July 11, 2016
David Chi
Stanford University
476 Lomita Mall
McCullough Bldg, room 203
Stanford, CA 94305
Subject: TXRF Report, CEA #C03H0107
Dear Dr. Chi:
Attached is the final report on TXRF analysis of a 75 mm diameter Si wafer containing a thin
Hafnium Oxide film.
Number of Samples:
Date Received:
Analysis Request:
TXRF Analysis Date:
Fax or Email Date:
Analysis Cost:
Priority Surcharge (none)
Total Cost:
Purchase Order #:
Sample Disposition:
One
04/29/03
TXRF
04/29/03
05/02/03
$375
$0
$375
WG8283
Return after storage period
Please note that your samples will be retained for 8 weeks after their receipt. After this time they
will be disposed of unless you have specifically requested otherwise. Current instructions are listed
in the table above. We will maintain copies of the data and report for three years.
Thank you for using the analytical services of Charles Evans &Associates, a division of the Evans
Analytical Group. We appreciate your business and welcome any suggestions you may have for
improving the quality and efficiency of our service. Please do not hesitate to call me if you have any
questions regarding this work.
Sincerely,
Lori A. McCaig, Ph.D.
Manager
TXRF Analytical Services
(Phone: 408 530-3732)
TOTAL REFLECTION X-RAY FLUORESCENCE (TXRF)
SURFACE ANALYSIS REPORT
CEA #C03H0107
Prepared for:
David Chi
Stanford University
476 Lomita Mall
McCullough Bldg, room 203
Stanford, CA 94305
Prepared by:
_______________________________________
Lori A. McCaig, Ph.D.
Manager, TXRF Analytical Services
Reviewed by:
_______________________________________
Lori Bisaha
Scientist, XRF and TXRF Analytical Services
TOTAL REFLECTION X-RAY FLUORESCENCE (TXRF) ANALYSIS REPORT
CEA#C03H0107
Requester:
Analysis Date:
David Chi
04/29/03
PURPOSE OF ANALYSIS
The purpose of this analysis was to measure metal contamination on the surface of a 75 mm diameter Si
wafer containing a thin Hafnium Oxide film. The sample is identified by the description on the wafer
carrier.
SUMMARY OF RESULTS
All results are shown in Table 1. Ca, Ti, Cr, Fe and Cl are observed in spectra from the wafer. Results
will include contamination in the Hf film and at the Si interface.
PROCEDURES AND DISCUSSION
Analytical: TXRF measurements were made using a TREX 610-T TXRF instrument. The following
measurement conditions were used:
Source:
W Rotating Anode
Beam Power:
30 kV
Beam Current:
50 mA
Incident Angle:
0.04º
Integration Time:
1000 seconds
Due to its small size, the wafer was mounted on a carrier wafer for analysis. Measurements were made
on the polished front side of the wafer, at locations shown graphically on the data printouts and in the
diagram below.
Flat or Notch
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 2
Analysis area is 10 mm in diameter, and analysis depth is ~50Å for polished silicon surfaces (using W
primary beam). Analysis depth for this HfOx/Si sample is not known. Analyses were performed with
the samples in a nitrogen ambient at ~1 Torr.
Results and Discussion:
Results are summarized in Table 1.
Ca, Ti, Cr, Fe and Cl are observed in spectra from the wafer.
Signal from the Hf in the film interferes with measurement of several other species that are measured by
TXRF on Si surfaces, as discussed below.
The intense Hf peaks from Hf L-lines interfere with detection of Co, Ni and Cu due to overlap between
the Hf L-peaks and the K-peaks for these elements. It is possible that Co may be present in these
spectra. The software fitting of the Hf Ll line is improved if Co is included. However, unambiguous
identification of Co is impossible due to the interference from the overlap of the Hf Ll peak with the Co
K peak and the overlap of the Hf L peak with the Co K peak.
Overlap of Fe K and Hf Ll peak interfere with quantification of Fe, making Fe results more uncertain.
Values reported here will overestimate true contamination levels of Fe due to this interference.
The intense Hf M-lines interfere with measurement of S.
The detection limits for other elements on this film are also higher than typically observed for a polished
Si surface due to the higher background observed from the HfOx film and the reduced power of TXRF
analysis necessary due to the intensity of the Hf fluorescence. The presence of Ar and the Hf sum peak
also increase the detection limits for K.
The film thickness was given as 5 Å. The analysis depth for a polished Si surface using a W primary
beam is ~50Å under normal power and angle conditions. The analysis depth for a Hafnium Oxide film
under these reduced power and angle conditions is not known, but should be less than this (50Å). Still,
the analysis results here will include the HfOx film as well as contamination on the Si interface below. It
is not possible to sort out the contribution from each in this data using the signal due to Si K-peaks and
signal due to Hf M-peaks due to overlap of the Si K and the Hf M-peaks and the instrument software
difficulties in fitting M-peaks.
Quantification of TXRF data depends on the index of refraction at the surface of the sample. Since the
results will include both the film and Si interface, results were calculated using two quantification
factors. The results for this wafer are reported using quantification factors estimated for a thick
Hafnium Oxide surface (top half of Table 1) and for a Si or Si oxide substrate (bottom half of Table 1).
