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Optically Determining the Doping Depth in Semiconductors

IP.com Disclosure Number: IPCOM000089524D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Briska, M: AUTHOR [+3]

Abstract

The etch rate of semiconductors is known to depend on the concentration of doping atoms (or free carriers). By continuously monitoring the thickness of a sample (e.g., a wafer), the decrease in wafer thickness vs. time (or, equivalently, the etch rate vs. the etching depth) may be plotted (Fig. 1). It has been found that such plots exhibit a distinct and sharp discontinuity representative of the doping depth, i.e., the transition from the doped to the undoped regions.

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Optically Determining the Doping Depth in Semiconductors

The etch rate of semiconductors is known to depend on the concentration of doping atoms (or free carriers). By continuously monitoring the thickness of a sample (e.g., a wafer), the decrease in wafer thickness vs. time (or, equivalently, the etch rate vs. the etching depth) may be plotted (Fig. 1). It has been found that such plots exhibit a distinct and sharp discontinuity representative of the doping depth, i.e., the transition from the doped to the undoped regions.

It is proposed to measure the thickness variation, using infrared (IR) rays and bringing to interference the beams reflected by the two wafer surfaces. In the experimental set-up (Fig. 2), wafer 1 is made to form the bottom of a TEFLON* etchant beaker 2 and is illuminated by an IR beam (Lambda = 3.39 Mu He-Ne laser 3& with oblique incidence. The interferences are recorded by an IR detector 4 together with beam chopper, lock-in amplifier and plotter. The decrease in the wafer thickness is calculated from the interference condition, utilizing the known refractive index n of silicon. The plots obtained may be evaluated directly or after appropriate smoothing by a computer algorithm.

For ready adjustment, a second laser 5 in the visible range (0.63 Micron, He- Ne) is provided.

This method is independent of the etching conditions. The IR beam is not disturbed and absorbed by the etchant. The measuring spot can be made small for high spatial resolut...