 exp(
- XgL)
interface.
oxygen strongly affects the solid-liquid interfacial energy and changes the systemfrom non-wetting to wetting. This can be attributed to the decreasein surface tension of liquid copper. It can clearly be attributed to the existenceof oxygen since that was the only variable and strongly suggeststhat the reaction between AlN and Cu,O was responsible for this wettability improvement. However, the reaction between oxygen dissolved in the molten copper, not in the form of Cu,O, should also be considered. The further increase in the contact angle with higher Cu,O content in Fig. 8 can be explained by the increase of the liquidus temperature and therefore liquid viscosity in the G-0 phase diagram as the oxygen content increases(see Fig. 1).
3.2. Nettability of AlN by copper containing oxygen
3.3. Effect of osygen on the AlN-Cu
Copper powder mixtures with various oxygen contents were prepared with Cu,O additions and the wettability was assessed.The effect of Cu,O in copper on the contact angle is shown in Fig. 8. It is obvious that
A secondary electron image (SM.) micrograph of a typical AlN-Cu interface is shown in Fig. 9. A very thin reaction zone exists and, even at high magnification, no distinct interfacial phase is observed. One interesting observation about this microstructure is that there are no flaws at the interface whereas, considering Eq. (5), pores from NZ evolution might be expected. Possibleexplainations for the lack of pores may be (a) little or no reaction occurred at the interface resulting in no gas evolution, (b) oxidation of AIN changes the systemto an Al,O,-Cu systemor, (c) the gas evolution had already occurred and was liberated from the inter-
200
r
=I50 Et t? E a,
........ .& ............................ ............................. ..............................
0100 5
% E 8
interjhcncc
Table 2 The results of peel tests on metallized substrates
50
System
Average peel strength (MPa)
Max. peel strength (MPa)
A&O,-Cu-0 AIN-Cu-0 AIN-A&O,-Cu-0 (pre-oxidized)
49 42.1 14.7
66.6 58,s 39.2
0 0
.l
1
1.5 2 2.5 Oxygen content (at.%)
3
4
Fig. 8. The effect of oxygen on the wettability of AIN by Cu at 1100 “C.
M.
Emerariarl,
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/ Materials
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and Engineering
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Cc) Fig. 10. Fracture surfaces of (a) Al,O,-Cu,
face area through the molten copper during bonding. Among these scenarios, the latter is more likely and the result of the peel test reported below confirmed this. Another feature of the microstructure is the absence of cracks at the interface. The most common cracks at metal-ceramic interfaces occur owing to their CTE mismatch. The lack of this kind of cracking in this system can be attributed to the high ductility and low yield strength of pure copper. In fact, thermal stresses would be relieved by plastic deformation of copper during cooling from the bonding temperature. 3.4. Mechanical
evaluation of the bond stsengths
The results of the peel test done on samples bonded at 1070 “C for 5 min under a flowing nitrogen containing 500 ppm oxygen are summarized in Table 2. The fracture surfaces obtained from peel tests are shown in Fig. 10. The high strength in the Al,O,-Cu system can be attributed to the formation of a strong CuAlO, bonding phase at the interface. The fracture
(b) AlN-AlzO,-Cu,
and (c) AIN-Cu
systems.
