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AuSn metallization scheme for bonding of InP-based laser diodes to chemical vapor deposited diamond submounts
Materials
Chemistry
and Physics,
33 (1993)
281
281-288
Ti/Pt/Au-Sn metallization scheme for bonding of InP-based laser diodes to chemical vapor deposited diamond submounts A.
Katz,
R.
R.
AT&T
K.-W.
Varma
Wang, and
Bell Laboratories,
(Received
H.
F. A.
Baiocchi,
W.
C. Dautremont-Smith,
E.
Lane,
H.
S. Luftman,
Curnan
600 Mountain
Avenue,
Murray
HiIl, NJ 07974-0636
(USA)
July 30, 1992)
Abstract A layered structure (consisting of Ti(100 nm)/Pt(200 nm)/Au(SOO nm) and AuSn (2.5 pm total in multiple alternating layers)) was studied as a bonding scheme for InP-based laser diodes to chemical vapor deposited (CVD) diamond substrates. This structure provided a molten Au-G layer of eutectic composition (80:20 wt.%) on top of the Ti/Pt adhesion and barrier metals for about 6 s in the temperature range 300-350 “C and allowed for efficient bonding of the device to the substrate. Longer heating durations allowed a reaction between the Pt and Sn to consume significant amounts of Sn from the solder, thus elevating its melting temperature and resolidifying the solder. With optimum bonding conditions, a high-quality bond of the InP-based laser diode to the CVD diamond substrate was observed, and the electrical performance of the diode was superior to that of diodes that were bonded with the standard In/EkO configurations.
Introduction
A semiconductor laser device is typically mounted on a submount that provides mechanical stability, heat dissipation and electrical connections to one or more electrical sources (d-c. and a-c. control voltages or currents, or both). A reliable device bonding scheme is essential to provide shortand long-term laser die operating reliability. The quality of the bonding medium depends on the submount material, the solder metallurgical system, the barrier metallization scheme and the bonding conditions. Submounts for laser devices are traditionally manufactured from Si or ceramic materials such as BeO. Recently, owing to its desirably higher thermal conductivity and higher electrical resistivity, diamond has emerged as a potentially attractive alternative material with which to manufacture the submounts and, in particular, to use for bonding of devices that are operated under extreme conditions [l, 21. Thus, a natural application of diamond is one that takes advantage of its potentially high thermal conductivity, which can be as high as 22 W cm-’ “C-l (at room temperature). This is about five times better than an excellent conductor such as copper [3, 41. During
0254-0584/93/$6.00
the last year the chemical vapor deposited (CVD) diamond thick film plates have become commercially available, produced by a few vendors around the world, and offered at lower cost than the single-crystal parts, which made it an attractive technology for thermal management applications in the microelectronics environment. Solder preforms, paste or deposited solder have traditionally been used to realize bonding of microelectronic devices. Deposited solder, provides significant advantages over the other techniques, such as decreased oxide formation prior to the initiation of the bonding cycle, and more accurate control of the bonding media thickness and volume [5]. The few commonly used solders are the lowermelting-point elements such as In (183 “C), medium-melting-point binary Pb-Sn alloy (223 “C) or higher-melting-point alloys such as Au-Sn (278 “C), Au-Ge (361 “C) or Au-Si (363 “C), all of which have been used in the eutectic composition. Generally, the lower-melting-point solders have a better thermal conductivity, but are mechanically weaker, compared to the higher-melting-point solders. Thus, for better thermal stability and longterm reliability, one would prefer to use the highermelting-point solders [6-91. Au-Sn solder has been widely used in optoelectronic applications owing to its high, but practical, melting temperature,
0 1993 - Elsevier
Sequoia.
All rights
reserved
282
allowing for efficient bonding of devices that contain Au-based contacts. In this paper we report the first results on the bonding of InP-based laser devices to CVD diamond submount, using an Au-Sn solder and Ti/ Pt barrier metal systems. Since the metallurgical characteristics of the Ti/Pt/Au-Sn system have been studied earlier [lo, 111, only those aspects of the metallization that are important to the bonding properties studied in this work are discussed.
Au/Au-Sn/Pt/Ti
metallurgical
It is apparent that the Au-Sn binary eutectic system (Fig. l(a)) is a reactive system with two eutectic melting points. The commonly used Au-Sn eutectic alloy with 80 wt.% gold is a eutectic mixture of the compounds Au,Sn and AuSn, with a melting point of 278 “C. The Pt-Ti system (Fig. l(b)) is also an active system at temperatures as low as 250 “C. The Ti,Pt inter-metallic phase was reported to be the predominant phase in this system formed mainly by diffusions of Ti into the original Pt volume [B-15]. This suggests that some level of Ti-Pt inter-metallic reaction is to be expected through the executed Au-Sn bonding cycle, which is realized at temperatures of 300-350 “C. At the Au-Sri solder interface with the Pt barrier layer (Fig. l(c) and (d)), a preferred reaction between Sn and Pt is to be expected. While Au and Pt are entirely inert to each other (Fig. l(c)),
system
The binary phase diagrams for the elements that constitute the Ti/Pt/Au-Sn metallurgical scheme are given in Fig. 1 and have been discussed in the literature [5, 8, 11, 121.
, 2000
10 20 III
30
n
ATOMIC 40 50 I I
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283
the Sn-Pt system is extremely reactive even at temperatures lower than 100 “C, having a wide mutual miscibility range and creating about five inter-metallic phases. Thus, reaction between the Au-Sn and the Pt barrier metal is expected to proceed via the diffusion of Sn in the Au-Sn layer toward the barrier metal interface. The driving force for this phenomenon is clearly twofold: the intrinsic diffusivity of Sn through Au and the thermodynamic stability of Au-Sn versus Pt-Sn alloys. In spite of its reactive nature, the Ti/Pt/Au-Sn system is sufficiently thermally stable throughout the relevant chip bonding cycle temperature and time to allow its use in the bonding of optoelectronic devices to the appropriate submounts.
