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SEMI M41-1101 © SE MI2000, 2001 13 Ex ampl es: For Cu; HF -H 2 O 2 (HF : H 2 O 2 : H 2 O = 1 : 17 : 82) For Au and Pt; aqua reg ia (HNO 3 : HCl = 1 : 3) 8.4.4 ICP-MS (Inductively Coupl ed Plasma Mas s Spectrometry) — IC …

SEMI M41-1101 © SEMI 2000, 2001 12

Tolerance = Maximum value – Minimum value

NOTE 6: The location and numbers of measuring points

should be determined between users and suppliers.

8.1.3.2 In case of multi-points measurements (ex. a

few hundreds) within an SOI wafer, the tolerance is

defined as;

Tolerance = 3σ (3 times of the standard deviation)

8.1.3.3 Measurement exclusion area such as wafer

edge should be determined between users and suppliers.

NOTE 7: Recommendable metrology

a) Reflective spectroscopy or FT-IR is recommended

• for top silicon film of less than several µm (about 7 µm),

• and FT-IR for top silicon film of more than several µm

(about 7 µm)

b) Tolerance is defined as the difference between the

maximum and the minimum value after measuring several

(ex. 9) points.

c) It is not necessary to measure the BOX thickness of SOI

wafer after bonding. It is OK to measure it before wafer

bonding.

d) In case of the above 1) ~ 3), the number of measurement

points and their location should be specified in case of several

points measuring, and the measurement exclusion area should

be specified in case of multi-points measuring.

8.2 Crystal Defect of Top Silicon Film

8.2.1 OSF (Oxidation induced Stacking Fault) — This

technique is applicable to the top silicon film of thicker

than 1.5 µm. OSF density is measured by preferential

chemical etching and microscopic observation.

Preparation of samples and measurement of OSF

density are as follows:

8.2.1.1 Sample Preparation — SOI wafers are oxidized

at 1,100°C, 1 h, in H

/ O ambient after the SC-1 and

SC-2 cleaning. Oxide is removed by ca. 25 % HF and

then the SOI wafers are preferentially etched by 1 µm,

applying JIS H 0609:1994(B), and then rinsed

thoroughly in distilled water and blown dry. JIS H

0609 defines the chromium-free preferential solution,

which is composed of HF, HNO

, CH

COOH and H

8.2.1.2 Measurement of OSF Density — Samples are

examined by an optical microscope. The sample

surface is observed by magnification of 200 X, and

OSF is counted on SOI wafer within the scope along

the two lines, which are parallel and perpendicular to

the orientation flat (so called cross scanning). OSF

density is calculated from the count number and

scanning area.

8.3 Buried Oxide Defect

8.3.1 Cu Decoration Method — In case of Bonded

SOI, the buried oxide is usually formed by thermal

oxidation. Therefore, the defect of buried oxide is

taken into consideration only for the thin oxide cases.

Buried oxide defect such as pinholes can be evaluated

by Cu decoration method. This method has been

applied to the buried oxide film of less than 400 nm

thickness. Sample preparation and Cu decoration are

conducted by the following procedure. The top silicon

film on the buried oxide is removed by KOH solution,

and then cleaned and rinsed. The sample is set on a

gold-plated brass (Cathode) in the methanol solution.

On the other side, a copper plate (Anode) is placed 5

mm above the sample surface. Positive constant bias of

1–3 MV/cm (ex. 40–120 V for 400 nm oxide) is applied

to the copper plate for 5 minutes. Small leakage current

passes through the buried oxide defect, and

consequently copper precipitates on the defects.

Typical allowable defect density is < 0.1/cm

8.4 Metal Contamination

8.4.1 The surface metal contamination can be

measured by TXRF, AAS and ICP-MS methods.

8.4.2 TXRF (Total X-Ray Fluorescence) — Total X-ray

Fluorescence uses a low angle incident, and a tightly

collimated X-ray beam excites the characteristic X-rays

from impurity atoms near the sample surface. Usually,

the angle of X-ray incident is less than 0.1 degree. The

element identification and the amount of the element

can be obtained by measuring energy and intensities of

fluorescence X-ray. The instrument provides a map of

impurity element distribution. The surface metal

contamination (typically from Na to Zn) shall be less

than 10

-2

in total.

NOTE 8: This TXRF method is conveniently used to detect

the metals on the SOI wafer surface.

