GaAs Circuit Restructuring by Multi-Level Laser-Direct-Written Tungsten Process
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GaAs CIRCUIT RESTRUCTURING BY MULTI-LEVEL LASER-DIRECT-WRITTEN TUNGSTEN PROCESS JERRY G. BLACK, SCOTT P. DORAN, MORDECHAI ROTHSCHILD, JAN H.C. SEDLACEK, and DANIEL J. EHRLICH Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173 ABSTRACT Laser-direct-writing processes are employed to fabricate a GaAs digital integrated circuit. The lithography-free techniques deposit and etch conductors and resistors, and remove insulating layers, thus enabling multilevel interconnections. These combined directwrite processes provide the flexibility of clip-lead prototyping on a micrometer scale. INTRODUCTION An important application of laser-direct-write deposition is in interconnecting microelectronic devices, and specifically in the case of complex integrated circuits, in discretionary repair and restructuring for enhanced yield and functionality [1-3]. When combined with an interlayer dielectric, further metallization is formed above the original circuit, with electrical contact made only where vias are opened in the insulator by a laserdirect-write etch or ablation process. We have achieved generalized restructuring of GaAs direct-coupled digital logic circuits by combining direct write metal with discretionary via formation. Multiple-layered intarconnection networks are readily prepared by this technique. In addition, pull-up resistors of precisely prescribed values were directly written by laser deposition of polysilicon of the appropriate doping level. In this work functional digital latching circuits were created from pairs of enhancement-mode GaAs transistors. The laserwriting processes will be discussed in the order in which they were performed. The circuit and the processes required to construct it are schematically shown in Figure 1. The original sample is part of a GaAs test chip which includes enhancement-mode MESFETs of various dimensions. For these experiments a region was selected where two such MESFETs are located 250 pm apart, with bare substrate in the space between them. The two devices have
channel widths of 80 gm and 25 gm, respectively.
Fig. 1. Schematic summary of the laser processes employed in fabrication of the GaAs digital integrated circuit, and a schematic electrical diagram of the circuit.
Mat. Res. Soc. Symp. Proc. Vol. 75. '1987 Materials Research Society
652
DIRECT DISCONNECTION Since the MESFETs of interest are connected to other test structures on the chip, connections must first be cut to isolate the needed devices. This is readily accomplished by single-shot pulsed 193-nm radiation from an ArF-excimer laser. Figure 2 schematically shows the cuts in the circuit, as well as showing a cutaway view and two scanning-electron micrographs (SEMs). A microscope fitted with all-reflective optics was used because of the short wavelength employed. As seen in the SEMs of Figure 2, the Ti/Au lines are cleanly cut, while a region of approximately 10 gtm diameter sentered on the cut zone has the underlying polyimide partially ablated. At the 1 J/cm fluence level used here, abou
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