Experimental The diamond films that were used in this study were nucleated and grown on metal substrates by microwave-enhanced plasma chemical vapor deposition at 2.45 GHz. Depositions were done with a proprietary gas mixture of hydrogen, hydrocarbon and an oxygen-bearing precursor that results in optically transparent films. Except for some modifications in growth times, which led to changes in the CVD diamond plate thickness, the deposition conditions were held constant from sample to sample. Later the self-standing CVD diamond plates were disconnected chemically from the substrate. The plate surfaces were subsequently polished to mirror finish, metallized and laser diced to heat-sinks with dimensions of about 0.65 X 1.25 mm and a thickness of about 0.25 mm. These CVD diamond films had a thermal conductivity greater than 11 W cm-’ “C- ‘_ An adhesion layer of 100 nm of Ti was d-c. magnetron sputtered onto the mirror-finished CVD diamond plates, followed by deposition of 200 nm of Pt, which serves as a barrier layer. After deposition of the metal bilayer, a number of alternating layers of gold and tin were deposited. The number of Au-Sn layers varied from at least three to (preferably) nine and were capped with a layer of gold. The layers were chosen to provide an overall average composition of 80:20 wt.% Au:Sn and a total thickness of 3 pm. The structure that was found to give the best results consisted of a gold layer (0.5 pm) deposited onto the Ti/Pt barrier metals, topped with four pairs of alternating Sn(0.35 pm)/Au(0.2 pm) layers, and covered with a final layer of Au (0.5 pm) [16] (10 layers of Au and Sn for a total thickness of 3.2 pm).
InP-based laser devices were then bonded to the metallized CVD diamond submounts at thermal cycles of 320-350 “C and 5-10 s. The strength of the bond was assessed by shearing the device from the submount and analyzing the exact ruptured interface. Composition and microstructure analysis of the different metallurgical schemes were performed by a variety of means. Auger electron spectroscopy (AES) was used to obtain information on surface impurities (such as tin oxides) and, in conjuction with sputter depth profiling, to study the reaction of the Ti/Pt layer with the Au-Sn alloy after various heat treatments. Rutherford backscattering spectrometry (RBS) was used to measure the solder composition and thickness in the as-deposited and annealed layer structures. Scanning electron microscopy (SEM) was used to obtain microstructural information, both of the surface morphology of the solder pad (in plan view) and of the quality of the interfaces, obtained by cleaving the solder pad and examining the cross-section. Optical microscopy provided a quick way to screen the surface morphology of the solder pad area. Finally, electrooptical analysis of the bonded laser devices was carried out to ascertain the final laser performance.
Results
and discussion
Optimal TilPtlAu-Sn layer structure Bonding of the laser device to the diamond submount requires a solder with (1) the eutectic composition Au:Sn =80:20 wt.%, so that it melts at the correct temperature, and (2) a total thickness of 3 pm or less, to guarantee a sufficient thermal conductivity through the solder and eliminate technical problems associated with the presence of excess solder on the device. Given the unstable nature of the PtiAu-Sn metallurgical system, a number of Au-Sn solder structures with the required average composition and thickness were investigated to determine which system gave the best bonding results. Bonding is carried out in a rapid thermal anneal (RTA) station, in which the temperature is quickly ramped up to 320-350 “C and shut off after a period of 5 s. Clearly the differences in solder layer structure will affect the kinetics of the melting and bonding process, so that over the short time scale of the bonding process, different layer structures will lead to different results. For longer melting times (greater than 15 s), thermodynamic considerations become important. Then, no matter what the initial as-deposited structure may be, the behavior of the melt over time will depend on the
284
initial Au, Sn and Pt average composition, since these are the main interacting species. Presumably, the Pt barrier layer will consume an appropriate amount of Sn from the Au-Sn solder to form Pt-Sn species. This has the detrimental effect of destroying the barrier nature of the Pt layer and altering the Au-Sn composition so that the melting behavior is adversely affected. The optimal metallization structure should allow a significant portion of the Pt barrier to remain intact during the RTA bonding step, with the Au-Sn solder remaining molten long enough to bond to the laser device. Note that the optimal metallization structure depends in a critical way on the time scale of the bonding process. The shelf life of unbonded parts and the longterm reliability of the bond are issues that still need to be addressed, since the Pt-Sn interaction occurs over long time periods even at very low temperatures. This could change the bonding behavior of parts that have been waiting for a long time to be processed, or degrade the integrity of the bond after it has been formed. The effect of layer structure on bonding properties is summarized using the results obtained from two samples. In the first sample (referred to hereafter as sample I), a simple three-layered 17.0, (a,)
AS DEPOSITED ---
SIMULATECJ MEASUREMENT
Au(0.8 pm)/Sn(l.l4 pm)/Au(0.8 pm) structurewas used on top of the Pt/Ti barrier metals. This represents a simple processing sequence, with only two metal interfaces within the solder. The second sample (referred to hereafter as sample II) contains nine Au/Sri alternating layers and thus requires a relatively complicated processing sequence. Both samples were prepared to have the same overall thickness and Au:Sn ratio. Both samples also have at least 0.5 pm of gold directly on top of the Pt layer - an attempt to minimize the Sn-Pt interreaction. Each as-deposited wafer was cut into several pieces for temperature processing. Thus, for each sample, three annealing conditions were available for analysis: as-deposited; 320 “C, 5 s; and 320 “C, 10 s. RBS data on the two samples were used to determine the solder composition and thickness. The data for the as-deposited sample and 320 “C, 5 s annealed sample are summarized in Fig. 2. Interpretation of the data from the as-deposited samples shows that the overall thickness and Au:Sn composition of samples I and II are quite similar. The total thickness in both cases is quite close to 2.7 pm and the average Au:Sn ratio is close to 80:20 wt.%. Differences in the Au:Sn ratio with depth are observed between the two as-deposited samples, and this is expected, owing to the different
r
13.6:
: --_,
:
‘-.-_____/’
5Q”.
I
t MEASURED:
(bp)
AFTER
RIP
STRUCTURE MEASURED:
Au 0.70 Sn 0.30 >ZOOOOA
Ii
0
100
200
300
400
500 CHANNEL
Fig. 2. RBS spectra of metallized submounts: layer structure); (b,) and (b2) are as-deposited details.
