**PAM-XIAMEN can supply GaAs wafer with EPD less than 5000/cm ^{2}.**

Q: Could you please advise guaranteed EPD for below substrate and epi?

Gallium Arsenide wafers, P/E 2″Ø×380±25µm,

LEC SI undoped GaAs:-[100]±0.5°, n-type Ro=(0.8E8-0.9E8)Ohmcm,

One-side-polished, back-side matte etched, 2 Flats,

LT-GaAs EPI: 1-2µm, Resistivity >1E7 Ohm-cm, Carrier lifetime <1ps,

Sealed under nitrogen in single wafer cassette.

A: Dislocation Density<1×10^6cm-2

The EPD (etch pit density) is a measure for the quality of semiconductor wafers. It is well known in the gallium arsenide (GaAs) industry that the EPD level of a substrate is very important to the reliability of minority carrier devices and to the yield of fabricated devices. In the production of GaAs electronic devices such as heterojunction bipolar transistors (HBTs) and pseudo-high electron mobility transistors (pHEMTs), the EPD of GaAs substrate is one factor that determines the device yield.

So, how to measure the EPD of GaAs substrate? A standard method is suggested for testing the etch pit density in low dislocation density GaAs polished wafer as follows:

This method applies to the measurement of EPD of circular GaAs wafers with diameters of 2 inches and 3 inches and EPD less than 5000/cm^{2}.

## 1. Method Principles for Measeuring EPD of Polished GaAs Substarte

After the GaAs wafer is etched with molten potassium hydroxide, the number of corrosion pits is observed and recorded with a microscope.

## 2. Instruments for Determining EPD

Microscope: The measurement field of view area should be 0.01 cm^{2} or larger.

## 3. Chemical Reagents

# Sulfuric acid (p=1.84 g/mL), concentration 95%~98%, excellent grade pure;

# Hydrogen peroxide (p=1.00 g/mL), concentration>=30%, excellent grade;

# Potassium hydroxide, concentration >=85%, excellent grade pure.

## 4. Measurement Steps of EPD

1) The counting positions for 2-inch diameter GaAs wafers and 3-inch wafers are shown in Figures 1 and 2, respectively. The count point is at the center of each grid. For a 2-inch diameter wafer, the grid side length is 5 mm, the total number of counting points is 69, and the 35th point is located in the center of the wafer. For a 3-inch diameter wafer, the grid side length is 10 mm, the total number of counting points is 37, and the 19th point is located in the center of the wafer.

2) Count and record the number of corrosion pits whose center is within the measurement field of view. If the pits are too dense to count, increase the magnification. Then, count the number of corrosion pits and record the results along with the microscope magnification.

3) Repeat the operation described in 2) for all other counted points, namely points 2 to 69 for 2 inch gallium arsenide wafers and 2 to 37 points for 3 inch wafers.

Fig. 1 Counting Positions for 2″ Diameter GaAs Wafers

Fig. 2 Counting Positions for 3″ Diameter GaAs Wafers

## 5. Calculating Etch Pit Density

The EPD of each measurement field is equal to the number of corrosion pits in that field divided by the field area, that is:

EPD=W/S…………………………… (1)

In the formula:

W — the number of etch pits, number;

S — field of view area, cm^{2}.

Example: A measurement field size is 0.1cmx0.1cm, and then its field of view area is 0.01cm^{2}.

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