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SEMI MF1618-1104 © SEMI 2004 1 SEMI MF1618-1104 PRACTICE FOR DETERMINATION OF UNIFORMITY OF THIN FILMS ON SILICON WAFERS This guide was technically approved b y the Global Silicon Wafer Com mittee and is the direct respo…
SEMI MF1617-0304 © SEMI 2003, 2004 6
calculated as 2.8 times their respective standard
deviations (s
r
, S
R
).
14.1.1 SIMS measurements were made on samples
taken from an iron contaminated silicon wafer to
estimate the precision of the surface iron measurement.
Two SIMS instruments were used: a CAMECA IMS 3f
and a CAMECA IMS 4f. All measurements used a 3-
keV oxygen ion beam with an oxygen flood.
Measurements were taken over a one-year period. For
the CAMECA IMS 3f, 23 measurements were taken
with an average iron reading of 1.51 × 10
11
atoms/cm
2
and a 1 standard deviation of 0.148 × 10
11
atoms/cm
2
.
For the CAMECA IMS 4f, 20 measurements were
taken with an average iron reading of 1.71 × 10
11
atoms/cm
2
and 1 standard deviation of 0.188 × 10
11
atoms/cm
2
.
14.2 Bias — Bias cannot be estimated because there
are no accepted absolute standards. However, for
comparison only, the VPD/AAS results for the sodium,
aluminum, and potassium test samples are included in
Tables 1–3. A correlation between SIMS and TXF
quantitive results for iron is shown.
8
15 Keywords
15.1 aluminum; iron; potassium; silicon; SIMS;
sodium; surface contamination
Table 1 Summary Statistics for Sodium (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 10–15 10.68 2.240 2.791 6.273 7.816
B 29–32 33.64 3.339 4.931 9.348 13.81
C 115–121 112.1 16.19 19.79 45.32 55.41
E 0.6–4 0.665 0.359 0.861 1.005 2.410
Table 2 Summary Statistics for Aluminum (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 3 3.504 0.6865 1.107 1.922 3.100
B 7–8 9.425 0.7684 2.239 2.152 6.269
C 22–25 28.80 2.298 6.107 6.434 17.10
E not detected 0.795 0.390 0.760 1.093 2.128
8 Smith, S. P., and Metz, J., “Understanding the Correlation of
Surface SIMS and TXRF Measurements of Surface Metal
Contamination on Silicon Wafers,” Science and Technology of
Semiconductor Surface Preparation, edited by G. S. Higashi, M.
Hirose, S. Raghavan, and S. Verhaverbeke, Material Research
Society Symposium Proceedings, Vol 477 (Materials Research
Society, Pittsburgh, PA, l997) pp. 305–310.
Table 3 Summary Statistics for Potassium (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 7–8 7.829 1.642 2.761 4.596 7.731
B 22–23 24.11 3.607 5.943 10.10 16.64
C 92 82.37 12.77 19.63 35.74 54.97
E 0.1–2 0.407 0.162 0.344 0.454 0.965
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SEMI MF1618-1104 © SEMI 2004 1
SEMI MF1618-1104
PRACTICE FOR DETERMINATION OF UNIFORMITY OF THIN FILMS
ON SILICON WAFERS
This guide was technically approved by the Global Silicon Wafer Committee and is the direct responsibility
of the North American Silicon Wafer Committee. Current edition approved for publication by the North
American Regional Standards Committee on August 16, 2004. Initially available at www.semi.org
September 2004; to be published November 2004. Original edition published by ASTM International as
ASTM F 1618-95. Last previous edition SEMI MF1618-02.
1 Purpose
1.1 The fabrication of semiconductor, dielectric, and
metal thin films is a critical part of silicon integrated
circuit production. The variation of film properties
across the surface of a wafer can have significant
impact on the further processing of the wafer and on the
ultimate yield of acceptable chips from the wafer, as
well as on their reliability.
1.2 The purpose of this practice is to promote
commonality of approach to the analysis of uniformity
among all parties needing to generate or assess such
information, including manufacturers of the basic test
instrumentation to be used.
1.3 This practice is intended for process control,
research and development, and process equipment
evaluation purposes. It is intended for the benefit of
semiconductor device and equipment manufacturers
alike so that acquisition, reduction, and communication
of thin film data is consistent among various parties
who may need to concur on interpretation of the results
of a thin film fabrication-process step.
1.4 Measurement of the uniformity of one or more thin
film properties such as thickness, sheet resistance,
reflectivity, dielectric constant or index of refraction
enables the monitoring of a critical aspect of the results
of a given process step. This information can be used
to determine the behavior of individual process steps; it
can be used with similar information from earlier
process steps to determine their interaction with respect
to final product uniformity. Also, it can be used in
conjunction with historical data from the same process
step to determine loss of control of the piece of
equipment or the process cycle being used with respect
to producing product having a required level of
uniformity. Further, it can be used to determine the
adequacy of new or modified process cycles, materials
or equipment for a given film deposition or film
modification requirement.
1.4.1 It is common practice to monitor the value of a
simple statistic, such as the standard deviation, that
results from layer uniformity measurement data and to
compare the current value of the statistic with historical
values for the same layer formation process. An
increase of the current value over historical values is
taken as an indication of possible deterioration of the
quality of the layer formation process. It is then
common to convert the data into a contour or similar
map of uniformity to aid in diagnosing changes in the
process that caused the increase in standard deviation
(or similar statistic). This practice does not treat the
interpretation of the statistic resulting from data
analysis, nor does it give procedures for converting the
data to a uniformity map.