100
32O”C,
,
,
1
10 s
Au 0.69 Sn 0.31 31000/i
11
200
I,
300
I
400
IO
#
(a,) and (a2) are as-deposited and annealed spectra from and annealed spectra from sample II (the 9-layer structure).
sample I (the simple 3See text for deposition
methods of preparation. For both as-deposited samples, the RBS data show that no strictly pure Sn layers are left. The general shapes of the spectra show that there is a predominantly pure gold layer at the bottom of the solder next to the Pt and another on top of the solder. In the middle, the Sn intermixes readily with the gold to form an approximately 5050 at.% alloy. The middle layer composition differs between samples I and II according to the method of preparation. In general, the picture that emerges is that, during deposition, the first gold layer goes down on top of Pt and forms a sharp interface. The layer deposited next, which is Sn, is known to be fairly porous. When the next Au layer is deposited it mixes readily with the Sn to form an Au--& alloy. The process continues in this manner until the final gold deposition, which is a fairly thick layer. Some portion of that may react with the previously deposited Sn layer, leaving a portion of mainly pure Au on top of the structure. After annealing, the RBS spectra of samples I and II are qualitatively different. Note that the RBS beam spot size used here is =250 pm, which covers most of the solder pad. Any lateral variations of thickness and composition will be averaged in the final spectrum. The data from sample 11 (Fig. 2(b,)) show a final solder composition of 8O:ZO wt.% and a fairly uniform thickness of solder over the pad. On the other hand, the data from sample I (Fig. 2(a2)) show a similar solder composition but a greatly smeared low-energy edge. This indicates very nonuniform solder thickness across the pad, and hence a nonuniform melting behavior across different areas of the pad. Figure 3 shows four SEM cross-sectional micrographs of as-deposited and 320 “C, 5 s annealed pieces from samples I and II. The microstructural information obtained from the SEM photos shows clearly the differences between samples I and II and the reason for the different melting behavior. The cross-sectional micrograph from sample I, which was the simple structure, shows the individual Ti, Pt and Au-Sn solder layers quite clearly. Note that within the solder layer the original Au/Sri// Au interfaces are separated in certain regions. From the RBS data we know that, on average, the middle portion consists of 5O:SO at.% Au-Sn with pure Au layers on top and bottom. Perhaps because of the greater strain present in the much thicker layers used in sample I, or because of a volume change in the layer during formation of the Au-Sn alloy, empty spaces and gaps occur in several places within the solder layer of sample I. This is in contrast to the more complex layering
Fig. 3. SEM cross-sectional micrographs of a metallized Au(0.8 ~m)/Sn(l.4 pm)/Au(0.8 pm)/Pt(0.2 wm)rTi(O.l pm) submount [(a) as-deposited and (b) after a bonding cycle at 320 “C for 5 s] and of a metalked Au(O.5 pm)/{3 x )/Sn(0.35 ~m)/Au(O.2 ~m)/Au(O.S ~m)/Pt(O.Z pm)DX(O.l Mm) submount i(c) as-deposited and (d) after a bonding cycle at 320 “C for S sf.
used in sample II. In that case, the SEM micrographs show closely packed layers of Au:Sn with no gaps or separations at any of the interfaces. On the basis of this information, the difference in melting behavior of the two samples is understood. For sample II, the uniform layering of Au-Sn leads to uniform melting of the solder, which during the 5 s RTA forms a final layer of 80:20 wt.% solder that has uniform thickness throughout the area of the pad. On the other hand, in sample I, the observed gaps in the solder mean that during a short 5 s anneal, there is not enough time for uniform mixing of the Au:Sn solder, and this Ieaves areas that are very gold rich and areas that are very tin rich. Since the melting is nonuniform across the pad, areas of different thickness are formed, with greatly differing local compositions. This nonuniformity in melting for sample I is most easily observed in the plan-view SEM photos shown in Fig. 4. The dendritic structures found after RTA of sample I are thick regions that were shown to be gold rich by EDS analysis in the SEM. Notable differences in the feature size are observed depending on the melt duration, as dem-
286
Fig. 4. SEM plan-view micrographs of a metallized bonding cycle-at 320 “C for
Au(0.8 r.Lm)/Sn(l.4 pm)/Au(0.8
onstrated by a comparison of the 5 s and 10 s melted samples in Fig. 4. Resolidification of these localized features was observed for this sample already after 2-3 s of melting at temperatures above the eutectic melting point, and is consistent with the gold-rich composition of these regions measured in the SEM. By contrast, the plan-view photo of sample II after RTA shows a uniform surface morphology across the pad. Thus, a multiple layered structure consisting of thinner but more Au/Sri layers is superior to a simple layered structure consisting of very thick layers, with respect to melting behavior during RTA. The main reason is that a higher degree of strain and/or volume change occurs in the simple structure, which produces gaps in the integrity of the solder layer. During RTA, these gaps interfere with uniform melting behavior and create localized areas of Au-rich solder, which freeze prematurely. During a 5 s anneal, interaction of Sn with the Pt layer does not adversely affect the bonding process, especially if the as-deposited structure has 0.5 pm or greater of pure gold directly in contact with the Pt. AES spectra of specially deposited test structures were obtained to determine how much of the Pt barrier layer would be consumed during the typical RTA times of 5-10 s (Fig. 5). Basically, they show that, for short time anneals, Sn does tend to segregate toward the Pt interface, presumably forming Pt-Sn compounds, but that a significant amount of the original Pt layer is left intact during the bonding process. This does not mean that the Pt-Sn interaction is unimportant. As mentioned above, understanding
Fm)/Pt(0.2
~m)/Ti(O.l
I
,
-Ti
1
I
pm) submount
after a
-
_
Fig. 5. Auger spectra for test structure with Au(0.2 pm)/Sn(0.25 pm)/Au(O.l pm) deposited on Ti(O.1 pm)/Pt(0.2 pm) barrier metals: (a) as-deposited; (b) 320 “C, 5 s anneal.
of this interaction will be very important for establishing the long-term shelf life of the submounts and the long-term reliability of the bond once it is formed. Bonding per$ormance Based on the above information, we have established the multilayer solder structure as our commonly used solder [16] for the purpose of this report, for bonding InP-based laser devices to
287
Fig. 6. Metallized CVD diamond heat-sink submount (a) as processed, and (b) after bonding an InP-based laser device onto it and shearing it off (note the device front side metal/dielectric film, which was sheared off the device and left bonded to the submount).