surface for this system is shown in Fig. 10(a) where relatively fine grains of copper are observed. The presence of these particle are attributed to the copper bonded to Al,O, through copper aluminate spinel’formation at the inteface. These grains are entirely located within the reaction, or eutectic layer zone. Larger grains of copper with a multi-faceted morphology can also be seen. The shape of these grains confirm the pull out of copper grains from the copper foil. The reason for this can be explained by the formation of copperoxygen eutectic at the grain boundaries of copper which lowers the strength of copper and causes intergranular fracture. In the case of AlN-A&O,-Cu, (Fig. 10(b)), according to the EDS analysis, there was no evidence of copper at the interface confirming that fracture occurs at the AlN-Al,O, interface. The low strength for the pre-oxidized AlN substrate joined to Cu (AlN-A&O,-Cu) system can be explained by the stresses existing between the A1,03 and AlN, owing to the thermal expansion mismatch between the
212
ICI. Entezarian, R.A.L. Drew / Materials Science and Engineering A212 (1996) 206-212
various layers. The thermal expansion coefficient of A&0,(-9 x lOA ‘C-l) is almost double that of AlN (5 x 10V6 ‘C-l). The cracks produced at the AINA&O3 interface and on the surface of the Al,O, after oxidation suggest the existence of high thermal stresses. These cracks result from shrinkage during the formation of A&O3 by surface oxidation and subsequent differential contraction between AlN and Al,O, on cooling. The fracture surface of this system in Fig. 10(b) suggests that the fracture occurs at the A&O,AlN interface since there is no sign of copper. The relatively high strength for the AlN-Cu-0 system can be explained by the integrity of the interface between copper and AlN, as shown in Fig. 10(c). It should also be noted that the AlN-Cu-0 system demonstrated the best wetting characteristics. As oxygen in the AlN-Cu-0 system acts as an active element and can substitute for nitrogen in the AlN lattice. The fracture surface of the AlN-Cu-0 system shown in Fig. 10(c) displays a fine dispersion of copper particles at the interface confirming the role of oxygen in this system. The fracture surface observed for AlN-Cu-0 system was more extensively covered with copper than the one observed for the Al,O,-Cu system with the expectation of a higher interfacial strength however the results of the peel test did not confirm this. This might be attributed to the cracks observed on the fracture surface of Fig. 10(c), which were found propagating perpendicular to the interface. This kind of bond produces a very thin reaction layer at the interface (a few nanometres) and the relatively high strength confirms the above proposed mechanism of bonding.
4. Conclusions (1) AlN is not wetted by pure copper and the nonwetting condition is not a strong function of time and temperature. It was concluded from the wettability results that a modification either of the surface of the AlN or by altering the chemical composition of copper is required to achieve a bond, and hence improved wettability. (2) It was found that oxygen can be used as an active element with copper. The oxygen content of copper reacts with AlN forming A&O, which is more stable
than AlN. Chemical simulation of the interface using powders of AIN and C&O confirmed formation of elemental Cu. This was experimentally verified within a temperature range from 700 to 1000 “C with an activation energy of 117 kJ mol-*. (3) The addition of oxygen to the AIN-Cu system (about l-l.5 at.%) changes the system from a non-wetting to wetting, making bonding of Cu to AlN possible. (4) The mechanical properties of the interfaces using a standard peel test showed that the strength of the bond for AlN-Cu (42.14 MPa) is between that of Al,O,-Cu (50 MPa) and pre-oxidized AlN-Cu (14.7 MPa) fulfilling the minimum required strength of 19.6 MPa for microelectronic applications. (5) The low strength for the pre-oxidized AlN-Cu bond is a result of the CTE mismatch between A&O, and AlN. In contrast, the high strength for A&O,-Cu bond is attributed to the formation of CuAlO, phase at the interface.
References [I] W. Werdeckerand F. Aldinger,IEEE: T~rls. Cornpo,ter~s, Hybrids MatuJ
Technol., CHMT-7[4]
(1984) 399.
[2] G.A. Slack,J. Ploys.C/len?. Solids, 34 (1973) 321. [3] J.H. Enloe,R.W. Rice,J.W. Lau, R. Kumarand S.Y. Lee,J. Am. Ceratn. Sot., 74 (9) (1991) 2214.
[4] N. Kuramotoand H. Taniguchi,J. Mater. Sci. Left., 3 (1984) 471. [5] J.W. Balde,Overviewof multichiptechnology,Elecuordc Molerials Hodbook, ASM International,MaterialsPark, Ohio, vol. I, 1982,p. 297. [6] R. Brown,Thin film substrates, in L.I. Maisseland R.Glang (eds.),Handbook of Thin fibn Technology, McGraw-Hill, 1989. [7] P. Kluge-Weiss andJ. Gobrccht,Mater. Res. Sot. Symp. Proc., 40, MaterialsResearch Society,1985. [S] E.G. West, Copper and its alloys, Ellis Horwood Series, Chichester PublisherIndustrialMetals,1982. [9] N. Iwase,A. TsugeandY. Sugiuga,Proc. ht. Microelectronics Conf.Tokyo, Japan,vol. 5, 1984. [IO] A.D. Westwood and M.R. Notis,Mater. Res. Sot. Symp. Proc., 167,(1990)295. [ll] F.S. Ohuchi,J. Phys., Colloq~re CS, 10 (49) (1988). 1121M. Courbiere,J. Phys., Colloqlte Cl, 2 (47) (1986)187. 1131M. Courbiere,J. Phys., Collogue Cl, 2 (47) (1986)187. 1141E.G. West, Copper otrd its Alloys, Ellis Horwood Series, Chichester PublisherIndustrialMetals,1982,p, 8. [15] T. Young,Trans. R. Sot., London, 94 (1908) 65. [16] S.K. Rhee,J. Afn. Ceram. Sot., 53 (12) (1970)939. [17] J. Newton,Metallurgy of Copper, Wiley, London,1942.