In both cases, the quantification is based on standards of known impurity on a Si wafer. These values
provide a range for contamination estimates.
Other techniques might be useful in evaluation of these thin Hafnium Oxide films. A wet chemical or
vapor method may be useful for dissolving the film or leaching contamination from the surface. Jim
Norberg (operations coordinator) would be the contact person to discuss analysis of this type. TOFSIMS may also be used to look at these surfaces. The contact person to discuss this technique is Dr. Ian
Mowat (manager, TOF-SIMS Analytical Services).
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 3
7/11/2016
Table 1. TREX 610-T TXRF Results for W source measurements
(units of 1010 atoms/cm2).
S
Cl
Hf MOS-11 5A Hf
Assuming thick Hf Oxide
Center
<6500 2700
11,-11
<3500 1270
-11,11
<3700 1350
Assuming dominated by Si or Si oxide
Center
<2800 1160
11,-11
<1500 540
-11,11
<1600 570
K
Ca Ti Cr Mn Fe Ni Co Cu Zn
Ar
<170 350 90 290 <84 1350 I
<50 270 32 160 <50 780 I
<80 240 60 110 <36 780 I
I
I
I
I <15 7300
I <10 4000
I <11 4000
<70 150 38 124 <36 580
<20 114 14 67 <21 330
<35 103 26 47 <15 330
I
I
I
I
I
I
I
I
I
<6 3100
<4 1700
<5 1700
Hf fluorescence interferes with any practical detection of Ni, Co, and Cu. Detection limits for S and Mn are also increased due to interference
from Hf. Overlap of Fe K and Hf Ll peak interfere with quantification of Fe, making Fe results more uncertain (values reported here
overestimate true contamination).
Uncertainty values (±) reported are 1 standard deviations calculated from instrumental reproducibility and background signal-to-noise ratios.
These numbers do not include uncertainties from interferences, if any. Practical detection limits (indicated by <) are calculated for each
spectrum and may vary between analyses.
Elements in the ranges S-Zn and Mo-Hf can be detected by W-source TXRF. If an element in this range is not reported in the table, it was not
detected. X-ray lines for elements in the range Mo-Hf overlap x-ray lines for elements in the range S-Zn, making identification and/or
quantification difficult if they are present at high levels. For example, it is not possible to distinguish S and Mo; what is reported as sulfur
may be in part molybdenum.
Correction factors: corrections have been applied to Fe and Cu to compensate for detector background.
Measurement locations: Locations are given in mm from the center of the wafer, assuming that the wafer is oriented with the notch at the
bottom. Positive values are to the right and up, and negative values are left and/or down from the wafer center.
I indicates the presence of an interference.
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 4
7/11/2016
Appendix A
Brief Explanation of the Total Reflection X-Ray Fluorescence (TXRF) Technique
Total Reflection X-Ray Fluorescence (TXRF) is a nondestructive method of analyzing flat, smooth
samples. It is a survey technique, measuring a range of elements in a single analysis.
detector
primary
wafer
beam
X-rays from a primary beam impinging on the surface of a flat, smooth sample at a low angle (less than
the angle of total external reflection) excite impurity atoms on or near the surface of the sample. The
excited atoms emit x-ray photons of characteristic energies which are detected and recorded by a largearea Si(Li) detector and the associated electronics. The photon energies are used to identify the
elements present. The intensity of the x-ray fluorescence for each element is used to quantify the areal
density of that atomic species.
Reflection of the primary x-ray beam from the sample surface is maximized by use of mirror polished
samples. Less penetration of the primary beam into the sample results in fewer excited matrix atoms
and a proportionally larger number of photons from surface impurity atoms, leading in turn to enhanced
detection capability.
Analysis of rough surfaces such as wafer backsides, unpolished wafers, and patterned wafers, is possible
however. Degradation of detection limits, accuracy, and precision of the analysis depend on the
roughness of the sample.
TXRF analysis is performed on unbroken full wafers from 100 to 300 mm in diameter. Smaller samples
and partial wafers can be analyzed by mounting on carrier wafers. At Charles Evans & Associates,
TXRF samples are handled and analyzed in a Class 10 environment to minimize chances of accidental
contamination.
The range of elements that can be analyzed in a single analysis depends on energy of the primary beam.
TXRF instruments used at Charles Evans & Associates use either a rotating W anode or a fine focus Mo
x-ray tube, each with a monochromator, to produce and refine the primary x-ray beam. The two primary
beams excite different ranges of elements:
Tungsten source (W L) - sulfur through zinc, molybdenum through the lanthanide series
Molybdenum source (Mo K) - gallium through bromine; tantalum through lead.
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 5
7/11/2016
Appendix B
Quantification, Detection Limits, Precision, and Accuracy
QUANTIFICATION
Quantification is based on the measurement of a reference sample with known areal density of impurity
atoms on the surface. However, no universally accepted standards for TXRF quantification exist. Each
laboratory must develop and provide its own reference wafers, which may lead to inter-laboratory
discrepancies.