CVD diamond submounts. Figure 6 presents optical micrographs of the metallized CVD diamond submount (Fig. 6(a)) and of the same submount after the InP-based laser device was bonded onto it and sheared off (Fig. 6(b)). The center metal geometry is the Au-Sn multilayered solder bonding pad, which is deposited onto the barrier Pt-Ti larger-area metallization. The areas on the righthand side of this submount are used as a ribbon bonding pad to bring the laser device into contact with the submount and the submount into contact with the laser package. The InP-based laser device was bonded to the CVD diamond submount using a thermal cycle of 320 “C for about 5 s. The device was subsequently sheared off the submount at a force of about 500 g. Figure 6(b) shows the almost complete dielectric and metal pattern of the InPbased laser device. One can conclude that the wetting quality of the device to the CVD diamond submount is excellent, leading to adhesion of almost 100% of the area of the device to the submount. Thus, as a result of the shearing, the adhesion is lost in the semiconductor-metal interface contained in the laser structure, rather than in the device metal/dielectric-solder interface, leaving the bonded metal/dielectric film bonded to the submount, as is observed in the micrograph. Electrical performance The quality of the bonding is evaluated by measuring the electrical and optical performance of the bonded laser devices. Thermal impedance is an excellent parameter to use for judging the bonding quality of the laser diode and comparing it with a device bonded with the standard configuration of indium solder onto the Be0 submount. The previously used configuration has a typical
thermal impedance of about 15 K W-‘. Measuring the thermal impedance, however, is time consuming and the data are often influenced by various device parameters such as device resistance and leakage, which make the results ambiguous. Thus a new method was applied in this study to characterize the nature of the bonding. The standard method of bonding InP laser devices is to use In solder on a Be0 substrate. The influence of this bonding on the performance of the bonded device has been widely studied and is used as a reference line to the influence of the Au-Sn/CVD diamond bond on the device performance. The one property that reflects the quality of the bonding is the rollover current measured at the light-current graph. This parameter is influenced by the device resistance, leakage and external quantum efficiency. Thus, a comparison of the rollover values of similar devices that were bonded in two different modifications, such as In/Be0 and Au-Sn/CVD diamond, indicates which one is of superior quality. Figure 7 shows the full statistical optical and electrical performance distribution of one of the lots of the InP-based laser device bonded onto Au-Sn/Pt/Ti metallized CVD diamond on top of a Be0 testing stud, compared to InP-based lasers taken from the same wafer that were bonded using In solder onto a Be0 submount. The superior performance of the device that is bonded to the CVD diamond is reflected in both the rollover current and power dissipation measurements. A 5-7% increase in the rollover current performance is achieved by using the Au-Sn metallized diamond, which can definitely be correlated to the 10%
COMPARISON OF LASER PERFORMANCE IN Au-SnlCVD-DIAMOND AND IN In/Be0
Acknowledgements
WHILE BONDED CONFIGURATION
We would like to acknowledge the support of V. D. Mattera and D. P. Wilt, the discussions with W. V. Werner and the contributions of A. Kachidurina, C. H. Lee, T. Pernell, J. Shmulovich, K. L. Tai, Y.-M. Wong and J. E. Graebner.
I-
I-
References
I-
I-
1 K. Das, V. Venkatesan, I-
,-
2 3
I-
4 5
/-
'0
I
I
I
I
20
40
60
80
CUMULATIVE
POPULATION
6 1
(%)
Fig. 7. Rollover current and power dissipation statistical performance of an InP-based laser device bonded onto Au-Sn/Pt/ Ti metallized CVD diamond submount on a Be0 testing stud, and of an InP-based laser device bonded onto an In/Pt/H metallized Be0 submount.
improvement mond.
that was achieved by using the dia-
7 8 9 10 11 12 13
Conclusions 14
In this paper we have given the first report on the successful use of a Au-Sn/Pt/Ti metallized CVD diamond submount in microelectronics thermal management application as a heat-sink submount for InP-based laser devices. A 9 Au-Sn thin multi-layer solder structure (3 pm thick) was deposited onto a Pt(200 nm)/Ti(lOO nm) barrier metal structure on top of the CVD diamond submount.
15
16 17
18
K. Miyata, D. L. Dreifus and J. T. Glass, in Y. Tzeng, M. Yoshikawa, M. Murakawa and A. Feldman (eds.), Applications of Diamond Films and Related MateriaZs, Elsevier, Amsterdam, 1991, pp. 301-308. A. T. Collins, Semicond. Sci. Technol., 4 (1989) 605. J. E. Graebner, J. A. Mucha, L. Seibles and G. W. Kammlott, personal communication. J. E. Graebner, personal communication. C. C. Lee, C. Y. Wang and G. S. Matijasevic, IEEE Trans. Comp. Hybrids, Manuf: Technol., 14 (1991) 407. J. H. Lau and D. W. Rice, Solid State TechnoZ., 28 (1985) 91. S. Knecht and L. R. Fox, IEEE Trans. Comp. Hybrids, Manuf Techno/., 9 (1986) 423. 0. Wada and T. Kumai, Jpn. J. Appl. Phys., 40 (1991) L1056. G. S. Matijasevic and C. C. Lee, Proc. IEEE Int. Reliability Physics Symp., Phoenix, AZ, 1989, p. 11. 0. Wada and 0. Ueda, Mater. Res. Sot. Symp. Proc., I81 (1990) 273. 0. Wada and T. Kumai, Appl. Phys. Lett., 58 (1991) 908. C. Y. Wang and C. C. Lee, IEEE Trans. Comp. Hybrids, Manuf: TechnoL, 14 (1991) 874. A. Katz, P. M. Thomas, S. N. G. Chu, W. C. DautremontSmith, R. G. Sobers and S. G. Napholtz, J. Appl. Phys., 67 (1990) 884. S. N. G. Chu, A. Katz, T. Boon, P. M. Thomas, W. C. Dautremont-Smith, V. G. Riggs and W. D. Johnston, Jr., J. Appl. Phys., 67 (1990) 3754. A. Katz, in A. Katz (ed.), InPand Related Materials: Processing, Technology and Devices, Artech House, Norwood, 1991, pp. 307-335. A. Katz, D. D. Bacon, C. H. Lee, K. L. Tai and Y.-M. Wong, AT&T Patent Case No. 104784 (1991). J. I?. E. Baghn and J. M. Poate, in J. M. Poate, K. N. Tu and J. M. Mayer (eds.), Thin Films: Interdiffirsion and Reactions, Wiley, New York, 1978, p. 351. M. A. Nicolet, 7% Solid Films, 52 (1978) 415.
