Dag and Vladimir M. Rubinstein, Tevet Process Control Technologies;
and Yitzhak Gilboa and Steven Hedayati, Cypress Semiconductor
oxide-layer thickness following chemical-mechanical polishing (CMP)
is critical to die yield and device reliability. In addition to experiencing
normal process variations, oxide thickness in shallow-trench isolation
(STI) structures may vary greatly depending on trench width and trench
etch uniformity. Conventional methods for measuring oxide-layer thickness
involve pointing a small (<50-µm) light spot at a large, nonpatterned
wafer area, such as a large pad or scribe line. In order to point the
beam at the nonpatterned area, a vision system and pattern-recognition
algorithms are used in conjunction with an accurate mechanical stage
to move the wafer to the target measurement coordinates.
the need to isolate the measurement site from its surrounding films
by accurately positioning a small spot of light confines oxide-thickness
measurements to relatively wide (>50-µm) trenches. In the case
of STI structures, oxide and nitride thicknesses in narrow (0.21.0-µm)
trenches and dense patterns do not always correlate with thicknesses
measured in wide trenches. Furthermore, thickness problems appearing
in narrow trenches on the die may not appear in the pad areas monitored
by conventional instrumentation. Therefore, the performance of accurate
process control requires a technique that can measure film thicknesses
within the die and on dense structures.
article discusses an in situ thickness measurement system (IsTMS) from
Tevet Process Control Technologies (Yokneam Moshava, Israel) which uses
a large-spot-size broadband Fourier-transform reflectometry (LSBFTR)
method for measuring silicon nitride and silicon oxide film thickness
in STI applications. The IsTMS integrated metrology module was the subject
of experiments in which it was compared with stand-alone and analytical
architecture is formed during wafer processing as part of the effort
to electrically isolate adjacent transistors. Forming a dielectric isolation
layer between the source/drain parts of two neighboring transistors
prevents current leakage (crosstalk) between them. The STI process is
used to manufacture most advanced integrated circuits at technology
nodes below 0.25 µm.
typical STI process flow includes several steps.1
oxide growth. A thin (100-Å) layer of silicon oxide is grown
using thermal oxidation furnaces. The silicon oxide serves as an intermediate
layer between the silicon substrate and the silicon nitride that is
deposited on top of the oxide.
nitride deposition. A 1500-Å layer of of silicon nitride is
deposited on top of the pad oxide layer using chemical vapor deposition
(CVD) or low-pressure CVD. The silicon nitride forms an etch-stop layer
and acts as a hard mask for the trench etch steps.
layer lithography. Following nitride deposition, a lithography process
is implemented to form a photoresist mask for the trench etch steps.
etch. A nitride dry-etch step is followed by a silicon etch step
to create deep (5000-Å) trenches within the silicon.
fill. After the removal of the photoresist mask, CVD is used to
fill the trenches with silicon oxide. The silicon oxide layer forms
a thick cover over the nitride area between the trenches.
CMP oxide removal. CMP removes the oxide and stops after all
oxide above the nitride has been removed.
Nitride and pad oxide strip. The nitride is removed, leaving
the trenches filled with oxide and a clear silicon area between them
for transistor formation.
|Figure 1: Schematic drawing of
an STI structure before nitride strip.
1 shows a cross section of a typical STI structure. The silicon substrate
after trench etch is shown in gray. The silicon trench usually has sloped
walls as a result of the etching process. The silicon oxide remaining
after CMP is shown in blue. Although the schematic indicates that the
oxide surface is flat, the surface of the oxide remaining after CMP
may exhibit thinning at the center of the trencha phenomenon called
dishing. The figure also shows the nitride layer after CMP.
thickness measurements using classical spectroscopic reflectometry are
based on the interference between light beams reflected from the top
surface of the film and light reflected from the bottom surface. The
intensity of the reflected light, as a function of wavelength, depends
on the film thickness D and the complex index of refraction N of each
medium through which the light passes. The reflectance from a single
homogeneous film layer, for an illumination at normal incidence, is
described in the equation
= (ni jki),
= the refractive index of layer i,
= the extinction coefficient of layer i.2
|Figure 2: Schematic drawing of
light reflected from a single film layer with thickness D1
and complex index of refraction N1. The indexes
0 and 2 relate to ambient and substrate, respectively.
