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SEMI MF1392-1103 © SEMI 2003 2 (a) De plet ion De pth a s a Func tion of Do pant D ensit y w ith Appl ied R evers e B ias Volt age as a Para mete r. (b) Appli ed Reverse B ias Volt ag e as a Functio n of Dopant Densi ty …

SEMI MF1392-1103 © SEMI 2003 1

SEMI MF1392-1103

TEST METHOD FOR DETERMINING NET CARRIER DENSITY

PROFILES IN SILICON WAFERS BY CAPACITANCE-VOLTAGE

MEASUREMENTS WITH A MERCURY PROBE

This standard was technically approved by the Global Silicon Wafer Committee and is the direct

responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by

the North American Regional Standards Committee on July 12, 2003. Initially available at www.semi.org

October 2003; to be published November 2003. Originally published by ASTM International as ASTM

F 1392-92. Last previous edition ASTM F 1392-02.

1 Purpose

1.1 Net carrier density is a critical parameter in growth

of epitaxial layers of silicon. This test method provides

a means for determining net carrier density without

formation of a special diode structure on the layer. It

may also be used in characterizing net carrier density in

polished silicon wafers.

1.2 This test method can be used for research and

development, process control, and materials specifi-

cation, evaluation, and acceptance purposes.

1.3 In the absence of interlaboratory test data to

establish its reproducibility (see Section 14.2), this test

method should be used for materials specification and

acceptance only after the parties to the test have

established reproducibility and correlation.

2 Scope

2.1 This test method

covers the measurement of net

carrier density and net carrier density profiles in

epitaxial and polished bulk silicon wafers in the range

from about 4 × 10

to about 8 × 10

carriers/cm

−3

(resistivity range from about 0.1 to about 100 Ω·cm in

n-type wafers and from about 0.24 to about 330 Ω·cm

in p-type wafers).

2.2 This test method requires the formation of a

Schottky barrier diode with a mercury probe contact to

an epitaxial or polished wafer surface. Chemical

treatment of the silicon surface may be required to

produce a reliable Schottky barrier diode.

The surface

treatment chemistries are different for n- and p-type

wafers. This test method is sometimes considered

destructive due to the possibility of contamination from

1 DIN 50439, Determination of the Dopant Concentration Profile of

a Single Crystal Semiconductor Material by Means of the

Capacitance-Voltage Method and Mercury Contact, is technically

equivalent to this test method. DIN 50439 is the responsibility of

DIN Committee NMP 221, with which SEMI maintains close liaison.

DIN 50439 is available in German and English from Beuth Verlag

GmbH, Burggrafenstraße 4-10, D-10772, Berlin, Germany.

2 Severin, P. J., and Poodt, G. J.,“Capacitance-Voltage

Measurements with a Mercury-Silicon Diode,” J. Electrochem. Soc,

119, 1384–1388 (1972).

the Schottky contact formed on the wafer surface;

however, repetitive measurements may be made on the

same test specimen.

2.3 This test method may be applied to epitaxial layers

on the same or opposite conductivity type substrate.

This test method includes descriptions of fixtures for

measuring substrates with or without an insulating

backseal layer.

2.4 The depth of the region that can be profiled

depends on the doping level in the test specimen.

Based on data reported by Severin

and Grove,

Figure

1 shows the relationships between depletion depth,

dopant density, and applied voltage together with the

breakdown voltage of a mercury silicon contact. The

test specimen can be profiled from approximately the

depletion depth corresponding to an applied voltage of

1 V to the depletion depth corresponding to the

maximum applied voltage (200 V or about 80% of the

breakdown voltage, whichever is lower). To be

measured by this test method, a layer must be thicker

than the depletion depth corresponding to an applied

voltage of 2 V.

2.5 This test method is intended for rapid carrier

density determination when extended sample

preparation time or high temperature processing of the

wafer is not practical.

2.6 This test method provides for determining the

effective area of the mercury probe contact using

polished bulk reference wafers that have been measured

for resistivity at 23° C in accordance with SEMI MF84

(see Note 1). This test method also includes procedures

for calibration of the apparatus for measuring both

capacitance and voltage.

NOTE 1: An alternative method of determining the effective

area of the mercury probe contact that involves the use of

reference wafers whose net carrier density has been measured

using fabricated mesa or planar p-n junction diodes or

evaporated Schottky diodes is not included in this test method

but may be used if agreed upon by the parties to the test.

3 Grove, A. S., Physics and Technology of Semiconductor Devices

(John Wiley and Sons, New York, 1967) Sections 6.2 and 6.7c.

SEMI MF1392-1103 © SEMI 2003 2

(a) Depletion Depth as a Function of Dopant Density with Applied Reverse Bias Voltage as a Parameter.

(b) Applied Reverse Bias Voltage as a Function of Dopant Density with Depletion as a Parameter.

