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SEMI M46-1101 E © SEMI 2001 2 5.1.2 A rectify ing barrier, of kn own contact area, is form ed by placin g the sample in con tact with an electrolyte. 5.1.3 T he quality of the barrier is assessed usin g I- V and C-V m ea…

SEMI M46-1101

TEST METHOD FOR MEASURING CARRIER CONCENTRATIONS IN

EPITAXIAL LAYER STRUCTURES BY ECV PROFILING

This test method was technically approved by the Global Compound Semiconductor Committee and is the

direct responsibility of the European Compound Semiconductor Materials Committee. Current edition

approved by the European Regional Standards Committee on June 29, 2001. Initially available at

www.semi.org December 2001; to be published March 2002.

This document was editorially modified in November 2001 to correct a typographical error. A change was

made to Section 7.3.3.

1 Purpose

1.1 The purpose of this document is to specify a

method to measure the carrier concentration and carrier

concentration vs. depth profile of epitaxial layers by

Electrochemical Capacitance Voltage ECV profiling.

2 Scope

2.1 This test method covers a procedure for measuring

the carrier concentration of epitaxial layers by ECV

profiling. This method focuses on improving the

accuracy and repeatability of the measurement by

standardizing the test conditions and reporting and by

routine calibration of the measurement.

2.2 This test method is intended to cover the majority

of routine samples measured. However, because of the

number of different materials encountered it cannot

cover every contingency.

2.3 This standard may involve hazardous materials.

This standard does not purport to address safety issues,

if any, associated with its use. It is the responsibility of

the users of this standard to establish appropriate safety

and health practices and determine the applicability of

regulatory limitations prior to use.

3 Referenced Standards

3.1 SEMI Standards

SEMI C1 — Specifications for Reagents

3.2 ASTM Standards

D1125-95 (1999) — Standard Test Methods for

Electrical Conductivity and Resistivity of Water

4 Terminology

4.1 bias — the potential applied to the sample with

respect to a reference electrode.

4.2 capacitance voltage CV measurements — electrical

measurements where the capacitance of a rectifying

barrier is measured as a function of applied bias and is a

measure of the net fixed ionized charge per unit

volume.

4.3 carrier concentration — the net fixed ionized

charge per unit volume. Equal to the free carrier

concentration if the dopant is fully ionized and the

material is free of traps.

4.4 current voltage IV measurements — electrical

measurements where the current through the rectifying

barrier is measured as a function of applied bias.

4.5 dissipation factor — the ratio of the real part to the

imaginary part of the complex admittance. It is a

measure of the non-ideality of the barrier.

4.6 electrochemical — chemical reaction in which

charge transfer takes place via an external circuit.

4.7 epitaxial layer — a layer of single crystal

semiconductor material grown on a host substrate

which determines its orientation.

4.8 excess area — the difference between the wetted

and illuminated areas.

4.9 flatband potential — the intercept on the voltage

axis of the 1/C

vs V plot. A measure of the built in

field or barrier height.

4.10 illuminated area — the area of the sample which

can be illuminated during electrochemical etching.

4.11 rectifying barrier — a potential gradient formed

at the junction between two materials which permits the

flow of charge in one direction only.

4.12 reference electrode — half cell which has a

constant electrode potential, such as a saturated calomel

electrode, SCE.

4.13 rest potential — the open circuit potential of the

sample with respect to the reference electrode.

4.14 wetted area — the area of contact between the

electrolyte and the sample.

5 Summary of Method

5.1 In this method, the carrier concentration of the

epitaxial layer is measured by the CV method.

5.1.1 Ohmic contacts are formed on the sample.

SEMI M46-1101

5.1.2 A rectifying barrier, of known contact area, is

formed by placing the sample in contact with an

electrolyte.

5.1.3 The quality of the barrier is assessed using I-V

and C-V measurements.

5.1.4 From the measured C-V data the carrier

concentration of the layer is determined.

5.1.5 The sample is anodically etched to produce a new

barrier deeper in the material. The etch depth is

determined using Faraday’s law of electrolysis.

5.1.6 Sections 5.1.4 and 5.1.5 are repeated to generate

a carrier concentration vs. depth profile.

6 Interferences

6.1 Thin Layers — Diffusion of carriers into adjacent

regions set a fundamental resolution limit to the

measurement. In addition if the layer has a high sheet

resistivity this can also affect the value obtained.

6.2 Double Layer Capacitance — A limitation

imposed by the physical nature of the

electrolyte/semiconductor barrier which sets an upper

limit to the measured carrier concentration, for accurate

measurements, of 1 × 10

-3

. The technique can be

used beyond this value, but with progressive

degradation.

6.3 Etch Well Uniformity

6.3.1 Etch Well Flatness — Non-uniformity increases

the effective area and may result in the measurement

not being wholly made in the layer of interest. The

main causes of non-uniformity are gas bubbles,

inadequate electrolyte circulation and uneven

illumination.

6.3.2 Etch Well Roughness — indicates poor etching

and has a deleterious effect on the measurements.

Generally overcome by selecting a more appropriate

electrolyte and/or etching conditions.

6.4 Non-Ideal Electrical Characteristics

6.4.1 Series Resistance — From the contacts, sample

and electrolyte affect the accuracy of the measurements.

