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SEMI M34-0299 © SEMI 1999 3 7.1.2 S pectroscopic Ellipso metry Measurement — In this measurement, white light from a xenon arc lamp passes through a polarizing rotating filter and illuminates the sample site under study;…

SEMI M34-0299 © SEMI 1999 2

7. Test Methods - Dimensions

7.1 SIMOX Layer Thickness Measurements

7.1.1 Measurements Methods  Two non-contact,

non-destructive optical characterization techniques,

spectroscopic reflectometry and spectroscopic

ellipsometry, have proven useful for SIMOX layer

thickness measurements. Both techniques use reflected

light to allow deduction of the thickness and refractive

index of thin film layers. In both cases, film thickness

and index of refraction data must be " backed out" of the

measured optical data by a process of successive

approximation. In both cases, the fitting procedure is

more straightforward and more accurate the as-annealed

SIMOX wafers with abrupt silicon/oxide interfaces than

for the implanted SIMOX wafers with extended

interface zones. Silicon islands and interface

nonuniformities make these techniques problematic for

SIMOX wafers with oxygen implant doses below the

" stoichiometric dose" , roughly 1.6 × 10

/cm

for a

375 nm oxide layer.

7.1.1.1 The measurement strategy is to make a detailed

measurement with an accurate fit on at least five wafer

sites, including the center as illustrated in Figure 1. The

number of wafer sites to be monitored should be agreed

on between customer and vendor. Generally, the greater

the variability relative to the mean, the larger the

number of sites that should be monitored. In each case,

the measurement system supplies a " goodness-of-fit"

parameter which indicates a level of confidence in the

fit to the measured data.

Figure 1

Schematic of Measurement Sites, Cleavage Lines, and Edge Exclusion for 100 mm SIMOX Wafer Inspection

SEMI M34-0299 © SEMI 1999 3

7.1.2 Spectroscopic Ellipsometry Measurement — In

this measurement, white light from a xenon arc lamp

passes through a polarizing rotating filter and

illuminates the sample site under study; reflected light

passes through an analyzer to a monochromator and

photomultiplier detector. For each wavelength,

reflectivity oscillates with polarizer rotation; the

magnitude and phase of reflectivity changes are

measured to determine ellipsometric angles, δ and ψ.

The two measured spectra are fit by successive

approximation to allow determination of the silicon and

oxide layer thickness and oxide composition.

Ellipsometry measurements are only specified for the

annealed SIMOX wafers. For spectroscopic

ellipsometry, the choice of instrument and associated

model and fitting parameters affect the confidence-of-

fit, so they should be taken into account in the

user/supplier agreement. For example, with the SOPRA

SE system, the goodness-of-fit error estimator shall be

less than 0.025 for accurate structural models.

7.1.3 Optical Reflectance Measurements — In this

measurement, light from a xenon arc lamp passes

through a grating monochromator and illuminates the

sample site under study; reflected light is gathered by

an intrinsic silicon detector. Specular reflectivity is

plotted as a function of wavelength from 0.4 micron to

1.1 micron. The analysis proceeds by making

successively better approximations to the oxygen

content, index of refraction, and absorption of each

layer until an acceptable fit is achieved. Measurements

are made with a reflectance mode optical

interferometer.

7.1.4 Optical Model Fitting and Correlation — There

are slight, systematic differences between layer

thickness measured by reflectance and by spectroscopic

ellipsometry (SE). Because of this, user and supplier

should specify the actual measurement method to be

used. The two methods offer results which are

reproducible and well-correlated with each other over a

wide range of conditions. If both measurement

techniques are used, it is recommended that the

reflectance system measurements be calibrated to fit the

results of the SE. Figure 2 shows conversion curves for

top silicon and oxide layer thickness measurements

made with the two measurement techniques.

7.1.5 Top Silicon Layer Thickness — See Section

7.1.4 on correlation of reflectance and spectroscopic

ellipsometry (SE) measurements. Optical measure-

ments will be made on five wafer sites as shown in

Figure 1. Spectra for each site will be fit independently

with both the top silicon and oxide layer thickness as

adjustable parameters. Both the mean thickness and the

uniformity should be specified. Depending on the type

of SIMOX wafer being specified, the mean thickness of

the top silicon layer will be from 50 nm to 500 nm.

Following is an example specification: Mean top silicon

layer thickness for the five sites will be 215 ± 10 nm

with acceptable goodness-of-fit at all five sites. Top

silicon non-uniformity will be less than ± 5 nm.

7.1.6 Buried Oxide Layer Thickness — See Section

7.1.4 on correlation of reflectance and spectroscopic

ellipsometry (SE) measurements. Optical measure-

ments will be made on five wafer sites as shown in

Figure 1. Spectra for each site will be fit independently

with both the top silicon and the buried oxide layer

thickness as adjustable parameters. Both the mean

thickness and the uniformity should be specified.

Depending on the type of SIMOX wafer being

specified, the mean thickness of the buried oxide layer

is from 50 nm to 500 nm. Following is an example

specification: Mean buried oxide layer thickness for the

five sites will be 380 nm ± 20 nm with acceptable

goodness-of-fit at all five sites. Buried oxide uniformity

will be less than ± 10 nm.

7.2 Crystallographic Defect Measurements - Test

Methods — The evaluation of threading dislocation

density in the top SI layer will be made by destructive

chemical etching and microscopic etch pit density

measurements. The appropriate evaluation procedure

for given SIMOX wafer, which depends on the

threading dislocation density and the thickness of top Si

and buried oxide layers, will be determined by the

agreement between user and supplier.

7.2.1 Optionally, other crystallographic defects which

may be formed in the top Si layer of SIMOX wafers,

such as oxide precipitates or micro stacking faults,

silicon crystal defects induced by surface particles prior

to epi growth when epi is used to increase the thickness

of the top silicon. A sampling plan should be

established based on experience with the supplier.

7.2.2 Following are examples of the evaluation of

threading dislocations in two kinds of SIMOX wafers:

7.2.2.1 Example 1. Threading dislocation evaluation in

SIMOX wafers with 200 nm thick top Si layer and 400

nm thick buried oxide: Samples are handled with plastic

tweezers throughout the etching procedure. Samples are

first stripped of native oxide by dipping in Bell 2 or HF

stripping solutions. Immediately after stripping, wafers

are dipped in freshly prepared standard Secco Etch: one

part (by volume) of a 0.15 molar solution of K

distilled water and two parts HF (49%). Samples are

dipped in the Secco etch until 50 nm of silicon remains,

and then they are rinsed thoroughly in distilled water

and blown dry. The thickness of the remaining silicon

ensures that stacking fault pyramids found in multiple

implant material are counted. The threading dislocation

density may vary over a wide range, depending on the

type of material. The Secco Etch, as described, creates

etch pits that appear as dark circles roughly 50 nm in

diameter, and pictures should be taken at 2500X to

20,000X magnification in order to unambiguaously

identify and count the etch pits. This is suitable for high

density defect samples. For lower dislocation densities

typical of recent SIMOX material, a third etch in

buffered HF (1 HF (49%): 6 NH

F (40%)) for 10

minutes will etch the buried oxide under each Secco

etch pit, creating a char-acteristic circular shape 2 µm

in diameter that can be seen at 500X magnification.

Figure 2

Calibration Curves Relating Reflectance Measurements to Ellipsometry Measurements for Top Silicon and

Buried Oxide Layer Thickness

NOTE: Dashed lines are at ± 1 sigma and ± 3 sigma.