A
Materials Scienceand EngineeringA212 (1996) 206-212
Direct bonding of copper to aluminum
nitride
M. Entezarian, R.A.L. Drew Depnrtmetlt
of Mirlirzg
and Metallurgical
Engitzeering,
McGill
Unicersit)!,
Motweal,
Quebec
I-13.4 24 7 Cmtdr
Received 1.5August 1995;in revised form 4 January 1996 Abstract
Direct bonding (DB) of copper to aluminum nitride was studied. The process parameters of DB were optimized based on time, temperature and thickness of the Cu-foil for Cu-A&O, system in an N, atmosphere containing 500 ppm 0, in a temperature range of 1065 to 1075 “C. These conditions were then applied to the Cu-AlN system. Wettability of AIN by Cu was studied and improved through oxidation of AlN and modification of Cu by adding 1 at.% 0,. The interface of AIN and Cu containing 0, was then simulated using powder mixtures. The oxidation of AlN was found to be the driving force for improving the wettability of the AlN by copper. The activation energy for oxidation of AIN was found to be 94 kJ mol-‘. It was demonstrated that direct bonding of Cu to AlN can be performed without any intermediate layer. The average peel strength of AIN-Cu, A120,-Cu and
AlN-Al,O,-Cu
systems were 42, 49 and 14.7 MPa, respectively.
Kq~ords: Direct bonding; Copper; Aluminium nitride
1. Introduction
Aluminum nitride has recently been studied for electronic ceramic packaging applications because of its high thermal conductivity and non-toxic nature [l]. The estimated theoretical thermal conductivity at room temperature is 320 Wm-’ K-’ [2]. However, measured values vary from 30 to 260 Wm-’ K-’ [3,4]. AlN has a coefficient of thermal expansion (CTE) between 4 and 4.5 x low6 K-l [5], and is close to that of the silicon (2.7 x lO-‘j) and thus suitable for direct attachment to very large scale integration (VLSI) dies. The combination of high thermal conductivity and low CTE gives AlN good thermal shock resistance [6]. Moreover AlN possesses higher flexural strength than alumina or berylia. The lower hardness of AlN compared with AlsO3 also facilitates machining processes. Heat dissipation from silicon chips is becoming critical as a result of increasing circuit density and the higher power applied to these chips. In addition, metal-ceramic interfaces determine the heat dissipation through a circuit and, in order to minimize the effect of interfaces,
direct bonding
(DB)
of the substrate
to the
conductor has been suggested. Historically, copper has been the most common thin-film conductor material and is comparable in performance with Al and Au. Copper also has lower bulk resistivity than Al but is
cheaper than Au, however, corrosion protection is required in the form of a thin surface coating such as a gold flash. The direct bonding process was essentially developed for bonding copper to alumina. In this process, the metal is joined directly to the ceramic and only a few monolayers of transition layer occur between the copper and ceramic. Therefore, the thermal contact resistance between copper and ceramic is low and, as a result, high power and high frequency operation is possible. Another advantage of direct bonding copper is the elimination of low thermal conductivity solder (usually glass based) between the Cu-metallization and the ceramic substrates. It is also reported that the directly bonded substrates are almost free of thermal fatigue, since no plastic deformation occurs on temperature cycling below 100 “C [7]. The basic idea of the direct bonding is to form a liquid shell around the metal to be bonded. This is done by either rapid heating of the metal-ceramic assembly for short times at the desired temperature and subsequent cooling, or forming a liquid at the metal/ceramic interface with a lower melting point than the bulk metal. Fortunately, these requirements can be achieved by utilizing the oxygen-copper eutectic melt which can wet and bond to the ceramic [8]. Fig. 1 indicates that the melting temperature decreases with oxygen content 0921-5093/96/$15.000 1996- Elsevier ScienceS.A. All rights reserved
hf. Entezarian, 1250
"
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II
R.A.L.
