Title: Ion implant monitoring through measurement of modulated optical response
Abstract: A method for simultaneously monitoring ion implantation dose, damage and/or dopant depth profiles in ion-implanted semiconductors includes a calibration step where the photo-modulated reflectance of a known damage profile is identified in I-Q space. In a following measurement step, the photo-modulated reflectance of a subject is empirically measured to obtain in-phase and quadrature values. The in-phase and quadrature values are then compared, in I-Q space, to the known damage profile to characterize the damage profile of the subject.
Patent Number: 6,989,899 Issued on 01/24/2006 to Salnik, et al.
| Inventors:
|
Salnik; Alex (Castro Valley, CA);
Nicolaides; Lena (Castro Valley, CA);
Opsal; Jon (Livermore, CA)
|
| Assignee:
|
Therma-Wave, Inc. (Fremont, CA)
|
| Appl. No.:
|
387259 |
| Filed:
|
March 12, 2003 |
| Current U.S. Class: |
356/432; 356/445 |
| Current Intern'l Class: |
G01N 21/00 (20060101); G01N 21/55 (20060101) |
| Field of Search: |
356/432,445
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
A. Ehlert et al., "Selected applications of photothermal and photoluminescence
heterodyne techniques for process control in silicon wafer manufacturing," Opt.
Eng., vol. 36, No. 2, Feb. 1997, pp. 446-458.
S. Käpplinger et al., "Measurement of surface recombination of excess carriers
by use of the double modulation technique," Journal De Physique IV, Colloque
C7, supplément au Journal de Physique III, vol. 4, Jul. 1994, pp. C7-145-C7-149.
M. Hovinen et al., "Nondestructive analysis of ultrashallow junction implant
damage by combined technology of thermal wave and spectroscopic methods," J.
Vac. Sci. Technol. B, vol. 20, No. 1, Jan./Feb. 2002, pp. 431-435.
L. Chen et al., "Characterizing Modulated Reflectance Signal from Ion-Implanted
Silicon Wafers," 9th International Conference on PPP, (China 1996), Digest,
pp. 740-740.
A. Salnick et al., "Quantitative photothermal characterization of ion-implanted
layers in Si," Journal of Applied Physics, vol. 91, No. 5, Mar. 1, 2002,
pp. 2874-2882.
|
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Merlino; Amanda
Attorney, Agent or Firm: Stallman & Pollock LLP
Parent Case Text
PRIORITY CLAIM
This application claims priority from prior provisional application Ser. Nos.
60/365,237, filed Mar. 18, 2002, and 60/378,140, filed May 14, 2002, both of which
are incorporated herein by reference.
Claims
What is claimed is:
1. A method of optically inspecting and evaluating a subject, the method comprising:
identifying one or more points within an I-Q space that correspond to the photo-modulated
reflectance of a known damage profile;
measuring the photo-modulated reflectance of the subject to obtain in-phase and
quadrature values; and
comparing the in-phase and quadrature values obtained from the subject to the
identified points to compare the damage profile of the subject to the known damage
profile.
2. A method as recited in claim 1, wherein at least some of the one or more points
are identified empirically by analyzing the photo-modulated reflectance of a calibration subject.
3. A method as recited in claim 2, that further comprises:
periodically exciting a region on the sample;
directing a probe beam to reflect off the region on the sample surface that has
been periodically excited;
monitoring the reflected probe beam and generating output signals in response
thereto; and
analyzing the output signals with a phase synchronous detection system and generating
in-phase and quadrature signals.
4. A method as recited in claim 1, wherein at least some of the one or more points
are identified by extrapolating from empirically identified points.
5. A method as recited in claim 4, wherein the process of extrapolation includes
defining one or more calibration regions within the I-Q space, where each calibration
region includes all points within a specified distance of a set of empirically
identified points.
6. A method as recited in claim 4, wherein the process of extrapolation includes
defining one or more calibration lines within I-Q space, where each calibration
line is a linear fit of a set of empirically identified points.
7. A method of evaluating a sample comprising the steps of:
periodically exciting a region on the sample;
directing a probe beam to reflect off the region on the sample surface that has
been periodically excited;
monitoring the reflected probe beam and generating output signals in response
thereto;
analyzing the output signals with a phase synchronous detection system and generating
in-phase and quadrature signals; and
evaluating the sample by comparing a value of the in-phase versus the quadrature
signals to a predetermined reference value.
8. A method as recited in claim 7, that further comprises:
evaluating the sample by comparing the value of either the amplitude or phase
of the signals to a predetermined reference value.
9. A method as recited in claim 7, wherein the known reference value is identified
empirically by analyzing the photo-modulated reflectance of a calibration subject.
10. A method as recited in claim 7, further including the step of evaluating
a linear combination of in-phase and quadrature signals.