2 illustrates the reflection from a single layer with thickness D1
and complex index of refraction N1.
light illuminates a large wafer area where adjacent films with different
optical properties and thicknesses are present, the resulting reflectance
is the sum of the reflectance from each of the films. Figure 3 presents
a simplified example of a reflectance spectrum (excluding dispersion)
expected from two different adjacent films as a function of wave number
|Figure 3: Simulated reflectance
spectrum (excluding dispersion) as a function of wave number from
two adjacent films with different optical properties and thicknesses.
Fourier-transform spectrometric reflectometry method calculates the
thickness of layers by initially applying a Fourier transform (FT) to
the reflectance curvereflectance versus wave number R(λ1)and
by converting the result into a spectrum, denoted by the equation S()
= FT[R(λ1)]. The relative frequencies m
of the peaks in the S() spectrum are then determined, and the
peaks associated with specified thickness values are isolated for further
analysis using proprietary mathematical algorithms, which calculate
the corresponding thicknesses accurately.
example, applying the Fourier-transform technique and the fitting algorithms
to a reflectance curve such as that shown in Figure 3 results in a spectrum
with two peaks, representing the thicknesses of two films (see Figure
4). A change in a layer's thickness causes a shift of the peak relative
frequency associated with that layer, allowing direct thickness monitoring
from an analysis of the FT spectrum.
|Figure 4: Simulated Fourier transform
of reflectance curve from two films, represented by the two peaks.
this technique is applied to typical IC devices, additional mathematical
analysis is required for fine peak resolution, since film stacks and
structures are complex. Each film may have different thicknesses when
deposited on top of different underlying topographies. In such complex
cases, a thickness decomposition method can further separate thicknesses.
This method is based on high-resolution spectral analysis.3
IsTMS integrated metrology module employs several sensors to measure
multiple points across product wafers. Using parallel-beam optics, each
sensor radiates a light spot onto the wafer and collects the reflected
light, which is then transferred to a spectrophotometer for spectral
analysis. Performing thickness measurements using a wide (20-mm-diam)
spot size, which is similar to the size of the lithography field, simplifies
the integration of the thickness-measurement module into the process
tool by eliminating the need for pattern recognition, autofocusing,
and wafer alignment.
IsTMS can be operated in situ, or it can be seamlessly integrated into
the process tool if the measured wafer can be viewed directly. Simultaneous
measurements of multiple points across the wafer are accomplished using
the parallel optical sensors. The unit's wide spot size and simultaneous
measurement capacity enable it to measure multiple points across the
wafer in less than 2 seconds (data acquisition takes less than 1 second,
and data analysis takes another second). Because of its speed, the system
has no negative impact on process tool throughput.
of its large-spot-size measurement capability, the instrument can perform
on-product and in-die measurements. In STI applications, large-spot-size
measurements provide thickness information from dense patterns and from
materials of different thicknesses, such as oxide and nitride layers.
The results obtained from performing large-spot-size measurements represent
an average value of the specific film thicknesses under investigation
across the die (i.e., oxide within trenches or nitride between trenches).
Furthermore, the technique eliminates the sensitivity to micrononuniformities
associated with measuring small, nonpatterned areas using small-spot-size
optics. These features make the method suitable for performing process
monitoring and statistical process control.
five-sensor IsTMS measurement module was used to perform post-CMP thickness
measurements on 20 product wafers containing an STI structure. The STI
structure consisted of trenches with widths varying between 0.22 and
1.0 µm. The thicknesses of both silicon oxide and silicon nitride
films were measured simultaneously on the top, bottom, left, right,
and center of each wafer. In order to compare the performance of the
thickness-measurement module with other systems, measurements in all
cases were taken from the center of each wafer.
being measured using the IsTMS module, the wafers were remeasured using
a stand-alone metrology system. Stand-alone measurement sites across
the wafers were chosen so that the stand-alone tool's small light spot
was positioned in the center of the area that had been previously illuminated
by the much larger IsTMS light spot. Adhering to the stand-alone tool's
standard measurement procedure, the investigators performed each measurement
on a special test structure located in a nonpatterned area close to
the die. Because the stand-alone tool uses different measurement schemes
for different materials, separate recipes were used to measure oxide
and nitride films.
|Figure 5: Correlation between IsTMS
and stand-alone measurements from STI oxide layer (normalized thickness).
measurement results from both the IsTMS measurement module and the stand-alone
system were then compared. (All the results reproduced in this article
are normalized.). Figures 5 and 6 show the correlation between measurements
from the IsTMS module and the stand-alone tool for oxide and nitride
films, respectively. Although the IsTMS measurements were performed
on the die while the stand-alone measurements were performed on a test
structure outside the die, correlation coefficients of R2
= 0.88 for the oxide films and R2 = 0.84 for the silicon
nitride films were obtained. The good correlation between IsTMS and
stand-alone measurements demonstrates that the IsTMS's sensitivity to
thickness changes is at least as good as that of the stand-alone tool.