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1
Drew 8
/ Materials
('4 11 I /TWO , LIQUIDS -
Scierxe
and Exgirzeering
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(1996)
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207
of refractory metals to AlN has also been reported. The basic attraction of the latter is the low residual thermal stresses owing to their close thermal expansion coefficients to that of AlN. However, only mechanical interlocking has been found at the interface and no chemical reaction has been observed for these systems
POI.
1065 -a
C
‘C,
1050
-4-a
1000
l_G
a t cu20
400
0 -
EH 350
, 0
o.c.24@x08 c.O*
315
3
atcuo
, , , I , , , I , , , , , , 1 , , , 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Weight percentage oxygen Fig. 1. 01-0
binary phase diagram.
up to the eutectic point (1065 “C). Therefore, by heating copper containing dissolved oxygen to above 1065 “C, a Cu rich eutectic liquid will be formed. Two procedures have been suggested for controlling the eutectic layer [7]. These are (a) preoxidation of copper and (b) heating the assembly, copper foil and ceramic, in an oxidizing atmosphere with a given partial pressure of oxygen. Previous studies on the bonding of AlN to Cu have mainly concentrated on the surface oxidation of AlN by the formation of A&O3 [7,9], after which standard oxide-metal joining processes can be applied. Joining 170
160
150
140
130 1080
1065
1090
1100
1120
1140
1160
1160
1200
1220
Temperature (3) Fig. 2. Contact angles for AIN-Cu
system under flowing N,.
The nature of the bonding of AlN to Cu was studied by Ohuchi [l I] who investigated the electronic structure and adhesion of Cu-AlN interface. It was concluded that the adhesion strength of Cu to AlN is strongly influenced by the surface crystallographic orientations. In contrast, in the Cu-Al,O, system, Courbiere et al. [12,13] reported that CuAIO, forms at the beginning of joining and then is stabilized with time in favour of Al,O, and Cu,O. Active metals are available (e.g. titanium) that possess a higher affinity for aluminum than nitrogen. Addition of these metals to copper can improve both wettability and bonding in the Cu-AlN system. However, for electronic applications the conductivity of copper should also be considered when these elements are added to copper in order to improve its bonding to ceramics. Unfortunately, active metals affect the electrical conductivity of copper when they are added in small quantities and in solid solution [14]. On the contrary, oxygen has a less harmful effect on the conductivity of copper [8]. It can also be directly introduced at the interface and forms stable compounds such as oxides and oxynitrides. It would be very beneficial to the electronics industry to join AlN to Cu without diminishing the excellent physical properties of both materials. Therefore, the goal of the current study was to investigate the feasibility of direct bonding of Cu to AlN.
