semi合集-English.pdf - 第7509页

SEMI MF1724-1104 © SEMI 2004 4 7.3 Cl eanroom Cl othing — Analysts must be attired in full clean room cl othing, including hoods, masks, boots, and gloves. Cleanroom practices m ust be observed. 7.4 Acid Exhaust Fume Hoo…

SEMI MF1724-1104 © SEMI 2004 3

5.2.3 digested blank — samples of acid, with no

analytes added, taken through the digestion process and

analyzed to provide a monitor of the analytical process

that includes acid purity, digestion bottles cleanliness,

cross contamination, and environment purity.

5.2.4 digested control standard — samples prepared to

known concentrations of the analytes to provide

calibration checks on the instrument and the digestion

procedure.

5.2.5 digestion — holding the polysilicon chunks in the

acid mixture at temperature until the surface metal

contaminants are dissolved into solution.

5.2.6 polytetrafluoroethylene — an HF-resistant

material for sample bottles, lids, and tongs.

5.2.7 standard samples — samples prepared to known

concentrations of the analytes, typically 5 ppbw, 10

ppbw, and 20 ppbw to provide a calibration standard

and set absorption values for the GFAAS instrument.

6 Summary of Test Method

6.1 A standard weight and volume of chunk sample is

chosen in order to provide a consistent basis for the

analysis and provide a basis for interlaboratory

correlation of analytical values. For referee purposes, a

total sample weight of 300 g, taken as six chunks, each

approximately 3 by 3 by 3 cm, at 50 g each, is

recommended. A minimum of three of the six pieces

should have an outside surface. The outside surface, or

skin of the polysilicon rod, is thought to be the most

susceptible to contamination during the rod removal

and chunk processing steps. Choosing a minimum of

half of the chunk samples to have an outside surface is

assumed to be representative of the lot characteristics.

6.2 Making sure to avoid cross contamination, the

chunks are loaded into clean PTFE bottles, covered

with acid etchant, heated in a fume hood and the

surface of the chunks etched. The chunks are removed

from the etchant, and the etchant heated to dryness on a

hotplate.

6.3 The dried etchant residue is dissolved by adding 2

mL of 5% HNO

and 8 mL of H

O to make a total of 10

mL. This extract is then analyzed by GFAAS for trace

metals.

6.4 The sample carrousel tray of the GFAAS

instrument is loaded with a series of blanks, calibration

standards, and monitor standards, along with the acid

extract samples. The temperature program for the

graphite furnace is optimized for maximum sensitivity

and the absorption spectral lines chosen for maximum

sensitivity and minimum interferences for each

element.

6.5 Data from the GFAAS instrument is collected and

the value for each analyte in the lot sample calculated.

For each analyte, the digested blanks are averaged to

provide a zero reference. This average value is

subtracted from the lot sample value, and the remaining

value multiplied by the dilution factor to obtain the

reported result. The dilution factor is the final volume

of the acid extract, 10 mL, divided by the starting

weight of the polysilicon sample.

6.6 This acid mixture has a measured recovery

efficiency of 95% or greater for iron, chromium, nickel,

sodium, zinc, aluminum, copper, calcium, and

potassium, in the chemically-bound form usually found

on the polysilicon surface, after one etch cycle. By

measuring the analytes after a second etch cycle, the

recovery efficiency of the first cycle is determined.

Recovery efficiencies above 90% are required to verify

the accuracy of the analysis. Recovery efficiency was

confirmed by neutron activation analyses.

To ensure

that no analytes are lost through chemical reaction or

evaporation processes, digested control standards are

prepared and monitored for each analyses.

6.7 Contamination from the room environment,

apparatus, reagents, sampling techniques, and handling

techniques is monitored statistically by the absorption

values for the digested blanks and digested control

standards.

6.8 The detection limit depends on the dilution factor,

instrument sensitivity, spectral response of the analyte,

acid recovery efficiency, blank value, and method

interferences. Instrument and method variations are

given in Section 14.

7 Apparatus

7.1 GFAAS Instrument — Graphite furnace atomic

absorption spectrometer, with sufficient resolving

power to perform elemental analysis at the sub-ppb

level. A sample tray with selective sampling capability

is integral to the instrument. A computerized data

system calculates peak absorbance values, provides the

instrument calibration curve, and reports sample values.

7.2 Air Environment — The area for sample collection,

acid extraction, and GFAAS analysis must be enclosed

in a clean room with a minimum standard of Class 6 as

defined in ISO 14644–1.

