Program Organizers: Jagdish Narayan, Dept of Matls Sci & Engrg, Box 7916, NC State University, Raleigh, NC 27695; John Sanchez, Advanced Micro Devices, M3 160, PO Box 3453, Sunnyvale, CA 94088
Tuesday, AM Room: Orange County 5
February 6, 1996 Location: Anaheim Marriott Hotel
Session Co-chairs: J.M. Woodall, Purdue University, West Lafayette, IN 47907; M.O. Aboelfotoh, North Carolina State University, Raleigh, NC 27695
8:30 am Invited
PROPERTIES OF OHMIC CONTACTS AND SCHOTTKY BARRIERS ON GaN: H. Morkoc, University of Illinois at Urbana- Champaign, IL 61810
GaN and its alloys have gained considerable interest recently owing to the introduction of blue LEDs and potential applications to coherent sources and high power devices. Having experienced a tremendous improvement in the quality of the material paves the way for an examination of metal contacts in order to harness the device potential. Thus, research into ohmic and Schottky metals have already begun and rapid progress ensued. With Ti/Al/Ni/Au annealed at 900deg.C for 30 sec, specific resistivities in the high 8.9 x10- 8 cm2 for a doping level of 4 x 10l7 cm-3have already been obtained. On the Schottky barrier front, Pt/Au has been found to lead to low ideality factors and low leakage currents approaching nearly ideal behavior. The electrical and structural properties of metal/GaN structures will be discussed.
9:00 am Invited
OHMIC CONTACTS TO WIDE BANDGAP SEMICONDUCTORS HgSe AND Au-Pt-ZnTe/ZnSeMQW'S ON p-ZnSe AND Au ON GaN: P.H. Holloway, J. Fijol, J.Trexler, S.Miller, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611- 6400
Achieving ohmic electrical contacts to p- type semiconductors is non- trivial as the bandgap becomes large, because the top of the valence band, Ev, normally is far below the vacuum reference energy level. Even for semiconductors such as ZnSe and GaN with unpinned Fermi energy levels, metals with large work functions are required since Ev is so negative. As a result, we have studied an ex situ method of forming the large electron affinity semi- metal HgSe and its use for ohrnic contacts to p- ZnSe. This approach still results in Schottky barriers of 0.55eV at RT. Ohmic contacts may be achieved using MQW's of p- ZnTe/ZnSe, but these degrade with time while transporting currents at densities of 30A/cm2. The characteristics and mechanisms of this degradation will be illustrated and discussed. Finally, the lessons learned from ZnSe will be discussed relative to the search for ohmic contacts to wide bandgap GaN. Specifically, the ability to form ohmic contacts using Au- based metallizations will be illustrated and discussed. This work supported by ONR Grant N00 l4- 92- J- l895.
INFLUENCE OF PLATING PARAMETERS ON GROWTH MORPHOLOGY AND GOLD BUMP SHAPE DEPOSITED ON GaAs DEVICES: L- P. Lai, D. Ramirez, P. Goolsby, Motorola, Inc., MD EL609, 2100 E.Elliot Rd., Tempe, AZ 85284
The effect of pulsed- plating parameters such as voltage, frequency and duty cycle on the surface morphology and shape of plated gold bumps was investigated. The study was performed on the electroplating process to produce rectangular shaped micro- gold bumps deposited on GaAs devices to form interconnections and flip chip bond sites. An internally developed plating system was used which provides precise control of the charge and discharge characteristics of the electrochemical diffusion layer while maintaining a constant current density independent of varying plating area. A set of plating parameters was selected with high frequency (>3000 Hz) and low duty cycle (10%) while maintaining a large difference of "on" and "off" voltages. This resulted in uniform deposition across a wafer which produces very flat bumps (less than 1.5 um planarity) with smooth grain structure and soft gold to maintain adequate assembly yields. The process diminishes the effect of changing chemistry during a plating solution's effective lifetime and flow dynamics upon deposition morphology and rate, and greatly enlarges the process window.
10:00 am BREAK
10:30 am Invited
SPREADING OF CONTACTS: ORIGIN AND RAMIFICATIONS: S. Mahajan,
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213
State- of- the- art semiconducting devices impose stringent demands on contacts in terms of their dimensional and chemical stabilities. We have evaluated the dimensional stability of circular gold contacts deposited on InP surfaces of different orientations during annealing in the temperature range of 450- 600deg.C. Results indicate that the contacts spread during annealing and assume different shapes. The observed shapes reflect the crystallography of the underlying InP surface. Similar results have been obtained on the spreading of aluminum contacts on a (111) silicon surface. Arguments will be developed to rationalize the preceding observations. Ramifications of the contact spreading to the device technology will also be discussed.
11:00 am Invited
AN INVESTIGATION OF SnPbAg SOLDER FOR DIE ATTACH OF GaAs DEVICES: J.M. Parsey, Jr., W. Cronin, L- P. Lai, P. Mobley, B. Scrivner, S. Valocchi, L. Sue,, Motorola, Tempe, AZ 85284; J. Mohr, K. Monarch, B. Keller, Motorola MCL, Mesa, AZ 85202
The interactions of Sn62Pb36Au2 solder with NiV-Au and TiPtAu backmetal
systems were investigated. GaAs die were soldered to copper slugs (plated with
Ni (barrier) and Au). Die attach and reflow cycles were carried out in the
range of 190-250C. Optical microscopy, SEM cross-sections, EDS and X-ray
transmission were used for assessing the evolution of the microstructure
through die attach and subsequent reflow operations. Five alloy phases were
identified in the solder matrix. The Au-Sn phases produces a "girder like"
needle microstructure, and strongly influenced the void formation and
coalescence. There was no discernible difference in the microstructures for
NiV-Au or TiPlAu backmetals. Fine structure and dispersed voids found at die
attach were replaced by coarse structure and large voids after reflow. The void
volumes were in agreement with liquid-solid phase transitions. Ni was found to
provide robust barrier to diffusion and interactions at the die and slug
interfaces. We also identified the critical role of the thermal conductivities
of the slug and die on the solidification of the solder and void formation, and
deduced that the reflow process results in only a partial remelting of the
solder due to the high-melting point Au-Sn microstructure.
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