Chemistry
and Physics,
33 (1993)
281
281-288
Ti/Pt/Au-Sn metallization scheme for bonding of InP-based laser diodes to chemical vapor deposited diamond submounts A.
Katz,
R.
R.
AT&T
K.-W.
Varma
Wang, and
Bell Laboratories,
(Received
H.
F. A.
Baiocchi,
W.
C. Dautremont-Smith,
E.
Lane,
H.
S. Luftman,
Curnan
600 Mountain
Avenue,
Murray
HiIl, NJ 07974-0636
(USA)
July 30, 1992)
Abstract A layered structure (consisting of Ti(100 nm)/Pt(200 nm)/Au(SOO nm) and AuSn (2.5 pm total in multiple alternating layers)) was studied as a bonding scheme for InP-based laser diodes to chemical vapor deposited (CVD) diamond substrates. This structure provided a molten Au-G layer of eutectic composition (80:20 wt.%) on top of the Ti/Pt adhesion and barrier metals for about 6 s in the temperature range 300-350 “C and allowed for efficient bonding of the device to the substrate. Longer heating durations allowed a reaction between the Pt and Sn to consume significant amounts of Sn from the solder, thus elevating its melting temperature and resolidifying the solder. With optimum bonding conditions, a high-quality bond of the InP-based laser diode to the CVD diamond substrate was observed, and the electrical performance of the diode was superior to that of diodes that were bonded with the standard In/EkO configurations.
Introduction
A semiconductor laser device is typically mounted on a submount that provides mechanical stability, heat dissipation and electrical connections to one or more electrical sources (d-c. and a-c. control voltages or currents, or both). A reliable device bonding scheme is essential to provide shortand long-term laser die operating reliability. The quality of the bonding medium depends on the submount material, the solder metallurgical system, the barrier metallization scheme and the bonding conditions. Submounts for laser devices are traditionally manufactured from Si or ceramic materials such as BeO. Recently, owing to its desirably higher thermal conductivity and higher electrical resistivity, diamond has emerged as a potentially attractive alternative material with which to manufacture the submounts and, in particular, to use for bonding of devices that are operated under extreme conditions [l, 21. Thus, a natural application of diamond is one that takes advantage of its potentially high thermal conductivity, which can be as high as 22 W cm-’ “C-l (at room temperature). This is about five times better than an excellent conductor such as copper [3, 41. During
0254-0584/93/$6.00
the last year the chemical vapor deposited (CVD) diamond thick film plates have become commercially available, produced by a few vendors around the world, and offered at lower cost than the single-crystal parts, which made it an attractive technology for thermal management applications in the microelectronics environment. Solder preforms, paste or deposited solder have traditionally been used to realize bonding of microelectronic devices. Deposited solder, provides significant advantages over the other techniques, such as decreased oxide formation prior to the initiation of the bonding cycle, and more accurate control of the bonding media thickness and volume [5]. The few commonly used solders are the lowermelting-point elements such as In (183 “C), medium-melting-point binary Pb-Sn alloy (223 “C) or higher-melting-point alloys such as Au-Sn (278 “C), Au-Ge (361 “C) or Au-Si (363 “C), all of which have been used in the eutectic composition. Generally, the lower-melting-point solders have a better thermal conductivity, but are mechanically weaker, compared to the higher-melting-point solders. Thus, for better thermal stability and longterm reliability, one would prefer to use the highermelting-point solders [6-91. Au-Sn solder has been widely used in optoelectronic applications owing to its high, but practical, melting temperature,
0 1993 - Elsevier
Sequoia.
All rights
reserved
282
allowing for efficient bonding of devices that contain Au-based contacts. In this paper we report the first results on the bonding of InP-based laser devices to CVD diamond submount, using an Au-Sn solder and Ti/ Pt barrier metal systems. Since the metallurgical characteristics of the Ti/Pt/Au-Sn system have been studied earlier [lo, 111, only those aspects of the metallization that are important to the bonding properties studied in this work are discussed.
Au/Au-Sn/Pt/Ti
metallurgical
It is apparent that the Au-Sn binary eutectic system (Fig. l(a)) is a reactive system with two eutectic melting points. The commonly used Au-Sn eutectic alloy with 80 wt.% gold is a eutectic mixture of the compounds Au,Sn and AuSn, with a melting point of 278 “C. The Pt-Ti system (Fig. l(b)) is also an active system at temperatures as low as 250 “C. The Ti,Pt inter-metallic phase was reported to be the predominant phase in this system formed mainly by diffusions of Ti into the original Pt volume [B-15]. This suggests that some level of Ti-Pt inter-metallic reaction is to be expected through the executed Au-Sn bonding cycle, which is realized at temperatures of 300-350 “C. At the Au-Sri solder interface with the Pt barrier layer (Fig. l(c) and (d)), a preferred reaction between Sn and Pt is to be expected. While Au and Pt are entirely inert to each other (Fig. l(c)),
system
The binary phase diagrams for the elements that constitute the Ti/Pt/Au-Sn metallurgical scheme are given in Fig. 1 and have been discussed in the literature [5, 8, 11, 121.
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283
the Sn-Pt system is extremely reactive even at temperatures lower than 100 “C, having a wide mutual miscibility range and creating about five inter-metallic phases. Thus, reaction between the Au-Sn and the Pt barrier metal is expected to proceed via the diffusion of Sn in the Au-Sn layer toward the barrier metal interface. The driving force for this phenomenon is clearly twofold: the intrinsic diffusivity of Sn through Au and the thermodynamic stability of Au-Sn versus Pt-Sn alloys. In spite of its reactive nature, the Ti/Pt/Au-Sn system is sufficiently thermally stable throughout the relevant chip bonding cycle temperature and time to allow its use in the bonding of optoelectronic devices to the appropriate submounts.