2. Experimental
procedure
2.1, Sample preparatio~z/t~eatment
AlN was used in the form of both substrates and powder (obtained from Tokuyama Soda Co., Ltd.). The AlN powder was Grade F powder with nominal particle size of 3 pm, a surface area of 3.4 m2 g-l and 0.8 wt.% oxygen. The hot pressed substrates, with a surface roughness (Ra) 0.3-0.8 pm, a thermal expansion coefficient 4.4 lop6 ‘C-l, density of 3.3 g cmm3, and thermal conductivity of 180 W m-l K-’ were used for copper bonding experiments whereas the powder was used to investigate the reaction between AlN and Cu,O. Oxygen free high conductivity copper (OFHC) foil, supplied by Aldrich, was used for direct bonding of copper to AlN. The Cu,O powder was supplied by Aldrich (Aldrich) with an average particle size of 7 ym and a surface area of 0.3 m2 g-‘. Conventional thick
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film copper powder was supplied by Sherritt Gordon Inc. (Grade: 2M, Organic Treated). Two different sources of nitrogen gas were used, commercial and pre-purified nitrogen (Lindegas products UN No.: UN 1066) The former contains 500 ppm oxygen and 25 ppm moisture and was used for direct bonding. The latter contains only I ppm oxygen and was used as an inert gas for simulation of the reaction taking place at the interface. A controlled atmosphere tube furnace was used for both wettability and bonding experiments. The furnace was operated at 10m2 mbar or less. Photographs of sessile drops were then taken at the experimental temperature. Oxidation experiments were performed on substrate coupons (12 x 12 x 2 mm) in a tube furnace with flowing air at a rate of 15 ml min-‘. Resultant weight gains were then used to both study the oxidation kinetics of AlN and for calculating the thickness of the oxide layer. Substrates for wetting experiments were degreased ultrasonically with acetone. Both copper powder and mixtures of Cu/Cu,O were used to measure the wettability of each substrate by copper. The drop size was kept as small as possible (5 mg) to minimize the effect of gravity on the sessile drop. These experiments were performed under pre-purified N, for different times at the desired temperature. The oxygen content of the powder mixtures was determined by chemical analysis (ON-MAT 822, supplied by Strohlein Instrument Co., West Germany). The dimensions of the metal drop and contact angle were recorded as a function of time using a camera. In some cases, the samples were cut through the centre of the drop and the contact angles were measured using a scanning electron microscope (Jeol, JSM-840).
.;.~..............,,,.,,
120
,..,.(.:,
..,
I
/
,
I
1
/
I
/
0
1
5
IO
15
20
25
30
35
Time (min.)
Fig. 3. Contact angles for AIN-Cu flowing nitrogen at 1100 “C.
system in vacuum, nitrogen, and
using a mortar and pestle and X-ray powder diffraction (XRD) analysis was performed to identify the reaction products. In addition, reaction kinetics of hlN/Cu,O powders were studied by thermogravimetry. Microstructural and chemical analysis were performed by SEM and a Noran EDS analysis. A peel test was used to evaluate the adhesion of the copper to the base materials after bonding (ASTM, B 571-91). Three strips of copper measuring 5 x 5 x 0.1 mm3 were placed parallel and equally spaced on the top of the substrates and bonded. The peel tests were then made by pulling the copper perpendicular to the surface at a rate of 0.5 mm min-‘, using a tensile testing machine (Instron Model 1362).
3. Results and discussion 2.2. Bondhg procedure 3.1. Wettability of AlN by copper,
OFHC copper foils were cut into shape and degreased ultrasonically followed by etching in a 15% nitric acid solution. Substrates were then bonded to Cu by heating the assembly at 30 “C min-’ to 10651075 “C for various times (5-60 min.) in a chamber evacuated to 10m2 mbar and re-filled with commercial N, gas. A flow rate of a 15 ml min-‘, was maintained throughout the experiment. 2.3. Simulation of interfacial reaction
In order to determine the effect of oxygen content on the interaction of Cu and AlN, mixtures of AlN and Cu,O power were prepared. Samples of the mixture were formed by die-pressing approximately 2 g of the powder mixture at a pressure of about 4 MPa. These samples were fired at the bonding temperature (1065 “C) followed by pulverizing to a fine powder
The instantanous contact angles between Cu/AlN upon melting at various temperatures under nitrogen are shown in Fig. 2. Fig. 3 indicates a decreasing contact angle with time, particularly for still and flowing nitrogen, wit’h the lowest contact angle being under Table 1 Effect of time on wetting behavior Time (min) 1 5 10 15 20 25 30
0 o\r,)
y,, (N cm-‘)
It:, (N cm-‘)
160
0.02084 0.02062
0.00702 0.00926 0.00127 0.00156 0.00199 0.00246 0.00327
157 153 150 146 142 136
0.01928 0.01998 0.0195G 0.01902 0.01828
hf. Enierasian,
R.A.L.
Dyelv
1 Materials
Science
and Engineering
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(1996)
xAl,O,
x IO4 El
(
30.0
209
206-212
32.0
/I,
A;ec"i
34.0
36.0
,
38.0
40.0
,
42.0
K,
,
44.0
Fig. 5. X-ray diffraction pattern of mixed powders heated at 900 “C.