11. A device for evaluating a sample, the device comprising:
a first illumination source producing an intensity modulated beam for periodically
exciting a region on the sample;
a second illumination source producing a probe beam to reflect off the region
on the sample surface that has been periodically excited;
a detector for monitoring the reflected probe beam and generating output signals
in response thereto;
a lock-in amplifier for analyzing the output signals to generate in-phase and
quadrature signals; and
a processor for evaluating the sample by comparing a value of the in-phase versus
the quadrature signals to a predetermined reference value.
12. A device as recited in claim 11, wherein the processor evaluates the sample
by comparing the value of either the amplitude or phase of the output signals to
a predetermined reference value.
13. A device as recited in claim 11, wherein the predetermined reference value
is identified empirically by analyzing the photo-modulated reflectance of a calibration subject.
14. A device as recited in claim 11, wherein the processor evaluates a linear
combination of in-phase and quadrature signals.
Description
TECHNICAL FIELD
The subject invention relates to optical devices used to non-destructively evaluate
semiconductor wafers. In particular, the present invention relates to systems for
measuring dopant concentrations in semiconductor samples.
BACKGROUND OF THE INVENTION
As geometries continue to shrink, manufacturers have increasingly turned to optical
techniques to perform non-destructive inspection and analysis of semiconductor
wafers. The basis for these techniques is the notion that a subject may be examined
by analyzing the reflected energy that results when an optical beam is directed
at a subject. This type of inspection and analysis is known as optical metrology
and is performed using a range of different optical techniques.
One widely used type of optical metrology system, as shown in FIG. 1, includes
a pump laser. The pump laser is switched on and off to create an intensity-modulated
pump beam. The pump beam is projected against the surface of a subject causing
localized heating of the subject. As the pump laser is modulated, the localized
heating (and subsequent cooling) creates a train of thermal and plasma waves within
the subject. These waves reflect and scatter off various features and interact
with various regions within the sample in a way that alters the flow of heat and/or
plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the surface
reflectivity of the sample. Features and regions below the sample surface that
alter the passage of the thermal and plasma waves will therefore alter the optical
reflective patterns at the surface of the sample. By monitoring the changes in
reflectivity of the sample at the surface, information about characteristics below
the surface can be investigated.
To monitor the surface changes, a probe beam is directed at a portion of the
subject
that is illuminated by the pump laser. A photodetector records the intensity of
the reflected probe beam. The output signal from the photodetector is filtered
to isolate the changes that are synchronous with the pump beam modulation. For
most implementations, this is performed using a heterodyne or lock-in detector
(See U.S. Pat. No. 5,978,074 and in particular FIG. 2 for a discussion of such
a lock-in amplifier/detector). Devices of this type typically generate separate
"in-phase" (I) and "quadrature" (Q) outputs. These outputs are then used to calculate
amplitude and phase of the modulated signal using the following equations:
Amplitude=√{square root over (
I2+Q2)} (1)
Phase=arctan(
Q/I) (2)
The amplitude and phase values are used to deduce physical characteristics of
the sample. In most cases, this is done by measuring amplitude values (amplitude
is used more commonly than phase) for one or more specially prepared calibration
samples, each of which has known physical characteristics. The empirically derived
values are used to associate known physical characteristics with corresponding
amplitude values. Amplitude values obtained for test subjects can then be analyzed
by comparison to the amplitude values obtained for the calibration samples.
Systems of the type shown in FIG. 1 (i.e., those using external means to
induce thermal or plasma waves in the subject under study) are generally referred
to as PMR (photomodulated reflectance) type systems. PMR-type systems are used
to study a range of attributes, including material composition and layer thickness.
PMR-type systems and their associated uses are described in more detail in U.S.
Pat. Ser. Nos.: 4,634,290, 4,646,088, 4,679,946; 4,854,710, 5,854,719, 5,978,074,
5,074,669 and 6,452,685. Each of these patents is incorporate in this document
by reference.
Another important use of PMR-type systems is measurement and analysis of
the dopants added to semiconductor wafers. Dopants are ions that are implanted
to semiconductors during a process known as ion implantation. The duration of the
ion implantation process (i.e., total exposure of the semiconductor wafer) controls
the resulting dopant concentration. The ion energy used during the implantation
process controls the depth of implant. Both concentration and depth are critical
factors that determine the overall effectiveness of the ion implantation process.
PMR-type systems are typically used to inspect wafers at the completion
of the ion implantation process. The ion implantation damages the crystal lattice
as incoming ions come to rest. This damage is typically proportional to the concentration
and depth of ions within the crystal lattice. This makes measurement of damage
an effective surrogate for direct measurement of dopant concentration and depth.
PMR-type systems have proven to be adept at measuring damage and have been widely
used for post implantation evaluation.
As shown in FIG. 2, the relationship between dopant concentration and amplitude
measurements (i.e., as defined by Equation (1)) is monotonic for low dopant concentrations.
