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A TECHNIQUE TO PREPARE GaAs VCSEL SAMPLE USING A TRIPOD POLISHER FOR TRANSMISSION ELECTRON MICROSCOPIC ANALYSIS

IP.com Disclosure Number: IPCOM000008430D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-13
Document File: 5 page(s) / 206K

Publishing Venue

Motorola

Related People

Wenbin Jiang: AUTHOR

Abstract

Transmission electron microscope (TEM) has been used for high resolution imaging of semicon- ductor lattice structures. It's also an effective tool in failure mode analysis (FMA) to identify defect origins and failure mechanisms. The most time-consuming part of the TEM work is sample preparation. The procedure to prepare a bulk or a blank film TEM sample typically includes grinding, polishing, dimpling and ion milling. The same method does not work well if a particular failure site needs to be analyzed in a device. To precisely pinpoint a failure site, focused ion beam machine (FIB) is a conve- nient and efficient tool to use for the TEM sample preparation, but this approach requires an expensive initial capital investment and a high cost long term maintenance. There are also concerns that the high energy ion beam will cause undesired damage to the sample, causing artifacts difficult to resolve from what is to be analyzed. An alternative way of sample preparation to analyze a specific site in a device is the so-called tripod polishing method originally developed by a group of IBM researchers. The wedged sample is prepared by polishing using different grid size diamond films and diamond suspension in succession. Sometimes, blank ion milling is used in the final step for cleaning. This technique has been successful in preparing silicon based device samples, but has encountered difficulty in preparing GaAs material based samples.

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MOTOROLA Technical Developments

A TECHNIQUE TO PREPARE GaAs VCSEL SAMPLE USING A TRIPOD POLISHER FOR TRANSMISSION ELECTRON MICROSCOPIC ANALYSIS

by Wenbin Jiang

  Transmission electron microscope (TEM) has been used for high resolution imaging of semicon- ductor lattice structures. It's also an effective tool in failure mode analysis (FMA) to identify defect origins and failure mechanisms. The most time-consuming part of the TEM work is sample preparation. The procedure to prepare a bulk or a blank film TEM sample typically includes grinding, polishing, dimpling and ion milling. The same method does not work well if a particular failure site needs to be analyzed in a device. To precisely pinpoint a failure site, focused ion beam machine (FIB) is a conve- nient and efficient tool to use for the TEM sample preparation, but this approach requires an expensive initial capital investment and a high cost long term maintenance. There are also concerns that the high energy ion beam will cause undesired damage to the sample, causing artifacts difficult to resolve from what is to be analyzed. An alternative way of sample preparation to analyze a specific site in a device is the so-called tripod polishing method originally developed by a group of IBM researchers. The wedged sample is prepared by polishing using different grid size diamond films and diamond suspension in succession. Sometimes, blank ion milling is used in the final step for cleaning. This technique has been successful in preparing silicon based device samples, but has encountered difficulty

in preparing GaAs material based samples.

  GaAs TEM samples are difficult to prepare using the tripod polishing method because of two major reasons, First, GaAs material is very brittle. During the polishing, the sample tends to cleave by itself and is easily broken into pieces when being polished down to a thickness of a couple of microns. Second, GaAs is a direct bandgap material with large absorption coefficient. The material only starts to become transparent to the visible light when being thinned down to the submicron range. Transparency is important in the tripod polishing technique to determine the polishing uniformity. It also tells an operator how thick the sample is by judging the color transmitted. Silicon materials start to become transparent at about 10 urn. After reaching this stage, the required final sample thickness and the polishing uniformity along the wedge are achieved by monitoring the color and the interfer- ence patterns formed near the sample wedge tip under a transmission optical microscope. There is no such a luxury when working with GaAs materials. By the time the transparency becomes evident at about 1 pm, the GaAs sample may have broken into pieces. Not to mention the uniformity control since there is no more room for the polishing thickness adjustment at 1 pm thickness.

0 MOLorola, Inc. ,997 57 December 1997

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