IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001 1033
Backside Copper Metallization of GaAs MESFETs
Using TaN as the Diffusion Barrier
Chang-You Chen, Edward Yi Chang, Member, IEEE, Li Chang, and Szu-Hung Chen
Abstract—Backside copper metallization of GaAs MESFETs using TaN as the diffusion barrier was studied. A thin TaN layer of 40 nm was sputtered on the GaAs substrate before copper film metallization. As judged from the data of X-ray diffraction (XRD), Auger electron spectroscopy (AES), and cross-sectional transmis-sion electron microscopy (TEM), the Cu/TaN films with GaAs were very stable without interfacial interaction up to 550 C annealing. The copper metallized MESFETs were thermally stressed at 300 C. The devices showed very little change in the device character-istics( 3%) after thermal stress, and the changes of the electrical parameters and RF characteristics of the devices after thermal stress were of the same order as those devices without Cu metal-lization. These results show that TaN is a good diffusion barrier for Cu in GaAs devices and the Cu/TaN films can be used for the backside copper metallization of GaAs MESFETs.
Index Terms—Copper, GaAs MESFET, metallization, TaN.
I. INTRODUCTION
C
OPPER metallization has become a hot topic in silicon IC technology ever since IBM announced its success in silicon VLSI processes [1]–[3]. Now, copper metallization is widely used in 0.18- m VLSI technology. The advantages of copper metallization for Si VLSI include lower resistivity and higher electromigration resistance. Even though the use of copper as a metallization metal has become very popular in the Si industry, the use of copper as a metallization metal for GaAs FETs has not been reported yet. Traditionally, GaAs FETs and MMICs use Au as the metal for transmission lines and ground plane metallization. The gold used in transmission lines and ground planes are usually plated to more than 2 m. The use of copper as the metallization metal for transmission lines and ground plane metallization has the following advantages over gold: lower resistivity, higher thermal conductivity, lower cost, and better mechanical properties if plated thicker. Low thermal conductivity and fragile substrates have always been problems in GaAs devices, especially in GaAs power FETs which are required to dissipate a lot of heat. To provide a good thermal sink, the wafer of the power GaAs FETs is usually thinned to 2–5 mils thick, which makes the substrate very fragile. Therefore, the use of a thicker copper layer forManuscript received July 21, 2000; revised October 31, 2000. This work was supported jointly by the Ministry of Education and the National Science Council, R.O.C., under Contract 89-E-FA06-2-4. The review of this paper was arranged by Editor M. F. Chang.
C. Y. Chen, E. Y. Chang, and S. H. Chen are with the Department of Materials Science and Engineering and the Microelectronics and Information Systems Re-search Center, National Chiao-Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]).
L. Chang is with the Department of Materials Science and Engineering, Na-tional Chiao-Tung University, Hsinchu, 300 Taiwan, R.O.C.
Publisher Item Identifier S 0018-9383(01)03235-X.
backside metallization of GaAs FETs and MMICs to provide better mechanical strength and heat sink capabilities is a very attractive concept.
Copper diffuses very fast into Si when it is in contact with the substrate without any diffusion barrier [4]–[6]. Just like in the silicon case, copper also diffuses very fast into GaAs when deposited on the substrate without any diffusion barrier [7]. Since copper is a deep acceptor for GaAs, this causes degradation of electrical properties in GaAs devices. TaN is currently an effective diffusion barrier for Cu metallization in Si technology, and it also has good adhesion to GsAs. Therefore, using TaN as diffusion barrier should ensure the success of Cu metallization of GaAs. In this paper, the thermal stability of a Cu/TaN/GaAs film structure is investigated. Also, the electrical performances of the Cu/TaN metallized GaAs MESFETs are reported for the first time.