Note: The light dashed line represents the applied reverse bias voltage at which breakdown occurs in a

mercury-silicon contact; the heavy dashed line represents 80 % of this voltage, it is recommended that the

applied reverse bias voltage not exceed this value. The light chain-dot line represents the maximum

reverse bias voltage specified in this test method.

Figure 1

Relationship Between Depletion Depth, Applied Reverse Bias Voltage, and Dopant Density

SEMI MF1392-1103 © SEMI 2003 3

2.7 Warnings and precautionary notes regarding

potential safety hazards are provided throughout the

document.

NOTICE: This standard does not purport to address

safety issues, if any, associated with its use. It is the

responsibility of the user of this standard to establish

appropriate safety and health practices and determine

the applicability of regulatory or other limitations prior

to use.

3 Limitations

3.1 A poor Schottky contact, which is generally

indicated by an excessively high leakage current (see

Section 11.5) is the most common problem in

capacitance-voltage measurements made with mercury

probe instruments. It must be emphasized that the use

of a poor Schottky contact does not actually prevent a

carrier density determination but produces an

erroneous result.

3.2 Improper determination of the compensation

capacitance, C

comp

, (see Section 10.3) can cause

significant errors in the capacitance measurement. In

homogeneous material, improper zeroing or use of an

improper value for C

comp

results in an apparent

monotonic increase or decrease of carrier density with

distance from the Schottky barrier. In some fixtures,

inherently large stray capacitances exist; in such cases,

the value of C

comp

may depend both on the diameter of

the wafer and on the position of the wafer on the chuck.

If these dependencies are observed, they may be

reduced or eliminated by shielding the mercury probe

column. If shielding is not practical, probe calibration

procedures should be carried out with wafers of the

same diameter as the wafers being tested and care

should be taken to ensure that the geometry of wafer

and probe is the same during calibration and

measurement.

3.3 Alternating frequency test signals greater than 0.05

V rms may lead to errors in the measured capacitance.

3.4 Excessive series resistance in the capacitance

measurement circuit can cause significant errors in the

measured capacitance values. Series resistance values

greater than 1 kΩ have been reported to cause

measurement error in some cases.

4,5

The primary

source of excessive series resistance is generally a high-

resistance return contact; other possible sources are

bulk resistance in the wafer and wiring defects in the

4 Wiley, J. D., and Miller, G. L., “Series Resistance Effects in

Semiconductor CV Profiling,” IEEE Trans. Electron Devices ED-22,

265–272 (1975).

5 Schroder, D. K., Semiconductor Material and Device

Characterization (Wiley-Interscience, New York, 1990) Section

2.5.2.

mercury probe fixture or the test cables (see Section

11.4).

3.5 When exposed to air, a scum tends to form on the

exposed surface of the mercury used to form the

mercury probe contact. When freed from the surface,

this scum floats to the top of the mercury column. It is

necessary to make certain that the mercury that contacts

the wafer surface is clean by changing the mercury

periodically or by otherwise removing the scum from

the exposed surface.

3.6 A dirty or damaged capillary tube containing the

mercury column may also result in unstable

measurements (see Section 10.4.2.2).

3.7 If the reference wafer is not sufficiently uniform

throughout its thickness, the value of net carrier density,

ref

, determined by the four-point probe measurement

(see Section 8.4.3) may differ from the value of net

carrier density at the surface where the mercury probe

measurement is made. Use of erroneous values of N

ref

results in incorrect values for effective probe contact

area (see Section 10.3). Further, if the resistivity profile

of the reference wafer is not uniform near the surface,

an incorrect value of C

comp

may be obtained (see

Section 10.2). Incorrect values of probe contact area

and C

comp

result in incorrect values for the average net

carrier density of the test specimen.

4 Referenced Standards

4.1 SEMI Standards

SEMI C28 — Specifications and Guidelines for

Hydrofluoric Acid

SEMI C29 — Specifications and Guideline for 4.9%

Hydrofluoric Acid (10:1 v/v)

SEMI C30 — Specifications and Guidelines for

Hydrogen Peroxide

SEMI MF26 — Test Methods for Determining the

Orientation of a Semiconductive Single Crystal

SEMI MF42 — Test Methods for Conductivity Type of

Extrinsic Semiconducting Materials

SEMI MF81 — Test Method for Measuring Radial

Resistivity Variation on Silicon Wafers

SEMI MF84 — Test Method for Measuring Resistivity

of Silicon Wafers with an In-Line Four-Point Probe

SEMI MF672 — Test Method for Measuring

Resistivity Profiles Perpendicular to the Surface of a

Silicon Wafer Using a Spreading Resistance Probe

SEMI MF723 — Practice for Conversion Between

Resistivity and Dopant Density for Boron-Doped,

Phosphorus-Doped, and Arsenic-Doped Silicon