Fig. 4. Microstructure cu,o.
obtained from wettability study of AlN by
flowing nitrogen. This difference can be attributed to the impurity levels of the gas. In comparison with static gas, flowing gas (with high impurities) would provide more 0, and H20 absorption by the melt. No significant change in contact angle with time was exhibited for the experiments performed under vacuum. In all cases, contact angle between Cu and AlN was < 135”, i.e. non-wetting. Fig. 2 also shows that increasing the temperature does not significantly affect the contact angle. The work of adhesion (W,) and wettability in the Cu-AlN system were calculated using Young’s equations [15] (Table l), (1)
79” ~0s e = 75” - hr and W,=y,,*(1+cos8)
(2)
yS,, ySl, and ylv the interfacial surface energies are for solid-gas, solid-liquid and liquid-gas, respectively. The surface energy ylv, of Cu was estimated by Rhee [16], who found the relation between yrv and temperature to be: yrV(Cu)(dynes
cm-‘) = 1462-0.27T
(“C)
stable than AlN in air. Furthermore, copper can dissolve oxygen and form copper oxide as demonstrated by the Cu-0 phase diagram Fig. 1 [17]. It is postulated that AlN may react with copper containing oxygen. In order to confirm this assumption, the reaction between AlN and Cu,O was investigated and thermodynamic calculations suggest that the reaction occurs as follows: 0.66AlN + Cu,O = 0.33A1,03 + 2Cu + 0.33N2
(5)
AG” = - 178198-37.1T
(6)
This indicates that AlN will reduce Cu,O to elemental cu. It was found that AlN is well wetted by Cu,O with a 30” contact angle and microstructural examination also confirmed that a reaction takes place at the interface and Fig. 4, shows the microstructure of the Cu,O/AlN interface. The formation of elemental copper, where Cu,O is in intimate contact with AlN, is clearly evident and corroborates the thermodynamic prediction given in Eq. (5). These results were also confirmed when blended powders of AlN and Cu,O were pressed and heated in vacuum in the range of 700-1060 “C for 10 min. The presence of Cu was confirmed by X-ray analysis, as shown in Fig. 5. The X-ray showed that this reaction is complete at about 900 “C. Both Fig. 6 and 7, show the amount of copper formation and In rate of copper formation vs. l/r, respectively. The activation energy was found to be
(3)
The decrease of the contact angle as a result of increasing temperature in Fig. 2 can be attributed to the decrease in yIU, The decrease in Fig. 3 can be attributed to 0, and other impurities and improved bonding of Cu to the surface of AlN which results in lower values of ylS. The values of ySufor AlN were taken from Rhee [16] in order to calculate the liquid/solid surface tension yIS, particularly under flowing nitrogen where oxygen pick-up from the gas appears to be important. Thermodynamic calculations suggest that oxygen can react with AlN forming A&O, and nitrogen. The overall reaction can be written as: 2AlN + ;O, = A1,03 + N,
(4)
By comparing the free energy of formation of AlN and Al,O,, it can be easily concluded that A1,03 is more
8.5
9.0 Temperature
8.5
10.0
10.5
11.0
( 1/Tx104K)
Fig. 6. Amount of the elemental copper formed vs. temperature.
Fig. 9. The SE1 of AIN-Cu
8
8.5
8.9 Temperature
9.3 ( I/T
9.8
10
12
xl 04K)
Fig. 7. The Arrhenius plot for the rate of copper formation vs. l/T.
approximately 117kJ mol-’ with a reaction constant of about 3.92, accordingly the following equation can be derived: K = 3.92 exp(
- XgL)
interface.
oxygen strongly affects the solid-liquid interfacial energy and changes the systemfrom non-wetting to wetting. This can be attributed to the decreasein surface tension of liquid copper. It can clearly be attributed to the existenceof oxygen since that was the only variable and strongly suggeststhat the reaction between AlN and Cu,O was responsible for this wettability improvement. However, the reaction between oxygen dissolved in the molten copper, not in the form of Cu,O, should also be considered. The further increase in the contact angle with higher Cu,O content in Fig. 8 can be explained by the increase of the liquidus temperature and therefore liquid viscosity in the G-0 phase diagram as the oxygen content increases(see Fig. 1).