The thickness difference between the samples investigated in this experiment
was less than 1 Å.
|Figure 6: Correlation between IsTMS
and stand-alone measurements from STI nitride layer (normalized
optical thickness measurements were performed, two product wafers were
randomly selected for scanning electron microscope (SEM) cross-section
measurements. The wafers were cross-sectioned where the IsTMS measurements
were performed. Three SEM cross-section measurements were made on each
Figure 7: SEM cross section
of STI structure before silicon nitride strip that was compared
with the IsTMS thickness measurement of the same structure.
results from dense wafer structures were compared with the SEM cross-section
results. An example of a SEM cross section is illustrated in Figure
7. Table I compares IsTMS and SEM cross-section measurements for two
wafers. The left-hand table summarizes the measurements from wafer 1,
while the right-hand table summarizes the measurements from wafer 2.
Three SEM-based thickness measurements per wafer (W01-1, 2, 3 and W02-1,
2, 3, respectively), average SEM-based thickness values, and the difference
between the IsTMS and SEM average values are shown. The agreement between
the two methods was better than 2%. The discrepancy between them was
smaller than the SEM resolution for thickness measurements.
|Table I: Comparison between IsTMS
and SEM cross-section measurements from STI structures of wafer
1 (top table) and wafer 2 (bottom table). The data represent normalized
film-thickness experiments, adjacent silicon oxide and silicon nitride
films in STI structures located on product wafers were measured using
a large-spot-size broadband Fourier-transform reflectometry method.
Comparison tests showed that there is good agreement between the film-thickness
measurement results obtained from the IsTMS system and those obtained
from a stand-alone tool and a SEM.
the IsTMS method discussed in this article performs as well as conventional
systems in STI process control applications, it has the added advantage
of being an integrated system that can measure different films simultaneously
in dense patterns on product wafers.
Lee, Modeling of Chemical Mechanical Polishing for Shallow Trench
Isolation. PhD diss., Massachusetts Institute of Technology, 2002;
available from Internet: http://www-mtl.mit.edu/Metrology/PAPERS/Lee-PHD2002-Thesis.pdf.
Born and E Wolf, Principles of Optics (Oxford, UK: Pergamon Press,
Proakis and D Manolakis, Digital Signal Processing (Upper Saddle
River, NJ: Prentice-Hall, 1996).
Dag is an application manager at Tevet Process Control Technologies
in Yokneam Moshava, Israel. She received MS degrees in physics and in
quality assurance and reliability from TechnionIsrael Institute of
Technology. (Dag can be reached at +972 4 9591775 or firstname.lastname@example.org.)
M. Rubinstein, PhD, is chief scientist at Tevet Process Control
Technologies and a founder of the company. His main interests are in
the fields of signal and image processing, simulation methods, precise
optical measurements, and metrology. Rubinstein is a member of SPIE
and the International Association of Science and Technology for Development.
He received an MS in electrooptical engineering from the Moscow Institute
of Electronics and Mathematics and a PhD in applied physics from the
National Research Institute of Optic-Physical Measurements, also in
Moscow. (Rubinstein can be reached at +972 4 9591775 or email@example.com.)
Gilboa is manager of technology development in the process development
group of Cypress Semiconductor (San Jose), where he is responsible for
developing fabrication methods in the areas of CMP, thin films, and
diffusion. He received a BS in aerospace engineering and an MS in materials
science from TechnionIsrael Institute of Technology, and an MBA from
San Jose State University in California. (Gilboa can be reached at 408/943-2719
Hedayati previously worked as a process development engineer at
Cypress Semiconductor. He has extensive experience in the semiconductor
industry in the areas of CMP and substrate engineering. In addition
to his work at Cypress, he has worked at OnTrak and National Semiconductor.
He received a BS and MS in materials science from the Technical University
of Berlin, Germany. (Hedayati can be reached at 408/232-2556 or firstname.lastname@example.org.)