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SEMI C3-0699 © SEMI 19 86, 1999 10 2. What is t he effect of deep levels and intracenter transitions on the intensit y ratios? 3. What is the effect of crystallographic defects? Of point defects ? And of surf ace defects…

SEMI C3-0699 © SEMI 1986, 19999

Table 3 Detectors for Gas Chromatography

Detector Type Temp

Limit

Analytes

Analyzed*

Carrier Gases Selectivity Detectability Linear

Range

Thermal Conductivity

(TCD)

U 400 CF

, PH

, SF

Fixed Gases

He, H

, Ar, N

N/A

4 × 10

-10

g/mL

>10

Ultrasonic (USD) U CO

, O

, N

He, H

, Ar, O

, N

Air

N/A

1 × 10

-9

g/mL

Helium Ionization

(HID)

U 325 H

, O

, Ar,

, CO

He N/A

4 × 10

-14

g/mL

Discharge Ionization

(DID)

U same as HID He N/A

1 × 10

-12

g/mL

Flame Ionization

(FID)

S 420 C

-C

, C

, CO,

/Air**, He, N

Hydrocarbons

2 × 10

-12

g/sec

>10

Flame Photometric

(FPD)

S 420 H

/Air**, He, N

S/C 10

-10

P/C > 10

S<1 × 10

-11

g/mL

P<1 × 10

-12

g/mL

S ≥ 10

P ≥ 10

Reduction Gas

(RGD)

S 300 CO, H

, Ar, He, Air H

, CO

0.5 – 2 ppb

CO 0.5 - 4 ppb

-10

Electron Capture

(ECD) (Ni

63)

S 420 N

, P5 halogens

5 × 10

-15

(linearized)

Photoionization (PID) S 350

, H

AsH

Ar, H

, N

By ionization

energy

2 × 10

-13

g/sec

>10

Thermionic (TID) S 420 H

, 8% H

/He

N/P 1:5

N/C 5 × 10

P/C 10

N 1 × 10

-13

g/sec

P 5 × 10

-14

g/sec

Mass Spectroscopy

(MS)

S 320 He, H

variable with

mass range

EI: 10-100 pg

NICI: variable as low as

25 fg

-10

Mass Density (MSD) S

Atomospheric

Pressure Mass

Spectroscopy

(APIMS)

*this does not represent detector capability; rather those analytes analyzed by current SEMI procedures

**used as combustion gases

7.2.4 Photoluminescence Spectroscopy (PLS) of

single crystalline silicon specimens at liquid helium

temperatures exhibits sharp emission lines. Electron

hole pairs called excitons are formed by incident

radiation (514.5 nm line from argon laser). The decay

of these “exciton or multi-exciton” states yields the

lines characteristic of the impurity atoms. For elemental

detection, the observed emission lines must be assigned

as being extrinsic (due to impurities) or intrinsic (due to

silicon). The luminescence process itself appears too

complex to attain precision or accuracy from basic

calculations. The PLS emissions are analyzed as a

function of wavelength, suggested to be 1077 to 1139

nm, for the elemental determination of shallow donors

and acceptors in silicon. Quantitative measurements of

impurities are based on the extrinsic-to-intrinsic

intensity ratio and compared to standards. The PLS

emissions are analyzed using a monochromator and

detected by a photomultiplier. Sensitivity in the range

of 5 × 10

to 5 × 10

atoms/cc has been claimed for B,

Al, P, and As.

7.2.4.1 In order to obtain reliable mea surements, the

silicon specimens must be in single crystalline form

with a mirrored surface. Epitaxial wafers with layers of

thickness greater than 10 microns are suitable, since the

sample's depth appears to be less than 5 microns.

However, it is still in the development stage, and very

few laboratories have such capability. A number of

questions remain unanswered, including the following:

1. What is the effect of compensating impurities?

SEMI C3-0699 © SEMI 1986, 1999 10

2. What is the effect of deep levels and intracenter

transitions on the intensity ratios?

3. What is the effect of crystallographic defects? Of

point defects? And of surface defects?

4. Interference from other shallow impurities may

mask the detection of very low-level impurities.

For instance, PLS signal of Al at 1078.5 nm may

be affected by strong signals from As at 1079.0 nm

or at 1078.2 nm.

7.2.4.2 CVD-deposited films in single crystalline form

can be used for elemental detection of donors and

acceptors for gaseous silicon compounds. However,

PLS does not detect carbon, oxygen, or nitrogen. It is

also not useful for metallic impurities. That is, in

general, PLS is useful only for a very limited number of

impurities.

7.2.5 Based on this analysis, it was r ecommended that

the SEMI Gases Analytical Procedures Subcommittee

should wait for further development of innovative and

improved analytical techniques for the quantitative

determination of low-level elemental impurities based

on FTIR and PLS. Meanwhile, techniques such as ICP,

AA, and Ion Chromatography may be applicable to a

very large number of impurities of interest to the silicon

semiconductor industry. Development and adaptation of

these technologies should be encouraged.

7.2.6 AA (Atomic Absorption Spectrophotometry),

ICP (Inductively Coupled Plasma Emission

Spectrometry), and IC (Ion Chromatography) are

probably the most valuable instrumental techniques for

the detection and analysis of trace elemental impurities

in aqueous media. These techniques usually require that

the elements be dissolved in an aqueous medium. Due

to this fact, and the fact that the elemental impurities

are typically present in silane and the chlorosilanes at

the ppb and sub-ppb levels (which is beyond the

detection limits for these techniques for most elements)

it is usually necessary to concentrate the impurities

prior to analysis, such as by passing the gas through a

series of impingers (i.e., gas bubblers) containing either

DI water or, more commonly, acidic solutions. From

analyses of the impinger solutions together with

knowledge concerning impinger solution volume(s) and

both the flow rate and period of flow of the gas, the

concentration of the respective elements in the original

gas should be readily calculated to a fairly high degree

of accuracy.