Experimental The diamond films that were used in this study were nucleated and grown on metal substrates by microwave-enhanced plasma chemical vapor deposition at 2.45 GHz. Depositions were done with a proprietary gas mixture of hydrogen, hydrocarbon and an oxygen-bearing precursor that results in optically transparent films. Except for some modifications in growth times, which led to changes in the CVD diamond plate thickness, the deposition conditions were held constant from sample to sample. Later the self-standing CVD diamond plates were disconnected chemically from the substrate. The plate surfaces were subsequently polished to mirror finish, metallized and laser diced to heat-sinks with dimensions of about 0.65 X 1.25 mm and a thickness of about 0.25 mm. These CVD diamond films had a thermal conductivity greater than 11 W cm-’ “C- ‘_ An adhesion layer of 100 nm of Ti was d-c. magnetron sputtered onto the mirror-finished CVD diamond plates, followed by deposition of 200 nm of Pt, which serves as a barrier layer. After deposition of the metal bilayer, a number of alternating layers of gold and tin were deposited. The number of Au-Sn layers varied from at least three to (preferably) nine and were capped with a layer of gold. The layers were chosen to provide an overall average composition of 80:20 wt.% Au:Sn and a total thickness of 3 pm. The structure that was found to give the best results consisted of a gold layer (0.5 pm) deposited onto the Ti/Pt barrier metals, topped with four pairs of alternating Sn(0.35 pm)/Au(0.2 pm) layers, and covered with a final layer of Au (0.5 pm) [16] (10 layers of Au and Sn for a total thickness of 3.2 pm).
InP-based laser devices were then bonded to the metallized CVD diamond submounts at thermal cycles of 320-350 “C and 5-10 s. The strength of the bond was assessed by shearing the device from the submount and analyzing the exact ruptured interface. Composition and microstructure analysis of the different metallurgical schemes were performed by a variety of means. Auger electron spectroscopy (AES) was used to obtain information on surface impurities (such as tin oxides) and, in conjuction with sputter depth profiling, to study the reaction of the Ti/Pt layer with the Au-Sn alloy after various heat treatments. Rutherford backscattering spectrometry (RBS) was used to measure the solder composition and thickness in the as-deposited and annealed layer structures. Scanning electron microscopy (SEM) was used to obtain microstructural information, both of the surface morphology of the solder pad (in plan view) and of the quality of the interfaces, obtained by cleaving the solder pad and examining the cross-section. Optical microscopy provided a quick way to screen the surface morphology of the solder pad area. Finally, electrooptical analysis of the bonded laser devices was carried out to ascertain the final laser performance.
Results
and discussion
Optimal TilPtlAu-Sn layer structure Bonding of the laser device to the diamond submount requires a solder with (1) the eutectic composition Au:Sn =80:20 wt.%, so that it melts at the correct temperature, and (2) a total thickness of 3 pm or less, to guarantee a sufficient thermal conductivity through the solder and eliminate technical problems associated with the presence of excess solder on the device. Given the unstable nature of the PtiAu-Sn metallurgical system, a number of Au-Sn solder structures with the required average composition and thickness were investigated to determine which system gave the best bonding results. Bonding is carried out in a rapid thermal anneal (RTA) station, in which the temperature is quickly ramped up to 320-350 “C and shut off after a period of 5 s. Clearly the differences in solder layer structure will affect the kinetics of the melting and bonding process, so that over the short time scale of the bonding process, different layer structures will lead to different results. For longer melting times (greater than 15 s), thermodynamic considerations become important. Then, no matter what the initial as-deposited structure may be, the behavior of the melt over time will depend on the
284
initial Au, Sn and Pt average composition, since these are the main interacting species. Presumably, the Pt barrier layer will consume an appropriate amount of Sn from the Au-Sn solder to form Pt-Sn species. This has the detrimental effect of destroying the barrier nature of the Pt layer and altering the Au-Sn composition so that the melting behavior is adversely affected. The optimal metallization structure should allow a significant portion of the Pt barrier to remain intact during the RTA bonding step, with the Au-Sn solder remaining molten long enough to bond to the laser device. Note that the optimal metallization structure depends in a critical way on the time scale of the bonding process. The shelf life of unbonded parts and the longterm reliability of the bond are issues that still need to be addressed, since the Pt-Sn interaction occurs over long time periods even at very low temperatures. This could change the bonding behavior of parts that have been waiting for a long time to be processed, or degrade the integrity of the bond after it has been formed. The effect of layer structure on bonding properties is summarized using the results obtained from two samples. In the first sample (referred to hereafter as sample I), a simple three-layered 17.0, (a,)
AS DEPOSITED ---
SIMULATECJ MEASUREMENT
Au(0.8 pm)/Sn(l.l4 pm)/Au(0.8 pm) structurewas used on top of the Pt/Ti barrier metals. This represents a simple processing sequence, with only two metal interfaces within the solder. The second sample (referred to hereafter as sample II) contains nine Au/Sri alternating layers and thus requires a relatively complicated processing sequence. Both samples were prepared to have the same overall thickness and Au:Sn ratio. Both samples also have at least 0.5 pm of gold directly on top of the Pt layer - an attempt to minimize the Sn-Pt interreaction. Each as-deposited wafer was cut into several pieces for temperature processing. Thus, for each sample, three annealing conditions were available for analysis: as-deposited; 320 “C, 5 s; and 320 “C, 10 s. RBS data on the two samples were used to determine the solder composition and thickness. The data for the as-deposited sample and 320 “C, 5 s annealed sample are summarized in Fig. 2. Interpretation of the data from the as-deposited samples shows that the overall thickness and Au:Sn composition of samples I and II are quite similar. The total thickness in both cases is quite close to 2.7 pm and the average Au:Sn ratio is close to 80:20 wt.%. Differences in the Au:Sn ratio with depth are observed between the two as-deposited samples, and this is expected, owing to the different