8.4.3 AAS (Atomic Absorption Spectrophotometry) —

The elemental characteristic absorption of the atom is

measured by introducing sample solution as aerosol

into the flame and then spectral absorption through the

flame from the light source is detected by the

spectroscope. The flameless method, superior to the

flame method in the sensitivity, is now broadly used.

8.4.3.1 Sample Preparation — Careful sample

preparation is necessary for the precise measurement.

SOI wafer surface is exposed to HF vapor, and the

metals on the surface are collected as droplet. To

improve the sensitivity, the volume of collective

solution should be as tiny as possible and the HF drops

are rolled all over the surface in collective operation. In

case of precious metals, it is better to use other kinds of

collective solutions instead, since they are not dissolved

or collected by HF solution itself.

SEMI M41-1101 © SEMI2000, 200113

Examples:

For Cu; HF-H

(HF : H

: H

O = 1 : 17 : 82)

For Au and Pt; aqua regia (HNO

: HCl = 1 : 3)

8.4.4 ICP-MS (Inductively Coupled Plasma Mass

Spectrometry) — ICP-MS is composed of ICP

(Inductively Coupled Plasma) part as an ion source and

MS (Mass Spectrometer) part, which measures the ions

generated at ICP part. Usually, sample solution is

vaporized in the nebulizer and then finally introduced

into Argon plasma in the silica tube called torch

through the spray chamber. The sample is decomposed,

evaporated, atomized and then ionized in the Argon

plasma. Except for few atoms that have relatively high

ionization potential, most of the elements (> 90%) can

be ionized. Ions are identified and measured in amount

by the mass spectrometer.

8.4.4.1 Sample Preparation — The same method as

AAS method is applicable. In case of quantitative

measurement of Fe, since its mass weight is close to

that of ArO

, it is necessary to pay attention to the

degradation of detection sensitivity.

8.5 Particle Density (LPD : Light Point Defect )

8.5.1 Light Scattering Tomography — The particle

larger than 0.2 µm on the thick SOI wafers is counted

by Automated particle counter. The particles in the

order of 0.1 µm can be detected if top silicon film is

sufficiently thick.

8.5.2 Principle of measurement — By scanning the

laser beam on the wafer surface, the light scattered by

the particles on the wafer is detected. The scattered

light and the noise from the wafer surface is detected as

a direct current, on the other hand, the scattered light by

the particles can be detected as pulse components. The

particle size can be calibrated with standard polystyrene

latex spheres. Multi-layers of SOI wafers usually have

scattering noise from the layer interface. In case of less

than 1 µm of the top silicon film thickness, it is

necessary to reduce incident angle of the laser beam to

increase the reflective component from the surface. For

example, S/N ratio is improved when using S-polarized

light of 10 degree incident, 85% of its component is

reflected from silicon surface.

NOTE 9: In case of SOI wafer (Thickness > 1 µm)

Particle counter with a vertical incident laser, which is the

same one used for the bulk wafer, is applied. It should be

noted that bypass filter to erase the interference signals due to

thickness dispersion, and adjustment of photo-multiplier

sensitivity are necessary. By this technique, it is capable of

detecting particles (> 0.1µm) as much as on the bulk wafer.

NOTE 10: In case of SOI wafer (Thickness < 0.5 µm)

It is recommended to use S - polarized light or normal light

with low incident angle because of high scattering noise.

However, the adjustment of photo-multiplier sensitivity is

necessary to reduce the noise component. The sensitivity

depends on the magnitude of the noise and it is usually

possible to detect particles of around more than 0.5µm (in

bulk wafer, > 0.2 µm).

8.5.3 Visual Inspection — SOI wafer can be visually

inspected in accordance with ASTM F523. The

automatic inspection equipment is also used when

available. For visual inspection, the collimated high

intensity bright light (ex. 500,000 lux) is used. Under

using this light, SOI wafer is inspected for haze, slip,

scratches, chips, cracks, pits, dimples, mound, orange

peel, LPD and contamination.

8.6 Surface Roughness

8.6.1 AFM (Atomic Force Microscope) — By

contacting the probe equipped with the cantilever onto

the wafer surface of the sample, and by scanning the

cantilever and detecting the variation by i.e., optical

method, the roughness information is obtained.

NOTE 11: It is expected to set the observation area as > 20

µm × 20 µm to increase reliability of the data.