3.2. Nettability of AlN by copper containing oxygen
3.3. Effect of osygen on the AlN-Cu
Copper powder mixtures with various oxygen contents were prepared with Cu,O additions and the wettability was assessed.The effect of Cu,O in copper on the contact angle is shown in Fig. 8. It is obvious that
A secondary electron image (SM.) micrograph of a typical AlN-Cu interface is shown in Fig. 9. A very thin reaction zone exists and, even at high magnification, no distinct interfacial phase is observed. One interesting observation about this microstructure is that there are no flaws at the interface whereas, considering Eq. (5), pores from NZ evolution might be expected. Possibleexplainations for the lack of pores may be (a) little or no reaction occurred at the interface resulting in no gas evolution, (b) oxidation of AIN changes the systemto an Al,O,-Cu systemor, (c) the gas evolution had already occurred and was liberated from the inter-
200
r
=I50 Et t? E a,
........ .& ............................ ............................. ..............................
0100 5
% E 8
interjhcncc
Table 2 The results of peel tests on metallized substrates
50
System
Average peel strength (MPa)
Max. peel strength (MPa)
A&O,-Cu-0 AIN-Cu-0 AIN-A&O,-Cu-0 (pre-oxidized)
49 42.1 14.7
66.6 58,s 39.2
0 0
.l
1
1.5 2 2.5 Oxygen content (at.%)
3
4
Fig. 8. The effect of oxygen on the wettability of AIN by Cu at 1100 “C.
M.
Emerariarl,
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Drew
/ Materials
Science
and Engineering
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(1996)
206-212
Cc) Fig. 10. Fracture surfaces of (a) Al,O,-Cu,
face area through the molten copper during bonding. Among these scenarios, the latter is more likely and the result of the peel test reported below confirmed this. Another feature of the microstructure is the absence of cracks at the interface. The most common cracks at metal-ceramic interfaces occur owing to their CTE mismatch. The lack of this kind of cracking in this system can be attributed to the high ductility and low yield strength of pure copper. In fact, thermal stresses would be relieved by plastic deformation of copper during cooling from the bonding temperature. 3.4. Mechanical
evaluation of the bond stsengths
The results of the peel test done on samples bonded at 1070 “C for 5 min under a flowing nitrogen containing 500 ppm oxygen are summarized in Table 2. The fracture surfaces obtained from peel tests are shown in Fig. 10. The high strength in the Al,O,-Cu system can be attributed to the formation of a strong CuAlO, bonding phase at the interface. The fracture
(b) AlN-AlzO,-Cu,
and (c) AIN-Cu
systems.
surface for this system is shown in Fig. 10(a) where relatively fine grains of copper are observed. The presence of these particle are attributed to the copper bonded to Al,O, through copper aluminate spinel’formation at the inteface. These grains are entirely located within the reaction, or eutectic layer zone. Larger grains of copper with a multi-faceted morphology can also be seen. The shape of these grains confirm the pull out of copper grains from the copper foil. The reason for this can be explained by the formation of copperoxygen eutectic at the grain boundaries of copper which lowers the strength of copper and causes intergranular fracture. In the case of AlN-A&O,-Cu, (Fig. 10(b)), according to the EDS analysis, there was no evidence of copper at the interface confirming that fracture occurs at the AlN-Al,O, interface. The low strength for the pre-oxidized AlN substrate joined to Cu (AlN-A&O,-Cu) system can be explained by the stresses existing between the A1,03 and AlN, owing to the thermal expansion mismatch between the
212
ICI. Entezarian, R.A.L. Drew / Materials Science and Engineering A212 (1996) 206-212
various layers. The thermal expansion coefficient of A&0,(-9 x lOA ‘C-l) is almost double that of AlN (5 x 10V6 ‘C-l). The cracks produced at the AINA&O3 interface and on the surface of the Al,O, after oxidation suggest the existence of high thermal stresses. These cracks result from shrinkage during the formation of A&O3 by surface oxidation and subsequent differential contraction between AlN and Al,O, on cooling. The fracture surface of this system in Fig. 10(b) suggests that the fracture occurs at the A&O,AlN interface since there is no sign of copper. The relatively high strength for the AlN-Cu-0 system can be explained by the integrity of the interface between copper and AlN, as shown in Fig. 10(c). It should also be noted that the AlN-Cu-0 system demonstrated the best wetting characteristics. As oxygen in the AlN-Cu-0 system acts as an active element and can substitute for nitrogen in the AlN lattice. The fracture surface of the AlN-Cu-0 system shown in Fig. 10(c) displays a fine dispersion of copper particles at the interface confirming the role of oxygen in this system. The fracture surface observed for AlN-Cu-0 system was more extensively covered with copper than the one observed for the Al,O,-Cu system with the expectation of a higher interfacial strength however the results of the peel test did not confirm this. This might be attributed to the cracks observed on the fracture surface of Fig. 10(c), which were found propagating perpendicular to the interface. This kind of bond produces a very thin reaction layer at the interface (a few nanometres) and the relatively high strength confirms the above proposed mechanism of bonding.