II. DEVICEFABRICATION ANDMETALLIZATION
A blanket film structure of Cu/TaN/GaAs was evaluated first. After that, MESFETs with TaN and Cu as backside metallized layers were fabricated for evaluation of electrical performance. The GaAs substrate was cleaned with boiling acetone and iso-propyl alcohol for 5 min each and dipped in HF:H O :H O (1:2:20) for 20 s and HCl:H O (1:4) for 1 min before it was loaded into the vacuum system. A TaN film of 40 nm thickness was sputtered onto the 3-in (100) GaAs substrate, then 100-nm Cu and 10-nm TaN films were subsequently sputtered on top of the TaN film without breaking vacuum in a multitarget magnetron sputtering system. The top layer of TaN served as a protective layer to prevent oxidation and penetration of oxygen into the films during higher temperature annealing. Both TaN films were deposited by reactive sputtering of Ta in the N /Ar mixture with 20% N and 80% Ar. The base pressure was Pa before sputtering, and the total sputtering gas pressure was 0.8 Pa during deposition of the films. The samples were annealed for 30 min at temperatures ranging from 400 C to 600 C in an argon ambient for materials analysis. X-ray diffraction (XRD), Auger electron spectroscopy (AES), and cross-sectional transmission electron microscopy (TEM) were used for phase identification and study of the interfacial reactions. Both 150 m and 1 mm gate width MESFETs used in this study were typical epitaxial-material based MESFETs. The process steps included mesa isolation, Au/Ge/Ni ohmic metallization, recessed gate with Au/Pt/Ti metals, plasma enhanced chemical vapor deposition (PECVD) silicon nitride passivation and gold plated air bridges. The GaAs wafers were mechanically thinned to only 75 m before the backside copper metallization, consisting of 40 nm TaN and 10 m Cu. The struc-ture of the MESFETs with copper backside metallization is shown in Fig. 1, the superlattice in the structure is the superlattice buffer.
1034 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001
Fig. 1. Structure of the GaAs power MESFET with backside Cu metallization.
Fig. 2. XRD pattern of the TaN/Cu/TaN/GaAs samples after annealing at various temperatures.
The MESFETs with copper backside metallization were annealed at 300 C for 2 h under vacuum for thermal stress. The electrical parameterswerecomparedtothedeviceswithoutcopperbackside metallization (bare chip without backside metal). Devices with and without copper backside metallization were put through the same thermal stress cycle.
III. THERMALSTABILITY OFMULTILAYERS
Fig. 2 shows the XRD results of the samples as-deposited and after annealed from 400 C to 600 C. From the XRD data, it is clear that the peaks of TaN and Cu remain unchanged until 550 C, indicating that the Cu/TaN/GaAs structure is still quite stable at 550 C. After 600 C annealing, the intensity of Cu decreased, and new phases of TaAs and Cu Ga were identified, suggesting that reactions between the substrate and the metallization layers have taken place at 600 C.Additional evidence hasbeen obtained from AESdepthprofiles. Fig.3showstheAESdepthprofilesofthemet-allized samples as-deposited and after annealing. As can be seen from this figure, the distribution of the elements of the deposited films did not change after 550 C annealing, however after 600 C annealing, the results show that copper has diffused into the GaAs substrate. Fig. 4 shows the cross-sectional TEM micrograph of
(a)
(b)
(c)
Fig. 3. AES depth profiles of the TaN/Cu/TaN/GaAs samples (a) as deposited; (b) after 550 C annealing; and (c) after 600 C annealing.
the TaN/Cu/TaN/GaAs structure. After annealing at 550 C, grain growth of the Cu occurred, and the interfaces between Cu, TaN, and GaAs were still quite sharp, we found no intermixing of Cu and TaN barrier layers with the GaAs substrate. From the XRD, AES, and TEM data shown above, there is no interfacial reaction between Cu/TaN/GaAs up to 550 C.
IV. DEVICEELECTRICALCHARACTERIZATIONS
The results from the materials study in the blanket GaAs wafer mentioned above suggest that diffusion of copper into GaAs can be severely retarded to an extremely small level below 550 C with TaN as the diffusion barrier. To study the electrical perfor-mance of the device after copper metallization and the thermal stability of the metallized devices after thermal stress, MESFETs with and without copper metallization were annealed at 300 C for 2 h and tested for the electrical performance. The electrical
CHEN et al.: BACKSIDE COPPER METALLIZATION OF GaAs MESFETs 1035
Fig. 4. Cross-sectional TEM micrograph of the TaN/Cu/TaN/GaAs sample after 550 C annealing.