NOTE 12: Height calibration of concave and convex: Refer to

UC standard (“Calibration method of 1 µm order height in

AFM”, [Ultra Clean Technology, Vol. 7, No. 2, pp. 43,

1995]).

8.7 Inclusions

8.7.1 In bonded SOI wafer, there exists the

contaminants at the bonding Si/SiO

or SiO

/SiO

interface such as particles, metals, boron, and

hydrocarbon. Here, inclusions means the contaminants.

Although there has been no report on the influence of

contaminants to the device characteristics, the

improvement of the contamination level is required.

8.8 Void

8.8.1 Scanning Acoustic Topography — The void can

be detected by means of the traveling time difference of

the acoustic waves. The void mapping can be made by

scanning an ultrasonic wave and detecting the reflecting

wave from the both surfaces of the void. Measuring in

water improves the resolving power of location since

the ultrasonic wave can be tightened by acoustic lenses.

NOTE 13: It is not suitable to measure SOI wafer that is not

bonded firmly because measurement is conducted in water.

NOTE 14: It is not suitable to measure top silicon film (< 7

µm) because it is impossible to separate reflective waves both

from top silicon film surface and the bonding interface.

NOTE 15: Detectable void gap depends on acoustic wave

frequency. Detectable void diameter depends on the size of

the acoustic source and the receiver. For example, if using 75

MHz frequency, 5 nm void gap and 50 µm void diameter can

be detected.

NOTE 16: Void is defined as “empty space” that is due to the

imperfect bonding at Si/SiO

and SiO

/ SiO

interface. This

void should be discriminated from the splitting at bonding

strength test.

NOTE 17: Void can be only evaluated during SOI wafer

processing, not at the shipping.

8.9 Bonding Strength

8.9.1 Tensile Testing Method — Bonding strength is

defined and evaluated by tensile strength (kgf/cm

)

which is needed to split the bonding interface vertically.

Details of the test structure and the test method should

be determined by negotiation between wafer users and

wafer suppliers.

Table 6 Test Summary Table

Parameter Reference Method

Wafer Diameter F613-93 Optical Comparator

Wafer Thickness F533-96, F1530-94 Thick. Gage, Auto. Noncontact Scan.

Total Thickness Variation

LTV

F1530-94 Automated Noncontact Scanning

Warp F1390-92 Automated Noncontact Scanning

Crystal Orientation

Top Silicon Film (SOI)

Base Wafer

F26-87a (1993) X-ray Diffraction

Substrate Type / Dopant F42-93 Hot-Probe (Test Method A)

Substrate Resistivity F43-93, F84-93, F1527-94 4 Point Probe

Substrate RRG F81-95 4 Point Probe

Top Si Film Thickness Section 8.1 Reflective Spectroscopy or FTIR

Crystal Defect (OSF) Section 8.2, (JIS H 0609 :1994 B) Cr-free Etch / Optical Microscopy

Buried Ox defects Section 8.3, (SEMI M34) (a) Cu Decoration, (b) BOX Capacitor

Metal Contamination

(per unit area)

Section 8.4, (F1526-95) TXRF, AAS/ICP-MS

Particle Density ( LPD ) Section 8.5 (F1620-96) Light Scattering Tomography

( Automated Particle Counter )

Haze F523-93 (see NOTE 1), F154-94 Visual Inspection

Slip F523-93 (see NOTE 1), F154-94 Visual Inspection

Scratches F523-93 (see NOTE 1), F154-94 Visual Inspection

Chips / Cracks F523-93 (see NOTE 1), F154-94 Visual Inspection

Pits and Dimples F523-93 (see NOTE 1), F154-94 Visual Inspection

Mounds F523-93 (see NOTE 1), F154-94 Visual Inspection

Orange peel F523-93 (see NOTE 1), F154-94 Visual Inspection

Particle Density ( LPD ) F523-93 (see NOTE 1), F154-94 Visual Inspection

Contamination ( Both Side ) F523-93 (see NOTE 1), F154-94 Visual Inspection

Surface Roughness Section 8.6 AFM

Inclusions Section 8.7 SIMS

Voids Section 8.8 Scanning Acoustic Tomography

Bonding Strength Section 8.9 Tensile Strength

NOTE 1: Users and suppliers may agree on the non-SOI edge area for these specifications. For example the area within 6 mm proximity of the

wafer edge may be excluded.