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methods of preparation. For both as-deposited samples, the RBS data show that no strictly pure Sn layers are left. The general shapes of the spectra show that there is a predominantly pure gold layer at the bottom of the solder next to the Pt and another on top of the solder. In the middle, the Sn intermixes readily with the gold to form an approximately 5050 at.% alloy. The middle layer composition differs between samples I and II according to the method of preparation. In general, the picture that emerges is that, during deposition, the first gold layer goes down on top of Pt and forms a sharp interface. The layer deposited next, which is Sn, is known to be fairly porous. When the next Au layer is deposited it mixes readily with the Sn to form an Au--& alloy. The process continues in this manner until the final gold deposition, which is a fairly thick layer. Some portion of that may react with the previously deposited Sn layer, leaving a portion of mainly pure Au on top of the structure. After annealing, the RBS spectra of samples I and II are qualitatively different. Note that the RBS beam spot size used here is =250 pm, which covers most of the solder pad. Any lateral variations of thickness and composition will be averaged in the final spectrum. The data from sample 11 (Fig. 2(b,)) show a final solder composition of 8O:ZO wt.% and a fairly uniform thickness of solder over the pad. On the other hand, the data from sample I (Fig. 2(a2)) show a similar solder composition but a greatly smeared low-energy edge. This indicates very nonuniform solder thickness across the pad, and hence a nonuniform melting behavior across different areas of the pad. Figure 3 shows four SEM cross-sectional micrographs of as-deposited and 320 “C, 5 s annealed pieces from samples I and II. The microstructural information obtained from the SEM photos shows clearly the differences between samples I and II and the reason for the different melting behavior. The cross-sectional micrograph from sample I, which was the simple structure, shows the individual Ti, Pt and Au-Sn solder layers quite clearly. Note that within the solder layer the original Au/Sri// Au interfaces are separated in certain regions. From the RBS data we know that, on average, the middle portion consists of 5O:SO at.% Au-Sn with pure Au layers on top and bottom. Perhaps because of the greater strain present in the much thicker layers used in sample I, or because of a volume change in the layer during formation of the Au-Sn alloy, empty spaces and gaps occur in several places within the solder layer of sample I. This is in contrast to the more complex layering
Fig. 3. SEM cross-sectional micrographs of a metallized Au(0.8 ~m)/Sn(l.4 pm)/Au(0.8 pm)/Pt(0.2 wm)rTi(O.l pm) submount [(a) as-deposited and (b) after a bonding cycle at 320 “C for 5 s] and of a metalked Au(O.5 pm)/{3 x )/Sn(0.35 ~m)/Au(O.2 ~m)/Au(O.S ~m)/Pt(O.Z pm)DX(O.l Mm) submount i(c) as-deposited and (d) after a bonding cycle at 320 “C for S sf.
used in sample II. In that case, the SEM micrographs show closely packed layers of Au:Sn with no gaps or separations at any of the interfaces. On the basis of this information, the difference in melting behavior of the two samples is understood. For sample II, the uniform layering of Au-Sn leads to uniform melting of the solder, which during the 5 s RTA forms a final layer of 80:20 wt.% solder that has uniform thickness throughout the area of the pad. On the other hand, in sample I, the observed gaps in the solder mean that during a short 5 s anneal, there is not enough time for uniform mixing of the Au:Sn solder, and this Ieaves areas that are very gold rich and areas that are very tin rich. Since the melting is nonuniform across the pad, areas of different thickness are formed, with greatly differing local compositions. This nonuniformity in melting for sample I is most easily observed in the plan-view SEM photos shown in Fig. 4. The dendritic structures found after RTA of sample I are thick regions that were shown to be gold rich by EDS analysis in the SEM. Notable differences in the feature size are observed depending on the melt duration, as dem-
286
Fig. 4. SEM plan-view micrographs of a metallized bonding cycle-at 320 “C for
Au(0.8 r.Lm)/Sn(l.4 pm)/Au(0.8
onstrated by a comparison of the 5 s and 10 s melted samples in Fig. 4. Resolidification of these localized features was observed for this sample already after 2-3 s of melting at temperatures above the eutectic melting point, and is consistent with the gold-rich composition of these regions measured in the SEM. By contrast, the plan-view photo of sample II after RTA shows a uniform surface morphology across the pad. Thus, a multiple layered structure consisting of thinner but more Au/Sri layers is superior to a simple layered structure consisting of very thick layers, with respect to melting behavior during RTA. The main reason is that a higher degree of strain and/or volume change occurs in the simple structure, which produces gaps in the integrity of the solder layer. During RTA, these gaps interfere with uniform melting behavior and create localized areas of Au-rich solder, which freeze prematurely. During a 5 s anneal, interaction of Sn with the Pt layer does not adversely affect the bonding process, especially if the as-deposited structure has 0.5 pm or greater of pure gold directly in contact with the Pt. AES spectra of specially deposited test structures were obtained to determine how much of the Pt barrier layer would be consumed during the typical RTA times of 5-10 s (Fig. 5). Basically, they show that, for short time anneals, Sn does tend to segregate toward the Pt interface, presumably forming Pt-Sn compounds, but that a significant amount of the original Pt layer is left intact during the bonding process. This does not mean that the Pt-Sn interaction is unimportant. As mentioned above, understanding
Fm)/Pt(0.2
~m)/Ti(O.l
I
,
-Ti
1
I
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after a
-
_
Fig. 5. Auger spectra for test structure with Au(0.2 pm)/Sn(0.25 pm)/Au(O.l pm) deposited on Ti(O.1 pm)/Pt(0.2 pm) barrier metals: (a) as-deposited; (b) 320 “C, 5 s anneal.
of this interaction will be very important for establishing the long-term shelf life of the submounts and the long-term reliability of the bond once it is formed. Bonding per$ormance Based on the above information, we have established the multilayer solder structure as our commonly used solder [16] for the purpose of this report, for bonding InP-based laser devices to
287
Fig. 6. Metallized CVD diamond heat-sink submount (a) as processed, and (b) after bonding an InP-based laser device onto it and shearing it off (note the device front side metal/dielectric film, which was sheared off the device and left bonded to the submount).
CVD diamond submounts. Figure 6 presents optical micrographs of the metallized CVD diamond submount (Fig. 6(a)) and of the same submount after the InP-based laser device was bonded onto it and sheared off (Fig. 6(b)). The center metal geometry is the Au-Sn multilayered solder bonding pad, which is deposited onto the barrier Pt-Ti larger-area metallization. The areas on the righthand side of this submount are used as a ribbon bonding pad to bring the laser device into contact with the submount and the submount into contact with the laser package. The InP-based laser device was bonded to the CVD diamond submount using a thermal cycle of 320 “C for about 5 s. The device was subsequently sheared off the submount at a force of about 500 g. Figure 6(b) shows the almost complete dielectric and metal pattern of the InPbased laser device. One can conclude that the wetting quality of the device to the CVD diamond submount is excellent, leading to adhesion of almost 100% of the area of the device to the submount. Thus, as a result of the shearing, the adhesion is lost in the semiconductor-metal interface contained in the laser structure, rather than in the device metal/dielectric-solder interface, leaving the bonded metal/dielectric film bonded to the submount, as is observed in the micrograph. Electrical performance The quality of the bonding is evaluated by measuring the electrical and optical performance of the bonded laser devices. Thermal impedance is an excellent parameter to use for judging the bonding quality of the laser diode and comparing it with a device bonded with the standard configuration of indium solder onto the Be0 submount. The previously used configuration has a typical
thermal impedance of about 15 K W-‘. Measuring the thermal impedance, however, is time consuming and the data are often influenced by various device parameters such as device resistance and leakage, which make the results ambiguous. Thus a new method was applied in this study to characterize the nature of the bonding. The standard method of bonding InP laser devices is to use In solder on a Be0 substrate. The influence of this bonding on the performance of the bonded device has been widely studied and is used as a reference line to the influence of the Au-Sn/CVD diamond bond on the device performance. The one property that reflects the quality of the bonding is the rollover current measured at the light-current graph. This parameter is influenced by the device resistance, leakage and external quantum efficiency. Thus, a comparison of the rollover values of similar devices that were bonded in two different modifications, such as In/Be0 and Au-Sn/CVD diamond, indicates which one is of superior quality. Figure 7 shows the full statistical optical and electrical performance distribution of one of the lots of the InP-based laser device bonded onto Au-Sn/Pt/Ti metallized CVD diamond on top of a Be0 testing stud, compared to InP-based lasers taken from the same wafer that were bonded using In solder onto a Be0 submount. The superior performance of the device that is bonded to the CVD diamond is reflected in both the rollover current and power dissipation measurements. A 5-7% increase in the rollover current performance is achieved by using the Au-Sn metallized diamond, which can definitely be correlated to the 10%
COMPARISON OF LASER PERFORMANCE IN Au-SnlCVD-DIAMOND AND IN In/Be0
Acknowledgements
WHILE BONDED CONFIGURATION
We would like to acknowledge the support of V. D. Mattera and D. P. Wilt, the discussions with W. V. Werner and the contributions of A. Kachidurina, C. H. Lee, T. Pernell, J. Shmulovich, K. L. Tai, Y.-M. Wong and J. E. Graebner.
I-
I-
References
I-
I-
1 K. Das, V. Venkatesan, I-
,-
2 3
I-
4 5
/-
'0
I
I
I
I
20
40
60
80
CUMULATIVE
POPULATION
6 1
(%)
Fig. 7. Rollover current and power dissipation statistical performance of an InP-based laser device bonded onto Au-Sn/Pt/ Ti metallized CVD diamond submount on a Be0 testing stud, and of an InP-based laser device bonded onto an In/Pt/H metallized Be0 submount.
improvement mond.
that was achieved by using the dia-
7 8 9 10 11 12 13
Conclusions 14
In this paper we have given the first report on the successful use of a Au-Sn/Pt/Ti metallized CVD diamond submount in microelectronics thermal management application as a heat-sink submount for InP-based laser devices. A 9 Au-Sn thin multi-layer solder structure (3 pm thick) was deposited onto a Pt(200 nm)/Ti(lOO nm) barrier metal structure on top of the CVD diamond submount.
15
16 17
18
K. Miyata, D. L. Dreifus and J. T. Glass, in Y. Tzeng, M. Yoshikawa, M. Murakawa and A. Feldman (eds.), Applications of Diamond Films and Related MateriaZs, Elsevier, Amsterdam, 1991, pp. 301-308. A. T. Collins, Semicond. Sci. Technol., 4 (1989) 605. J. E. Graebner, J. A. Mucha, L. Seibles and G. W. Kammlott, personal communication. J. E. Graebner, personal communication. C. C. Lee, C. Y. Wang and G. S. Matijasevic, IEEE Trans. Comp. Hybrids, Manuf: Technol., 14 (1991) 407. J. H. Lau and D. W. Rice, Solid State TechnoZ., 28 (1985) 91. S. Knecht and L. R. Fox, IEEE Trans. Comp. Hybrids, Manuf Techno/., 9 (1986) 423. 0. Wada and T. Kumai, Jpn. J. Appl. Phys., 40 (1991) L1056. G. S. Matijasevic and C. C. Lee, Proc. IEEE Int. Reliability Physics Symp., Phoenix, AZ, 1989, p. 11. 0. Wada and 0. Ueda, Mater. Res. Sot. Symp. Proc., I81 (1990) 273. 0. Wada and T. Kumai, Appl. Phys. Lett., 58 (1991) 908. C. Y. Wang and C. C. Lee, IEEE Trans. Comp. Hybrids, Manuf: TechnoL, 14 (1991) 874. A. Katz, P. M. Thomas, S. N. G. Chu, W. C. DautremontSmith, R. G. Sobers and S. G. Napholtz, J. Appl. Phys., 67 (1990) 884. S. N. G. Chu, A. Katz, T. Boon, P. M. Thomas, W. C. Dautremont-Smith, V. G. Riggs and W. D. Johnston, Jr., J. Appl. Phys., 67 (1990) 3754. A. Katz, in A. Katz (ed.), InPand Related Materials: Processing, Technology and Devices, Artech House, Norwood, 1991, pp. 307-335. A. Katz, D. D. Bacon, C. H. Lee, K. L. Tai and Y.-M. Wong, AT&T Patent Case No. 104784 (1991). J. I?. E. Baghn and J. M. Poate, in J. M. Poate, K. N. Tu and J. M. Mayer (eds.), Thin Films: Interdiffirsion and Reactions, Wiley, New York, 1978, p. 351. M. A. Nicolet, 7% Solid Films, 52 (1978) 415.