TABLE I
AVERAGEELECTRICALPARAMETERCHANGES OF THE150-m DEVICE
(a) WITHCu METALLIZATION. (b) WITHOUTCu METALLIZATION. (TESTING
CONDITIONS:I ISMEASURED ATV = 2 V; G ISMEASURED AT
V = 0 V, V = 2 V; V ISMEASUREDWHENI = 150 A)
parameter changes of the 150- m devices before and after an-nealing are listed in Table I. As can be seen from Table I(a), for the Cu metallized device, the drain to source saturation current change is around 1.60%, the transconductance change is around 0.73%, and the pinchoff voltage change is around 1.35% after thermal stress. The changes in the parameters are of the same order as the devices without copper metallization as compared with the data shown in Table I(b).
There was no appreciable piezoelectrical related pinchoff voltage shift for the devices after copper metallization, which means that there is little stress effect for the copper film. Fig. 5 exhibits the changes in the curves before and after annealing for the devices with and without copper metallization. The pin-choff voltage difference of these two devices shown is due to the nonuniformity of the wet chemical etch in the gate recess process. As can be seen from this figure, for devices with or without copper metallization, the source resistance of both devices increases after annealing. The increase of source resistance after annealing is due to the aging effect of the ohmic metals and not related to the copper metallization. Fig. 6 shows the copper metallized device and
Fig. 5. Changes in theI0V curves for the 150-m device (a) for device with Cu metallization and (b) for device without Cu metallization.
Fig. 6. Changes in the transconductance(G ) and drain current (I ) versus gate bias voltage(V ) curves for the 150-m device with Cu metallization.
drain to source current versus gate bias voltage curves before and after 300 C/2 h annealing. The changes of and are very small for the copper metallized device, which implies that there is no copper diffusion into the GaAs substrate, consistent with the material analysis results. The RF characteristics of the devices with and without Cu metallization were also listed. As shown in Table II(a), for m mm device, the changes of (maximum oscillation frequency), (maximum power gain), and (unilateral power gain) after 300 C/2 h annealing are 0.34 GHz, 0.38 dB, and 0.69 dB, respectively, for devices with Cu metallization. For devices without Cu metallization, the changes are of the same order as shown in Table II(b). The deterioration of the properties after annealing is mainly caused by the thermal effect on the properties of the intrinsic device itself. Therefore, it
1036 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001
TABLE II
AVERAGERF PARAMETERCHANGES FOR THE1 m 2 1 mm DEVICE
(a)WITHCu METALLIZATION(b) WITHOUTCu METALLIZATION. (TESTING
CONDITIONS:V = 7 V; I = 100 mA)
clearly shows that copper backside metallization has a negligible effect on the GaAs device performance. The measured value of sheetresistanceforthe40-nmTaNbarrierand10- mthickCufilm on GaAs MESFETs is 1.9 m , and the corresponding values of electrical conductivity and thermal conductivity are estimated to be ( cm) and 3.47 W/cm K, respectively, at room temperature. It is noted that bulk copper has a electrical conductivity of ( cm) and a thermal conductivity of 4.01 W/cm K, whereas gold has a electrical conductivity of ( cm) andathermalconductivityof3.17W/cm K [8].
V. CONCLUSION
Backside copper metallization with TaN as the diffusion bar-rier layer was successfully applied to GaAs MESFETs. The Cu/TaN metallization layers on GaAs substrate were very stable up to 550 C. After 600 C annealing, however, the interfacial mixing of TaN and Cu with the GaAs substrate occurred, re-sulting in the formation of TaAs and Cu Ga phases. MESFETs metallized using these Cu/TaN layers showed small changes in the electrical parameters after 2 h annealing at 300 C. The changes of electrical parameters and RF characteristics of the
devices including and were
within 3%. The magnitude of the changes in the device param-eters is of the same order as those devices without copper met-allization. The results show that Cu/TaN layers are quite stable and can be of practical use for backside GaAs MESFETs and MMICs metallization.