1.5 The non-uniformities of a film property on a given
wafer are primarily systematic, not random, in their
spatial shape or distribution and arise from spatially
systematic variations in such process variables as
temperature, gas flow, pressure, or electric field. As a
result, the simple statistic standard deviation that is
specified for analysis of data acquired with this practice
will not generally have the normal interpretation for the
standard deviation of a sample from a random
population. It is a figure of merit for comparing data
sets of a similar type, but it cannot be used for
computing confidence or tolerance intervals.
2 Scope
2.1 This practice covers a set of site distribution
patterns for measuring the uniformity of a property of a
thin film on a silicon wafer, as well as simple
procedures for analyzing and reporting the results of
those measurements.
2.2 This practice is intended for use as a template for
the evaluation of the uniformity of intrinsic film
properties such as thickness or composition, and also
film functional characteristics such as sheet resistance
and reflectivity. The resulting information may be used
to assess the uniformity of the film itself or of the layer
formation process. This practice is not directly
applicable to evaluating wafer-to-wafer or lot-to-lot
variations.
2.3 This practice is intended for use with any thin film
or layer type, or formation technique, for which basic
measurement instrumentation and capability exists that,
is appropriate to the film parameter of interest. This
practice is intended for layer growth and deposition
techniques such as epitaxy, implantation, thermal and
SEMI MF1618-1104 © SEMI 2004 2
chemical vapor deposition (CVD) oxidation, and
metallization, as well as for layer modification such as
various means of layer etching.
2.4 This practice is intended for use with all silicon
wafer sizes and types when measuring uniformity of
film properties and characteristics. This practice
describes measurement site patterns and determination
of their spatial coordinates on the wafer, as well as the
statistics to be used when reducing the measurement
data to determine uniformity. For each of the sampling
plans, the exact number of measurement sites is chosen
based on the size of the wafer being used, the desired
spatial resolution of the measurement instrument, and
whether maximal, or somewhat lesser information
density is desired. However, in all such choices, the
pattern of measurement sites, the rules for selecting
their coordinates on the wafer, and the statistical
calculations of the results should remain consistent with
the procedures of this practice.
2.5 This practice can be used with any measurement
method, procedure or instrumentation that can measure
the needed film property or characteristic with
sufficient precision and spatial resolution to reveal the
needed information on spatial non-uniformity of the
film. This practice does not itself contain details on
performing any specific measurement.
2.5.1 Not all types of measurements that may need to
be used for evaluation of the uniformity of a thin film
have formal procedural standards. SEMI MF374,
SEMI MF576, SEMI MF1392, SEMI MF1393, and
SEMI MF1529 give details of measurement procedures
that may be applied to evaluating the uniformity of thin
film properties.
2.5.2 This practice does not deal with acquisition or
analysis of uniformity data where it is desired to take
more than one measurement per specified spatial cell
such as is commonly done for wafer site flatness
measurements.
2.6 This practice makes no recommendations regarding
the interpretation of the statistics that result from
analysis of the data acquired with regard to the
goodness or badness of given values of the test statistic,
nor does it make recommendations regarding decisions
about the process cycle or equipment used to produce
the thin film that was measured.
2.7 The principles of this practice may be adapted to
determine the uniformity of bulk silicon wafer
properties such as interstitial oxygen content and
resistivity, but depending on the desired property and
the chosen measurement technique, depth-dependent
variations may be misinterpreted as lateral variations.
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety and health practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Limitations
3.1 This practice is written for evaluation of planar or
blanket films, but it may be applied to patterned films if
the pattern size, shape, and distribution do not interfere
with the spatial resolution of the selected measurement
technique and the specified measurement site selection.
If either of these interferences occur, the user may adapt
the principles of the method to the needed application,
but the interpretation of the results may change.
3.2 The principles of this practice may be adapted to
other semiconductor wafers, such as gallium arsenide,
but particular concerns with those other materials may
not be addressed adequately in this practice.
3.3 Uniformity measurements from certain types of
measurements may be misinterpreted if the user is not
aware of the full nature of the basic measurement being
made. For example, sheet resistance is a function both
of the inherent resistivity of the film being measured
and of its thickness value. Non-uniformity in sheet
resistance values across the wafer may result from non-
uniformity in resistivity (layer composition or structure)
or layer thickness (deposition rate) alone, or it may
result from simultaneous variations of both parameters.
3.4 Changes in test equipment performance, or changes
in test procedure or conditions over time may
detrimentally affect the ability to compare test results
from this practice over time for a given film property or
fabrication step. It is the responsibility of the user to
ensure that the measurement system and process
remains in sufficient control to allow time-wise
comparison of uniformity results, if such comparisons
are needed.
3.5 Sampling area or sampling spot-size may cause
misinterpretation of the cause of thin film non-
uniformity if the magnitude of the non-uniformity and
its spatial scale, or rate of variation is not well matched
to the sampling spot size and the selected distribution of
measurement sites. While this should not be a serious
concern for most modern film deposition processes, and
process equipments, which are operating in control, the
principles of this limitation are worth elaborating.
3.5.1 If the sampling area or the spot size of the test
instrument is large compared to rates of film parameter
change (gradients) that are important to identify, it must
be recognized that such large sampling area instruments
generate some form of spatial average response that