Several SEM inspection techniques for the process development of a 0.25 p.m T-gate MESFET process have been successfully utilized. The transition of these techniques to a manufacturing process with SPC monitoring will also be discussed. The unique challenges of imaging several different Ebeam resists, and their interaction with the scanning electron microscope will also be discussed. BACKGROUNDThe production of state of the art GaAs MESFET devices (which operate at 4-35+ GHz) requires low input capacitance devices with small gate resistances. One method of accomplishing these goals is the production of a T-gate. A T-gate is a gate that has a cross-section with a top larger than the bottom, or shaped like a "T". The small dimensions required for the bottom of the "T' are patterned by E-beam, and because of extremely tight registration requirements, the top of the "T" is also written by Ebeam.The combination of using 2 different Ebeam resists (with significantly different sensitivities) and a very precisely controlled dose and shape for the patterning electron beam, allows this "1"' feature to be patterned with only one exposure/develop sequence. Because the metal used in gate formation (Ti, Pt, and Au) are not readily etched, the actual metal gate structure is formed via a "lift-off" process. In a "lift-off' process the desired patterns are exposed and developed on the wafer (exposing bare GaAs) and then metal is evaporated over the entire wafer. After evaporation the remaining photoresist is dissolved away, leaving only the original exposed features on the wafer. Because the gate is formed via this metal "liftoff' process, both the size and the profile of the photoresist are critical to making the process reproducible. The following figures illustrate the exposure mechanism, and the idealized profile produced after development.The gate resulting after liftoff (and nitride passivation) is shown in Figure 2. 0-8194-1 160-4/93/$6.00 SPIE Vol. 1926 / 295 I EXPOSURE CAP EXPOSURE Figure 1. (a) Resist structure LOW SENSITIVITY RESIST 4-HIGH SENSITIVITY RESIST LOW SENSITIVITY RESIST (b) used for T-gate formation before exposure (a), and idealized profile after exposure(b) Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/24/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
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
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.