Quantification for TXRF is based on measurement of one element (or several elements) present at a
known areal density. Levels of all elements on the unknown sample are calculated using measured
photon counts for each element on the unknown, photon counts for the standard element, and relative
fluorescence yield factors for the different elements.
Results can be quantified by extrapolation from measurement of a reference sample of known areal
density, as long as the reference and the unknown are measured using the same instrument conditions.
A separate reference sample for each element is not required because relative fluorescence yields are
known.
Signal response for a given element in TXRF analysis is sensitive to the way in which the element is
distributed, as shown in Figure 1. As the glancing angle is changed, signal intensity varies by a factor of
~5 if the element is thinly plated on the surface, but remains constant within the range of angles
normally used for analysis if the contamination is particulate or residue. If contamination is buried (e.g.
an implanted layer), signal intensity will rise with increasing angle in a manner similar to the curve for
plated contamination, but will not drop as sharply at higher angles. The matrix curve refers to signal
response for the element(s) of the bulk material.
Arbitrary Intensity
Plated
Residue
Matrix
0
0.05
0.1
0.15
0.2
Glancing Angle (degrees)
0.25
0.3
0.35
Figure B-1. Schematic representation of signal intensity variation with glancing angle.
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 6
7/11/2016
CE&A procedure is to assume that all contamination is present as a thin uniformly distributed plating on
the sample surface unless otherwise stated in the Discussion section of the report. If contamination is
present as particles greater than 0.1m diameter, as a thick residue, or as a buried layer, reported values
will be high by an unspecified factor.
DETECTION LIMITS
Detection limits are variable, depending on element, sample surface, analysis conditions, and
interferences. Interference free detection limits for first row transition metals (Fe, Ni, Cu) may be as
low as 3E9 atoms/cm2. Sample roughness and interferences from other elements adversely affect
detection limits.
PRECISION AND ACCURACY
Precision of TXRF measurement varies with the source, level of the element measured, and
measurement conditions. Using the CE&A standard conditions, precision on tungsten source TXRF
measurements ranges from 1% at the 1X1015 atoms/cm2 level to ~100% at the levels near detection
limit.
Precision of repeat measurements on our SPC sample, which is vanadium at a level of 2X1013
atoms/cm2, is 16% over a period of more than six months. The CE&A TXRF instrument is calibrated to
a nickel contaminated wafer which has been verified by a Cr standard analyzed with HIBS. Uncertainty
on the HIBS measurement is approximately 10%.
Precision of molybdenum source repeat measurements on the SPC sample, vanadium at a level of
2X1013 atoms/cm2, is 17% over a period of more than six months.
The molybdenum source is calibrated to a wafer with plated nickel contamination. The nickel level is
taken from tungsten source TXRF measurement, and verified by measurement of a commercially
available spin-coated reference wafer.
TXRF Analysis Report
CEA #C03H0107
David Chi
Stanford University
Page 7
7/11/2016
Appendix C
The CE&A TXRF Data Printout
You will receive a data printout for each measurement made on your samples. Each printout consists of
a listing of analysis identification and instrument conditions used for the measurement, graphical
presentation of the data, and a data table.
Analysis identification:
file --- name of data file stored on disk
sample --- sample name
sub --- substrate. Si Wafer or GaAs wafer
memo 1 --- company and name of client, CEA job identification number
Analysis conditions:
Slot --- position in the special cassette into which samples are loaded for analysis
M --- measurement type (manual, programmed, angle scan, etc.)
Size --- wafer size in inches
X: and Y: --- analysis position measured in mm from wafer center
Time --- integration time for the analysis
X-ray Voltage --- anode operating voltage in kV
X-ray Current --- anode operating current in mA
X-ray Target --- anode material
D.T.: --- Dead time: percent of real time that detector does not accept new data
Angle: --- glancing angle at which measurement is made
DP Cond: --- Name of reference file used for data acquisition and processing
Element Cond: --- Name of reference file used for quantification
Quant: --- Name of reference file used for quantification
Spectrum:
The spectrum is a graph of fluorescence x-ray energy (from 0 to 10 keV for W primary beam or
0 to 18 keV for Mo primary beam) versus photon counts per second. For analysis of silicon
wafers, the large peak at the low energy end of the spectrum is the silicon signal from the wafer
substrate. High counts at the high energy end of the spectrum are due to scattered primary x-rays
(W L or Mo K). Characteristic x-ray peaks of elements S through Zn (K) and Rb through
Re (L) (W anode) or elements S through Br (K)and Rb through Bi (L) (Mo source) have
energies between the silicon peak and the high-energy scatter.
Quantitative Results:
Spectrum: ---symbol for the element and the name of the X-ray line used in calculation of areal
density
Energy keV --- energy at which the line for that element appears on the spectrum
Peak Int. cps --- integrated counts per second for the identified peak
Conc *E10 --- calculated areal density for the element, in units of 1010 atoms/cm2
Stand.Dev cps -- standard deviation of counts per second in peak signal
BG Int cps --- background counts per second in peak region