As dopant concentrations increase (e.g., greater than 1E14 for As
+ or
P
+ ions or greater than 1E15 for B
+ ions) the monotonic relationship
breaks down. In fact, at high concentrations, the amplitude measurements are no
longer well behaved and as a result cannot be used to accurately derive corresponding
dopant concentrations. In FIG. 1, this is illustrated by the points A, B and C
all having the sane the same amplitude measurement, even though each point represents
a different dopant concentration. The same sort of breakdown occurs as the type
of implanted ions becomes heavier (e.g., As
+ or P
+ ions).
In both cases, this is attributable to the appearance of a Si amorphous layer resulting
in optical interference effects. Although not shown in FIG. 2, phase information
becomes flat or insensitive to changes in concentration at high dopant concentrations
or where heavy ions are implanted.
One approach for dealing with the problem of monitoring samples with high dopant
concentrations is to measure the DC reflectivity of both the pump and probe beams
in addition to the modulated optical reflectivity signal carried on the probe beam.
Using the DC reflectivity data at two wavelengths, some ambiguities in the measurement
can often be resolved. The details of this approach are described in U.S. Pat.
No. 5,074,669 (incorporated in this document by reference).
In general, PMR-type systems of the type described above have proven to be effective
methods for testing and characterizing semiconductor devices. Their ability to
function in a non-contact, non-destructive fashion, combined with their high-accuracy
and repeatability have ensured their widespread use as part of semiconductor manufacturing.
Still, there is an obvious need for methods to provide this type of measurement
capability for high dopant concentrations and ion implantation of relatively heavy ions.
SUMMARY OF THE INVENTION
The present invention provides a method of simultaneously monitoring ion implantation
dose, damage and/or dopant depth profiles in ion-implanted semiconductors. For
this method, a PMR-type optical metrology tool is used to record both quadrature
(Q) and in-phase (I) values for a series of specially prepared calibration subjects.
Each calibration subject is fabricated at the same implantation energy. As a result,
variations recorded by the PMR-type system are largely attributable to variations
in dopant concentration.
The measurement method performs a linear fit using the recorded points to define
a calibration line within an I-Q plane. The slope of the calibration line is defined
by the implantation energy used to create the calibration subject. Points along
the calibration line correspond to different dopant concentrations. The calibration
line is used to define a calibration region within the I-Q plane. The calibration
region includes all points within a specified distance (often defined in terms
of a percentage) of the calibration line. Typically, this is done by defining an
upper boundary line that has a slightly greater slope than the calibration line
and a lower boundary line that has a slightly smaller slope than the calibration
line. The calibration region is the area between the upper and lower boundary lines.
After creating the calibration region, the PMR-type optical system may be used
to inspect and analyze semiconductor wafers. For each subject wafer, the PMR-type
system makes one or more measurements. Measurements that fall within the calibration
region are known to share the damage profile of the calibration subject. Measurements
that do not fall within this region are assumed to deviate from the known damage
profile of the calibration subject. This test provides an effective method of accepting
or rejecting wafers that provide acceptable accuracy even when dopant concentrations
are high or where heavy ions have been implanted.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a photo-modulated reflectometer as used by the present invention.
FIG. 2 is a plot of amplitude measurements as a function of implant dose, from
the sample output of the photo-modulated reflectometer of FIG. 1.
FIG. 3 is a plot of quadrature (Q) and in-phase (I) values recorded by the photomodulated
reflectometer of FIG. 1 for a series of subjects within a specially prepared calibration set.
FIG. 4 shows a calibration line that best fits the I-Q points of FIG. 3.
FIG. 5 shows a calibration region centered around the calibration line of FIG. 4.
FIG. 6 shows the use of the calibration region of FIG. 5 to accepte or reject
measurements recorded by the photo-modulated reflectometer of FIG. 1.
FIG. 7 is a second plot of quadrature (Q) and in-phase (I) values recorded by
the photo-modulated reflectometer of FIG. 1 for a series of locations within a
specially prepared calibration subject.
FIG. 8 shows a series of calibration lines that best fit the I-Q points of FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a method for simultaneously measuring ion implantation
dose, damage and/or dopant depth profiles in ion-implanted semiconductors. The
measurement method is logically divided into two steps: a calibration and a measurement
step. During the calibration step, the photo-modulated reflectance of a known damage
profile is characterized. Typically, this involves identifying one or more areas
within I-Q space that correspond to the photo-modulated reflectance of the known
damage profile. All other areas within I-Q space are then assumed to be dissimilar
to the known damage profile. In the measurement step, I-Q measurements for a test
subject are obtained empirically. The empirically obtained I-Q measurements are
then compared to determine if they fall within an identified region of I-Q space.
