<title>Process control for 0.25 um GaAs microwave monolithic integrated circuits</title>

Abstract

Gallium Arsenide Metal Semiconductor Field Effect Transistors (GaAs MESFETs) are used in analog Microwave Monolithic Integrated Circuits (MMICs) because of their high frequency response.Common applications for MMICs include low noise and power amplifiers for use in satellite communication and missile guidance systems. The performance of MESFETs is improved with smaller gate lengths, but to consistently achieve the highest performance, control methods must be in place for the critical processes. Gate length control is the key parameter in maintaining the RF performance and a lack of gate pinch off is the major yield loss category. This paper will describe the process and the tools that Texas Instruments uses to monitor the critical parameters. It will also describe the control methods and review the major contributors to variations in the process. . INTRODUCTIONThe use of 0.25 m gate lengths is becoming common for production of the highest performing circuits. To avoid performance degradation associated with higher line resistance, the gates are formed with a "T' shaped cross-section. The top of the gate, or cap, is larger (0.7-1.0 jim) than the trunk, the portion making contact with the surface. Figure 1 shows an SEM micrograph of a cross-sectioned gate. The trunk is the most critical dimension of the gate; its length determines the input capacitance of the FET and therefore controls the speed of the device. Figure 1. SEM micrograph of T-gate cross-section. PROCESSThe gates are defined using electron beam lithography using an Hitachi HL 700 M/D Ebeam. Figure 2 shows the process flow schematic. Three layers of resist are used to create an opening for the gate. From bottom to top the layers are 4% PMMA, EBR-9 (1) and 2% PMMA. The structure is exposed with two passes. The first pass exposes the trunk and uses a relatively high dose. The second Ebeam pass exposes the cap utilizing a smaller dose than for the trunk. The structure is developed in MIBK. The developer dissolves the EBR9 much flister than PMMA leaving a 0.25 .tm opening in the bottom layer of PMMA and a larger opening fij. the cap. The top layer overhangs the EBR9 forming a lip that assists in lifting the evaporated metal that is used to form the actual gate. 0-8194-149 1-3/94/$6.00 278/SPIE Vol. 2196 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/24/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx