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SEMI MF2139-1103 © SEMI 2003 4 [ N b ] = contribution of backgro und nitroge n to the observed nitroge n concent ration under the higher sputter rate conditions, [ N B ] = contribut ion of backgr ound nitrogen to the obs…

SEMI MF2139-1103 © SEMI 2003 3

NOTE: Top curve (

!) Mass 30 (

Si); middle curve (!)

Mass 42 (

Si), bottom curve (

") Mass 40 (

Si)

Figure 1

SIMS Signals in Nitrogen-Doped Silicon, Log Scale

6.4 For the analysis of the test specimens, a cesium

primary ion beam is used to bombard each test

specimen at two different sputter rates, by reducing the

beam raster area for the second sputter rate, while at the

same time maintaining a constant detected area. This

beam raster change technique is used for two reasons.

Firstly, this technique reduces the background nitrogen

contribution from the sample surface. Secondly, this

technique allows the determination of both the

instrumental nitrogen background and the real bulk

nitrogen concentration in each test specimen

measurement without the use of a separate blank

specimen, and due to averaging is capable of separating

the bulk nitrogen from the instrumental nitrogen even if

the bulk nitrogen is less than the instrumental

nitrogen.

6.5 The two sputter rates, and the amount of time they

are applied in each measurement, are chosen to

optimize the detection capability; the values depend on

the instrumentation being used. However, the second

sputter rate is typically the maximum sputter rate of the

instrumentation, and the first sputter rate is typically

less than half that of the second sputter rate.

6.6 The negative secondary ion of

Si is mass

analyzed by a mass spectrometer, and detected by an

electron multiplier (EM) or equivalent high-sensitivity

ion detector as a function of time. The matrix negative

secondary ion count for silicon (

Si,

Si, or a

10 Ishitani, A., Okuno, K., Karen, A., Karen, S. and Soeda, F.,

“Improvement of Oxygen Detection Limit in Silicon by Use of the

Secondary Ion Energy Distribution and Background Subtraction,” in

Proceedings of the International Conference of Materials and

Process Characterization for VLSI, 1988 (ICMPC'88), edited by X-F

Zong, Y-Y Wang, and J. Chen, (World Scientific, New Jersey, 1988)

pp. 124-129.

dimer such as

) is measured by a Faraday cup (FC)

or appropriate detector during the profile. If multiple

detectors are used during the test, the relative

sensitivities of the detectors are determined by

measuring standard ion signals (either the same

negative secondary ion count rate or ion count rates of

known relative intensity such as natural

Si/

Si) on

each detector. Additional negative secondary ions of

Si,

C, and

Si may also be detected depen-

ding upon the boron and carbon concentration in the

test specimen.

6.7 For illustration purposes an example data profile is

shown in Figure 1 where the log of counts of

Si,

Si, and

Si is plotted versus sputter time. At

about 200 s, the Cs beam raster area was changed from

250 µm × 250 µm down to 50 µm × 50 µm, resulting in

an increase in signal intensities, because the volume of

material analyzed per second has increased. At about

350 s, the Cs beam raster area was reversed back to the

first condition, and the signal intensities are seen to be

reduced back to the original levels. Since the surface

analysis area was kept constant during the raster change

(that is, the actual surface area detected is independent

of the raster area), the signal intensities have two

separate contributions, one from elements that are

adsorbed to the surface and one from elements in the

volume of material analyzed each second. There is

some nitrogen (and carbon in this case) that is adsorbed

to the surface from the local environment outside of the

silicon, and there is some nitrogen that is from the bulk

silicon. A key point is that if there is no nitrogen in the

bulk silicon, the nitrogen signal intensity does not

change as a result of the change in sputter rate, because

the adsorbed nitrogen (that is, instrumental nitrogen

signal) is only dependent on the analysis area, which

has been held constant. This understanding leads to the

following equations:

][][ NNRSF

+=× (1)

][][ NNRSF

+=× (2)

RSF

×=][ (3)

RSF

×=][ (4)

where:

= secondary ion intensity of

Si under the

higher sputter rate conditions,

= secondary ion intensity of

Si under the

lower sputter rate conditions,

SEMI MF2139-1103 © SEMI 2003 4

] = contribution of background nitrogen to the

observed nitrogen concentration under the

higher sputter rate conditions,

] = contribution of background nitrogen to the

observed nitrogen concentration under the

lower sputter rate conditions,

= secondary ion intensity from the adsorbe

ackground nitrogen under both sputter rate

conditions,

= secondary ion intensity of the silicon matrix

(e.g.,

Si) under the higher sputter rate

conditions,

= secondary ion intensity of the silicon matrix

(e.g.,

Si) under the lower sputter rate

conditions,

RSF = relative sensitivity factor for converting the ion

intensity ratio to concentration, and

[N] =

ulk nitrogen concentration in the test

specimen.

6.8 Equations (1) through (4) can be used to determine

[N] and [N

] from measured data as follows:

)/(1

][

siSI

RSFN

−

××= (5)

][][ N

RSFN

−×= (6)

In these equations, the secondary ion intensities used

are averages of these signal intensities. In the example

shown in Figure 1, the lower sputter rate was used at

the beginning of the profile, and the higher sputter rate

in the middle of the profile. For this example, [N] was

determined to be 8 × 10

/cm

and [N

] was 2 ×

/cm

. The use of averages for I

and I

particularly critical for detecting low levels of nitrogen

where the signal intensity count rates can be low and

therefore noisy. By using averages, it is possible to

detect an [N] level that is less than [N

] in the analysis.

6.9 The SIMS measurement of nitrogen in bulk-doped

silicon sometimes reveals anomalous intensity spikes

that are greater than a random fluctuation of signal

intensity, even in silicon that has not been thermally

processed beyond crystal growth.

6,7

These nitrogen

intensity spikes are not well understood, but are

suspected to be associated with oxygen defects,

possibly a co-aggregation of nitrogen and oxygen in a

precipitate form. Similar SIMS effects have been well

known for oxygen and even carbon in thermally-

processed silicon.

Figure 2 shows an expanded view

of the carbon and nitrogen data taken from Figure 1.

11 Hockett, R. S., Fraundorf, P. B., Reed, D. A., and Wayne, D. R.,

“Oxygen and Carbon Defect Characterization in Silicon by SIMS,” in

Oxygen, Carbon, Hydrogen and Nitrogen in Crystalline Silicon,

edited by J. C. Mikkelsen, Jr., S. J. Pearton, J. W. Corbett, and S. J.

Pennycook, Materials Research Society Symposia Proceedings Vol

59 (Materials Research Society, Pittsburgh, PA, 1986) pp. 433-438.

The vertical scale is linear. The unusually high

nitrogen signal at about 325 s may be one of these

anomalous intensity spikes. The presence of such non-

random signals, if significant, can make the averaging

method used here questionable.

NOTE: Top curve (

!) Mass 42 (

Si), bottom curve

(

") Mass 40 (

Si)

Figure 2

SIMS Signals from Nitrogen-Doped Silicon, Linear

Scale

6.10 Depending upon the carbon concentration in the

test specimen and the level of nitrogen, it may be

necessary to correct the

Si signal for an interfer-

ence from

Si. This correction is done by co-

detecting

Si at mass 40, and calculating the

contribution of

Si from isotopic ratios. The

contribution to mass 42 from a carbon-related signal at

mass 40 is 0.0341 times the signal at mass 40. In the

example in Figure 1, where the actual carbon concen-

tration is about 1.5 × 10

/cm

by SIMS calibration, the

carbon interference is insignificant. A carbon concen-

tration of 5 × 10

/cm

(the detection limit of most

Fourier Transform Infrared (FITR) measurements of

carbon in silicon) corresponds to an interference equi-

valent to about 8 × 10

/cm

of nitrogen. Thus, in

detecting nitrogen in the low 10

/cm

level, a co-

measurement of the carbon interference is critical.

6.11 Depending upon the boron concentration in the

test specimen, the procedure to correct for the carbon

interference may need to be modified by detecting

Si at mass 39, and calculating the contribution of

Si and

Si at mass 40. This procedure

becomes important for test specimens with boron

concentrations at 1 × 10

/cm

and greater. In the

example in Figure 1, the boron concentration is on the

order of 10

/cm

and does not interfere.

7 Apparatus

7.1 SIMS Instrument — Equipped with a cesium

primary ion source, electron multiplier detector, and

Faraday cup detector, capable of measuring negative

secondary ions. The SIMS instrument should be

adequately prepared (that is, baked) so as to provide the

lowest possible instrumental background, since the

instrumental background can affect the detection

capability. High quality vacuum is desired. A liquid

nitrogen- or liquid helium-cooled cryopanel, which

surrounds the test specimen holder in the analysis

chamber, may be helpful.

7.2 Test Specimen Holder

7.3 Oven — For baking the test specimen holder.

7.4 Stylus Profilometer — Or equivalent device to

measure SIMS crater depths. This device is required to

calibrate depth scale for concentration profiles of

calibration standard samples.

8 Reagents and Materials

8.1 No reagents or materials are required for this test

method.

9 Safety Precautions

9.1 The preparation of silicon test specimens from

silicon substrates requires the crystal to be cleaved or

broken. This procedure may generate very sharp

silicon shards. Care must be taken to protect the body

and particularly the eyes from these shards.

10 Sampling, Test Specimens, and Test Units

10.1 Since this procedure is destructive in nature, a

sampling procedure must be used to evaluate the

characteristics of a group of silicon wafers. No general

sampling procedure is included as part of this test

method, because the most suitable sampling plan

depends upon individual conditions. For referee

purposes, a sampling plan shall be agreed upon before

conducting the test. See ASTM Practice E 122 for

suggested choices of sampling plans.

10.2 The calibration or reference material shall be

silicon crystal with boron concentration below 2 × 10

atoms/cm

and implanted with ions of

N or

N; an

ion implant of energy 200 keV and dose of about 1 ×

atoms/cm

is preferred.

10.3 Sample specimens must have a chemo-mechanical

polished surface on the side used for analysis.

11 Calibration and Standardization

11.1 Calculate the RSF for an element in a matrix

experimentally from the SIMS profile of a reference

material containing a known areal density of the

impurity of interest as follows:

)( nISId

tInD

RSF

−

= (7)

where:

D = a real density of the impurity, atoms/cm

n = number of data cycles in the profile,

= matrix isotope secondary ion intensity, counts/s,

d = sputtered depth, cm,

= sum of the impurity isotope secondary ion

(counts over the depth of the profile),

= constant background intensity of the impurity

isotope, counts/cycle and

t = analysis time for the species of interest, s/cycle.

11.2 Calculate the calibration of concentration in the

SIMS profile as follows:

RSF

××= (8)

where:

= impurity atomic concentration, atoms/cm

= isotope secondary ion intensity, counts/s,

= matrix isotope secondary ion-intensity,

counts/s,

RSF = relative sensitivity factor, atoms/cm

, and

= ratio of matrix intensities on the Faraday

cup (FC) to the electron multiplier (EM),

when two detectors are used.

11.3 Complete the conversion of data cycles or time

into depth by measuring the crater depth and the total

time (or data cycles) of the sputtering that formed the

crater.

NOTE 1: The depth scale is assumed to be linear with time or

data cycles.

12 Procedure

12.1 Cleave or break specimens from the silicon

substrates in order to fit within the SIMS sample holder.

Specimens must be handled under normal analytical

laboratory practices, but no special ambient is required.

12.2 Load the specimens into the SIMS sample holder,

checking to see that the specimens are flat against the

backs of the windows and cover the windows as much

as is possible. A specimen load includes one

calibration specimen, and one or more test specimens.

12.3 Bake the loaded sample holder at 100 ± 10° C for

a minimum of 1 h in air.

12.4 Turn on the instrument in accordance with the

manufacturer' s instructions.