This effect is minimized by using a highly conductive

electrolyte, making large area ohmic contacts and by

employing a low measurement frequency.

6.4.2 Leakage Currents — Parallel conduction can

affect the accuracy of the measurement. Its influence is

reduced by employing a high measurement frequency.

6.4.3 Large Excess Area — Can give rise to errors on

n-type material particularly when profiling Hi-Lo

structures. The excess area is electrolyte dependent but

is mainly affected by the quality of the seal used to

define the area. Large excess areas indicate a poor seal

and are corrected by replacing the seal.

6.5 Hi-Lo Structures — Measurement is subject to

inaccuracy due to carrier diffusion and excess area

errors. For accurate measurements, chemically etch to

the layer of interest before measurement.

6.6 Contact Quality — Non-Ohmic or high resistance

contacts have a deleterious effect on the measurement.

Contacts should be checked by measuring the resistance

between two similar contacts and then reversing the

current direction and repeating the measurement.

6.7 Surface and Deep States — Cause the measured

capacitance to be frequency dependent and/or

dependent on the rate at which the bias is changed.

Usually resolved by using a high measurement

frequency. This should not be confused with the

frequency dependence which is more usually attributed

to high series resistance.

6.8 Surface Films — Have a deleterious effect on the

capacitance measurement. Film formation should be

avoided by using an appropriate electrolyte, control of

the etching bias and time and by electrolyte circulation.

6.9 Surface Roughness — The surface of the sample

should be smooth and free of features that would affect

the action of the seal.

6.10 Light — Light can affect the capacitance

measurement, so the sample must be placed in a dark

environment during the measurement.

6.11 Sample Loading — The accuracy of the

measurement depends on the repeatability of the area. If

a compliant seal is involved, such as a plastic ring, the

sample should be held in contact with the seal by

applying a controlled and constant force. No relative

lateral movement should take place during the loading

process.

6.12 Ring Variability — The condition of the ring has a

large effect on the measurement. Good seals will have a

small to zero excess area and exhibit a well defined and

reproducible contact area.

7 Apparatus

7.1 Measurement of Etch Well and Ring Areas —

Measuring microscope with either an XY stage, with a

linear resolution of 0.01 mm or better, or a camera and

image processing system capable of measuring an area

of 0.1 cm

to better than 0.5%.

7.2 Measurement of Etch Well Flatness — (Optional)

A means such as a stylus profiler for checking the

flatness and roughness of the etch well. In most cases

visual inspection of the etch well, for a mirror like

appearance, is sufficient indication of low roughness.

SEMI M46-1101

7.3 Instrumentation

7.3.1 Capacitance Meter — Capable of measuring

values up to 300 nF, employing a test signal of

amplitude less than 100 mV RMS. The measurement

frequency should be typically between 1 and 100 kHz.

7.3.2 Equivalent Circuit Model — The meter should

assume a parallel circuit model for the

electrolyte/semiconductor interface and provision

should be made to compensate for the large series

resistance of the electrolyte and contacts.

7.3.3 Potentiostat — Capable of maintaining the

potential of the sample with respect to a reference

electrode constant to ±5mV during the measurement

and with a reference input impedance of > 10

Ohms.

7.3.4 Light Source — A source of uniform illumination

of above band gap energy for etching n-type materials.

7.4 Sample Holder

7.4.1 Electrochemical Cell — Used to hold the sample,

contain the electrolyte and support the sealing ring and

other electrodes, see Figure 1.

7.4.1.1 Sealing Ring — A corrosion resistant ring

which forms a seal between the electrolyte and the

sample. This ring defines the measurement area which

should be reproducible and well defined. The ring area

should not be less than 0.008 cm

and the

reproducibility between runs should be better than ±

3%. Large ring areas (typically ≥ 0.1 cm

) are preferred

for more accurate measurements.

7.4.1.2 Reference Electrode — Used to control the

sample bias via a potentiostat.

7.4.1.3 Counter Electrode — The bias and test signals

are applied between this electrode and the sample.

Figure 1

Electrochemical Cell

7.4.1.4 Sensing Electrode — Placed close to the

sample-electrolyte interface and used to measure the

admittance of the sample.

7.4.1.5 Sample Contacts — Ohmic contacts to the front

and/or back surface of the sample for the purpose of

applying bias.

8 Reagents and Materials

8.1 Purity of Reagents — All chemicals for which such

specifications exist shall conform to SEMI C1.

8.2 Purity of Water — Use either distilled or de-

ionized water having a resistivity greater than

2 MOhm•cm at 25

C as determined by the Non-Referee

Test of Test Methods D 1125.

8.3 Selection of Electrolyte — The choice of

electrolyte is not defined by this standard and is up to

the discretion of the user or by agreement between the

customer and the supplier.

8.3.1 InP, GaAs and Related Compounds — Preferred

electrolyte, Na

.EDTA (0.1 M) basified with

ethylenediamine to a pH of 10, although other

electrolytes can be satisfactorily used.

NOTE 1: Preferred electrolyte, see reference

“Electrochemical C-V Profiling of Heterojunction Device

Structures” A.C.Seabaugh et al. IEEE Trans on Electron Dev

36 No 2 (1989) 309-313.

9 Safety Precautions

9.1 The electrolytes used are potentially hazardous.

Read the Materials Safety Data Sheets before

attempting to prepare the electrolytes. Use safety