REFERENCES
[1] K. Holloway and P. M. Fryer, “Tantalum as a diffusion barrier between copper and silicon,” Appl. Phys. Lett., vol. 57, no. 17, pp. 1736–1738, Oct. 22, 1990.
[2] D. Gupta, “Diffusion in several materials relevant to Cu interconnection technology,” Mater. Chem. Phys., vol. 41, pp. 199–205, 1995.
[3] D. J. Kim and Y. T. Kim, “Nanostructured Ta-Si-N diffusion barriers for Cu metallization,” J. Appl. Phys., vol. 82, no. 10, pp. 4847–4851, Nov. 15, 1997.
[4] E. R. Weber, “Transition metals in silicon,” Appl. Phys. A, Solids Surf., vol. 1, pp. 1–22, 1983.
[5] A. Cros, M. O. Aboelfofotoh, and K. N. Tu, “Formation, oxidation, elec-tronic, and electrical properties of copper silicides,” J. Appl. Phys., vol. 67, no. 7, pp. 3328–3336, Apr. 1, 1990.
[6] C. A. Chang, “Formation of copper silicides from Cu(100)/Si(100) and Cu(111)/Si(111) structures,” J. Appl. Phys., vol. 67, no. 1, pp. 556–569, Jan. 1, 1990.
[7] P. H. Wohlbier, Diffusion and Defect Data. Zürich, Switzerland: Trans Tech, 1975, vol. 10, pp. 89–91.
[8] C. Kittel, Introduction to Solid State Physics, 7th ed. New York: Wiley, 1996, p. 126, 160.
Chang-You Chen was born in Taiwan, R.O.C.
He received the B.S. degree in materials science and engineering in 1977 from National Tsing Hua University, Hsinchu, Taiwan, and the M.S. degree in materials science and engineering in 1995 from National Chiao-Tung University, Hsinchu, where he is currently pursuing the Ph.D. degree in materials science and engineering.
He is currently with the Department of Materials Science and Engineering, National Hu Wei Institute of Technology, Yun-Lin, Taiwan. His research interest is copper metallization of GaAs devices.
Edward Yi Chang (M’84) received the B.S. degree
in materials science and engineering from National Tsing Hua University, Taiwan, R.O.C., and the Ph.D. degree in materials science and engineering from the University of Minneapolis, MN, in 1977 and 1985, respectively.
From 1985 to 1988, he was with Unisys Cor-poration GaAs Component Group, Eagan, MN, and from 1988 to 1992, he was with Comsat Labs Microelectronic Group. He worked on the GaAs MMIC programs in both groups. In 1992, he joined National Chiao-Tung University, Hsinchu, Taiwan. In 1994, he helped set up the first GaAs MMIC production line in Taiwan, and became president of Hex-awave Inc., Hsinchu, in 1995. He returned to the teaching position in National Chiao-Tung University in 1999 and is currently an Associate Professor in the Department of Materials Science. His research interests include new device and process technologies for GaAs MMICs for wireless communication.
Dr. Chang is a member of the IEEE Electron Devices Society.
Li Chang received the B.S. and M.S. degrees in
ma-terials science and engineering, from National Tsing Hua University, Hsinchu, Taiwan, R.O.C., in 1977 and 1981, respectively, and the D.Phil. degree in ma-terials from the University of Oxford, Oxford, U.K., in 1986.
He is currently with the Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu. His interest is in the area of thin-film processing and microstructural character-ization.
Szu-Hung Chen was born in Taiwan, R.O.C. He
received the B.S. degree in materials science and engineering in 1997 from Tsing Hua University, Hsinchu, Taiwan. He is currently pursuing the Ph.D. degree in materials science and engineering at Chiao-Tung University, Hsinchu. His research interests are new materials and process technology for GaAs microwave devices.