This comparison indicates whether the test subject has a damage profile that is
similar to the known damage profile. The following sections describe several possible
implementations for both the calibration and measurement steps.
For a first implementation of the calibration step, a PMR-type optical metrology
tool is used to record both quadrature (Q) and in-phase (I) values for a series
of specially prepared calibration subjects. Each calibration subject is fabricated
at the same implantation energy. As a result, variations recorded by the PMR-type
system arc largely attributable to variations in dopant concentration. Each measured
value is treated as a point within an I-Q plane. FIG. 3 shows a representative
series of measured values plotted as points within an I-Q plane. The calibration
step uses a linear fitting algorithm (such as least squares) to define a calibration
line that best fits the points within the I-Q plane. FIG. 4 shows a calibration
line that corresponds to the representative points of FIG.
3. The slope
of the calibration line is defined by the implantation energy used to create the
calibration subjects. Points along the calibration line correspond to different
dopant concentrations.
The calibration line is used to define a calibration region within the I-Q plane.
The calibration region includes all points within a specified distance (often defined
in terms of a percentage) of the calibration line. As shown in FIG. 5, this is
typically accomplished by defining an upper boundary line and a lower boundary
line. The upper boundary line has a greater slope and the same Q-intercept as the
calibration line. The lower boundary line has a smaller slope and the same Q-intercept
as the calibration line. The calibration region is the area within the I and Q
space that is bounded by the upper and lower boundary lines.
For the associated measurement step, the PMR-type optical system is typically
used to inspect and analyze a series of semiconductor wafers. For each subject
wafer, the PMR-type system makes one or more measurements. Each measurement includes
both I and Q values and defines a point within the I-Q plane. For the measurement
method, the proximity of each point to the calibration line measures the similarity
of that point to the damage profile of the calibration subject. Points that are
close to the calibration line represent minor departures from the dopant depth
and concentrations of the calibration subjects. Points that are further away represent
larger departures. Points that fall outside of the calibration region (shown as
black dots in FIG. 6) represent even larger departures from the calibration subject.
These points are assumed to represent large deviations from the known damage profile
of the calibration subject resulting from channeling effects, wafer/beam nonuniformities,
etc. It should be noted that points that inside of the calibration region or even
on the calibration line (such as point X) can be very similar in amplitude to points
that fall outside of the calibration regions (such as point X
1). In
a prior art system, that examines only amplitude, the difference between these
two points would be undetectable. Similar ambiguities can also arise where points
that have different amplitudes correspond, in fact, to the same damage profile.
Prior art techniques would be unable to detect the similarity of such points.
For a second implementation of the calibration step, a PMR-type optical metrology
tool is again used to record both quadrature (Q) and in-phase (I) values for a
series of specially prepared calibration subjects. A representative series of points
of this type are shown in FIG.
7. As shown in FIG. 8, the calibration step
uses these points to create a series of calibration lines. Each line is localized
to fit a subset of the points measured by the PMR-type optical metrology tool.
In this example, three calibration lines have been defined. The first is localized
to fit the points that are local to point A. The second and third are localized
to fit the points near point B and C, respectively The slope of each calibration
line reflects its associated implantation energy. Points along each line reflect
different dopant concentrations.
For the associated measurement step, each point measured by the PMR-type optical
metrology tool is compared to see if it lies on or near any of the calibration
lines. Points on or nearby calibration lines share the damage profile of the associated
calibration line. Points that are not near (or on) calibration lines represent
major departures from the dopant depth and concentration of the calibration subjects
due to channeling effects, wafer/beam nonuniformities, etc. It should be noted
that points on different calibration lines (such as points A, B and C) can have
identical amplitudes. Using prior art techniques that examine only amplitude, the
difference between these points would be undetectable. Similar ambiguities can
also arise where points that have different amplitudes correspond, in fact, to
the same damage profile. Prior art techniques are unable to detect the similarity
of such points.
The preceding description has focused on the use of in-phase (I) and quadrature
(Q) signals. It is important to realize that there may be implementations that
use linear combinations of these signals, in place of the I and Q values. This
description and the following claims are specifically intended to cover all useful
linear combinations of this type, without limitation.
It should be noted that this approach is useful in systems that measure the modulated
reflectivity of the probe as well as systems that monitor other periodic surface
variations such as in interferometry systems or periodic angular variations ("pump"
type systems). To the extent these experiments are performed on semiconductor samples,
it should also be understood that a portion of the signal would be the result of
the modulated electron hole plasma as opposed to being a purely thermal signal.
The relative contributions of the plasma and thermal effects on the signals depends
on the dosage level and experimental conditions such as pump and probe beam wavelengths,
beam spot size and pump modulation frequency.
It should also be noted that the measurement method is useful both as described,
and as part of a more complex analysis. This means, for example that there may
be cases where the measurement method will be used in combination with related
measurements that analyze either or both of amplitude and phase information.
*