7.2.6.1 Atomic Absorption Spectroph otometry can be

used for elemental analyses for a multitude of elements

in the periodic table at detection limits ranging from

ppm to ppb, depending both on the exact configuration

or setup and the element being analyzed. The principle

of the technique involves conversion of an incoming

aerosol into an atomic vapor which can absorb light

from a primary light source, usually a hollow cathode

lamp or an electrodeless discharge lamp (EDL), the

latter providing improved sensitivity and lower

detection limits for certain elements. In addition, a

flameless (i.e., graphite furnace) technique can be used

to increase sensitivities and detection limits 50 to 100

times better.

7.2.6.2 Complementary to Atomic Ab sorption Spec-

trophotometry is Flame Emission Spectrophotometry,

which differs in the absence of a primary light source.

Again, a flame is utilized to convert an incoming

aerosol into an atomic vapor. However, the flame also

thermally elevates the atoms to an excited electronic

state, and, when the atoms return to the ground state,

they emit light detectable by a photomultiplier. This

technique is ideal for the analysis of Li, Ha, and K at

ppm and sub-ppm levels.

7.2.6.3 Inductively Coupled Plasma E mission

Spectrometry (ICP) is rapidly replacing Atomic

Absorption Spectrophotometry for elemental analyses

of liquids, since approximately 70% of the elements in

the periodic table can be determined with better

precision and equal or better sensitivity with minimal

sample preparation. Detection limits, in fact, for this

technique range from ppm to sub-ppb levels, depending

both on instrumental and/or experimental conditions

and the element being analyzed. ICP involves

nebulizing liquid samples either directly or in diluted

form into a spray chamber where a stream of argon gas

carries the smaller sample droplets into the axial

channel of an argon plasma which reaches temperatures

of 4000-8000°K. The common technique employs an

optical system for detection. Newer techniques are

being investigated using a mass spectrometer as a

detection system coupled to an ICP excitation source.

This is referred to as ICP/MS.

7.2.6.4 Ion Chromatography (IC) uses the principle of

ion exchange to separate anionic and cationic species at

detection limits ranging from ppm to ppb, depending

again both on the instrumental and experimental

conditions and on the element being analyzed.

7.2.6.5 Detection of ionic species in IC is most

commonly accomplished by an electrical conductivity

detector capable of measuring most non-transition

metal cations and anions to sub-ppm levels. An

electrochemical detector (for amperometric detection)

is available for measuring any electroactive species

having an oxidation/reduction potential near the applied

electrochemical cell potential. This latter detector is

capable of measuring many ionic species at ppb levels

(e.g., Br

–

, CN

–

, S

–

, etc.).

7.2.6.6 Of the three aforementioned techniques,

Atomic Absorption Spectrophotometry, Inductively

Coupled Plasma Emission Spectrometry, and Ion

Chromatography, the most sensitive method of

analyzing for a multitude of elements is, without doubt,

graphite furnace (i.e., flameless) Atomic Absorption

Spectrophotometry. However, this instrumental

technique suffers the disadvantage of being slow and

tedious. Consequently, Inductively Coupled Plasma

Emission Spectrometry is rapidly replacing Atomic

Absorption Spectrophotometry for elemental analyses

of liquids since it is a reasonably fast technique

requiring minimal sample preparation with practical

detection limits being superior to those of standard (i.e.,

flame) Atomic Absorption Spectrophotometry for most

elements and approaching those of graphite furnace

Atomic Absorption Spectrophotometry for many of the

elements. An added attractive feature of Inductively

Coupled Plasma Emission Spectrometry is that

eventually it may be used to analyze for elements

directly in gases. Preliminary work, in fact, is currently

being conducted in this area by several research groups.

Although Ion Chromatography can be used to analyze

for a multitude of non

transition metal cations (e.g., Li

, K

, Rb

, Cs

, Mg

, Ca

, Sr

, etc.) it is

limited by the fact that currently only a few transition

metals (e.g., Ni

, Cu

, Co

, Zn

, Fe

, Cd

etc.) can be analyzed by this instrumental technique. IC,

however, has the advantage in that it can be used for

analyzing for complex cationic and especially anionic

species directly (e.g., NH

, NO

, SO

-2

-3

, OC1

, SCN

, CN

, CO

-2

, etc.) and for different

oxidation states for certain elements (e.g., Fe

and

), unlike the other techniques, which are strictly

limited to elemental analyses only. Furthermore, IC is

extremely valuable for analyzing ionic species,

particularly anions (e.g., F

, Cl

, etc.) of many common

elements which otherwise cannot be analyzed directly

by either Atomic Absorption Spectrophotometry or

Inductively Coupled Plasma Emission Spectrometry.

8 Standards

8.1 Certified Standards shall be m ade by weight

traceable to the National Institute of Standards and

Technology.

9 Determination of Precisio n

9.1 To Be Determined.

10 Safety

10.1 Because of the continuing evolution of safety

precautions, it is impossible for this publication to

provide definite statements related to the safe handling

of individual chemicals. The user is referred to product

labels, product data sheets, government regulations, and

other relevant literature.