NOTE 1: This class is about the same as Class 1000 as

defined in Federal Standard 209E.

6 Maurits, J. E. A., Dawson, H. J., and Barker, T. H., “The

Analysis of Surface Metals in Polycrystalline Silicon,” Abstract No.

401, Extended Abstracts, Vol 94-2, Fall Meeting of the

Electrochemical Society, October 9–14, 1994, Miami Beach, FL.

SEMI MF1724-1104 © SEMI 2004 4

7.3 Cleanroom Clothing — Analysts must be attired in

full clean room clothing, including hoods, masks, boots,

and gloves. Cleanroom practices must be observed.

7.4 Acid Exhaust Fume Hood — An acid exhaust fume

hood, equipped to provide a clean air (ISO Class 6

minimum, Note 1) environment, and with hot plates for

the acid extraction and extract concentration steps.

7.5 Sample Bottles and Tongs — Sample bottles, size

500-mL, lids, and tongs are made of polytetra-

fluoroethylene (PTFE) or similar polymer material that

will not be attacked by hydrofluoric acid and can be

cleaned to avoid contamination interferences.

7.6 Analytical Balance — Balance capable of weighing

300 g to an accuracy of 0.01 g.

8 Reagents

8.1 Purity of Water — Reference to water shall be

understood to mean Type E-1 or better water as

described in ASTM Guide D 5127.

8.2 Nitric Acid (HNO

) — 65%, in accordance with

Grade 2 of SEMI C35.

8.3 Hydrofluoric Acid (HF) — 48%, in accordance

with Grade 2 of SEMI C28.

8.4 Hydrogen Peroxide (H

) — 30%, in accordance

with Grade 2 of SEMI C30.

8.5 Acid Cleaning Mixture — (1:1:1:25)

HNO

:HF:H

8.6 Acid Etching Mixture — (1:1:1:50)

HNO

:HF:H

9 Sampling

9.1 This test method is intended for sampling of

polysilicon lots. Typically, one 5-kg bag from a lot is

selected for sampling. The sample is taken from this

bag, and the surface metals values analyzed are

assumed representative of the lot. Statistical

techniques, such as process capability ratios, are used to

determine actual sampling plans for manufacturing

processes. A choice of sampling plans is found in

ASTM Practice E 122. For referee purposes, a

sampling plan shall be agreed upon before conducting

the test. Contamination during the sampling procedure

is a critical concern and must be avoided.

10 Calibration

10.1 Prepare calibration and control standards for each

analyte, according to instrument manufacturer's

instructions, from commercially-available atomic-

absorption elemental calibration standards, at 1000

ppm, traceable to NIST Reference Standards.

10.2 Prepare a series of calibration and control

standards for each analyte to determine the instrument

calibration curve, monitor recovery efficiency of the

acid etchant, and monitor reagent purity and purity of

the environment. Purpose of the calibration and control

standards is shown in Table 1.

10.2.1 Determine a calibration curve for each element

with a blank and a minimum of three elemental

standards. Dilute the 1000-ppm elemental standards to

1-ppm standards (0.1 mL/100 mL in acid etch mixture).

Prepare calibration standards of 5 ppb, 10 ppb, and 20

ppb by diluting 0.5 mL, 1 mL and 2 mL of the 1-ppm

standard to 100 mL with 18-mL acid etch mixture and

deionized (DI) water. Prepare the standards to provide

a range close to the suspected concentration of the

analyte. Determine the four-point calibration curve

according to the instructions of the instrument

manufacturer. Monitor any changes in the calibration

curve by checking the linearity against previous runs.

NOTE 2: These standards were chosen to provide calibration

for various samples over a range corresponding to 0.1 to 100

ppbw in the polysilicon sample. The use of a 1-ppb standard

will provide a lower method variation for analytes in the sub-

ppb range.

10.2.2 Collect the absorption values for the 5 ppb, 10

ppb, and 20 ppb calibration standards in a database, and

establish statistical rules to determine any drift or

excessive variation in the instrument readings. Repeat

the analysis if these values exceed the statistical limit.

Make corrections in the procedure or instrument if these

values continue out of statistical control.

10.2.3 Prepare two control standards, at 10 ppb, by

adding 10 mL of the 10-ppb calibration standards to

samples of clean polysilicon chunks. Digest these

standards along with the lot samples.