4. Conclusions (1) AlN is not wetted by pure copper and the nonwetting condition is not a strong function of time and temperature. It was concluded from the wettability results that a modification either of the surface of the AlN or by altering the chemical composition of copper is required to achieve a bond, and hence improved wettability. (2) It was found that oxygen can be used as an active element with copper. The oxygen content of copper reacts with AlN forming A&O, which is more stable
than AlN. Chemical simulation of the interface using powders of AIN and C&O confirmed formation of elemental Cu. This was experimentally verified within a temperature range from 700 to 1000 “C with an activation energy of 117 kJ mol-*. (3) The addition of oxygen to the AIN-Cu system (about l-l.5 at.%) changes the system from a non-wetting to wetting, making bonding of Cu to AlN possible. (4) The mechanical properties of the interfaces using a standard peel test showed that the strength of the bond for AlN-Cu (42.14 MPa) is between that of Al,O,-Cu (50 MPa) and pre-oxidized AlN-Cu (14.7 MPa) fulfilling the minimum required strength of 19.6 MPa for microelectronic applications. (5) The low strength for the pre-oxidized AlN-Cu bond is a result of the CTE mismatch between A&O, and AlN. In contrast, the high strength for A&O,-Cu bond is attributed to the formation of CuAlO, phase at the interface.
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[2] G.A. Slack,J. Ploys.C/len?. Solids, 34 (1973) 321. [3] J.H. Enloe,R.W. Rice,J.W. Lau, R. Kumarand S.Y. Lee,J. Am. Ceratn. Sot., 74 (9) (1991) 2214.
[4] N. Kuramotoand H. Taniguchi,J. Mater. Sci. Left., 3 (1984) 471. [5] J.W. Balde,Overviewof multichiptechnology,Elecuordc Molerials Hodbook, ASM International,MaterialsPark, Ohio, vol. I, 1982,p. 297. [6] R. Brown,Thin film substrates, in L.I. Maisseland R.Glang (eds.),Handbook of Thin fibn Technology, McGraw-Hill, 1989. [7] P. Kluge-Weiss andJ. Gobrccht,Mater. Res. Sot. Symp. Proc., 40, MaterialsResearch Society,1985. [S] E.G. West, Copper and its alloys, Ellis Horwood Series, Chichester PublisherIndustrialMetals,1982. [9] N. Iwase,A. TsugeandY. Sugiuga,Proc. ht. Microelectronics Conf.Tokyo, Japan,vol. 5, 1984. [IO] A.D. Westwood and M.R. Notis,Mater. Res. Sot. Symp. Proc., 167,(1990)295. [ll] F.S. Ohuchi,J. Phys., Colloq~re CS, 10 (49) (1988). 1121M. Courbiere,J. Phys., Colloqlte Cl, 2 (47) (1986)187. 1131M. Courbiere,J. Phys., Collogue Cl, 2 (47) (1986)187. 1141E.G. West, Copper otrd its Alloys, Ellis Horwood Series, Chichester PublisherIndustrialMetals,1982,p, 8. [15] T. Young,Trans. R. Sot., London, 94 (1908) 65. [16] S.K. Rhee,J. Afn. Ceram. Sot., 53 (12) (1970)939. [17] J. Newton,Metallurgy of Copper, Wiley, London,1942.