NOTE 3: These standards have a similar matrix to the lot

samples. The analysis of these standards provides a

quantification of the variation in metals retention efficiency,

instrument matrix effects, and variations due to cleanliness of

apparatus and contamination during handling and sample

processing.

10.2.4 Collect the absorption values for the 10-ppb

digested control standards in a database, and establish

statistical rules to determine if metals retention and

variations due to method interferences are in statistical

control. Repeat the analysis if these values exceed the

statistical limits. Make corrections in the procedure or

improvements in cleanliness of the apparatus and room

if these values continue out of statistical control.

Table 1 Sample Cup Loading Protocol with Function of Calibration and Control Standards

Sample Cup Function

Acid Blank

5-ppb calibration standard

10-ppb calibration standard

20-ppb calibration standard

To provide a 4-point calibration curve in the range of the analyte and set the 5-ppb

instrument absorption value.

Two 10-ppb digested control standards To monitor the dissolved metals retention of the method and quantify variations in the

method.

Two digested blanks, acid etch mixture To monitor reagent purity and purity of the environment and to provide a zero reference

for the lot sample.

Lot samples To determine concentrations of the analytes.

10.2.5 Prepare two acid blanks and digest along with

the lot samples. Measure these blanks to provide a

quantification of variations in reagent purity and

contamination that occurs from the room environment

during processing. Enter the values for the blanks into

a database and establish statistical rules to determine

any excessive variation in the current value. Repeat the

analysis if the current value is out of statistical control.

10.3 For purposes of interlaboratory correlation and

comparison of techniques, definitions of instrument

variation and this test method variation and detection

limit are as follows:

10.3.1 Measure the instrument variation by analyzing a

2-ppb standard for 15 runs, calculating the standard

deviation of the 15 values, then calculating the 3-sigma

value for each element. Multiply the 3-sigma value by

the dilution factor to define the instrument variation.

NOTE 4: Concentration of standards are chosen based on

instrument manufacturer recommendations for signal/noise

ratio and sensitivity values.

10.3.2 Measure the method variation by analyzing a

minimum of 30 values for the 10-ppb digested control

standards. Calculate the standard deviation for these 30

values and multiply the 3-sigma value by the dilution

factor for each element to define the method variability

of this test method.

NOTE 5: Method variation can be reduced by using lower

concentration control standards and by making improvements

in the procedure, room cleanliness, and acid purity. These

improvements are monitored by continually tracking the 3-

sigma values for the digested control standards.

10.3.3 Measure the detection limit by analyzing a

minimum of 30 values for the digested acid blanks.

Calculate the standard deviation for these 30 values and

multiply the 3-sigma value by the dilution factor for

each element to define the detection limit of this test

method.

11 Procedure

11.1 Clean bottles, lids and tongs between analyses.

Conduct additional cleaning and analyses of blanks

when using new bottles, when blank values indicate

contamination, and when performing referee analyses.

Prepare clean bottles and lids as follows:

11.1.1 Rinse three times with DI water.

11.1.2 Fill the bottles with 500 mL of the acid cleaning

mixture and heat on the hot plate, lids fit loosely, for 6

h at 100°C; refill the bottles with the acid cleaning

mixture and heat on the hot plate, no lids, for 6 h at

100°C.

11.1.3 Rinse bottles and lids three times with DI water;

11.1.4 Fill bottles with 250-mL acid etch mixture, no

lids, and heat at 130 to 150°C until dryness, about 10 h.

11.1.5 Dissolve residue with 2 mL of 5% HNO

and 8

mL DI water; analyze using the GFAAS instrument.

11.1.6 Repeat entire procedure if the GFAAS analysis

shows contaminants are present in the sample bottles.

11.1.7 Triple rinse bottles and lids.

11.1.8 Fill bottles with acid cleaning mixture and heat

for 6 h at 100°C.

NOTE 6: Other cleaning procedures may be used, but must

be monitored to prove effectiveness.

11.2 Choose one 5-kg bag of polysilicon from a lot to

be sampled. Deliver the bag to the clean room for

sampling. Choose six chunks from the bag as the lot

sample. At least three of the six chunks must have an

outside surface. Each chunk shall be about 50-g

weight, with an approximate size of 3 by 3 by 3 cm,

total weight about 300 g. If the sample must be taken

in a location other than the analytical laboratory, seal

the sample in double bags and send to the laboratory.

To minimize contamination from the room environment

and the analyst, all sample collection and