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Contract No. DE-AC36-99-GO10337 

B. Sopori, J. Amieva, B. Butterfield, and C. Li 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37478 

Rapid Mapping of AR Coating 

Thickness on Si Solar Cells 

Using GT-FabScan 6000 
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RAPID MAPPING OF AR COATING THICKNESS ON Si SOLAR CELLS USING GT-FABSCAN 6000 

Bhushan Sopori, Juana Amieva, Brian Butterfield, and Chuan Li 

National Renewable Energy Laboratory, Golden, CO 80401 

ABSTRACT

A new technique for rapid mapping of the thickness of

an antireflection (AR) coating on a solar cell is described. 

A filtered, reflectance (intensity) image of the AR-coated

wafer is generated by a CCD camera mounted on a

GTFabScan. This image is converted into a thickness

image using a transformation relating local AR thickness to

the local intensity in the image plane. The thickness map

is generated in <100 ms. 

INTRODUCTION 

Until recently, the objective of using an AR coating on

a solar cell has been to minimize its reflectance and

maximize the light trapping ability to obtain highest

photocurrent for the incident solar spectrum. Typically, the

AR coating for a textured Si solar cell should consist of a

uniform, 750 Å-thick, dielectric film of refractive index 2 to

2.4. However, the low-cost techniques used for deposition

of AR coatings on commercial Si solar cells do not yield

very uniform films. It is believed that a good trade off

between the cost and the cell performance would favor a

“reasonable” variation in the film parameters (thickness,

refractive index). Typically, AR coatings used in the PV

industry exhibit a variation of about 100 Å in the film

thickness. It is estimated that this nonuniformity leads to a

photocurrent density loss of about 1 mA/cm2

. 

In the current solar cell technology, AR coating 

deposition and its subsequent processing must combine

many additional functions that include: (i) a barrier to firethrough, screen-printed contacts, and (ii) participation in

the hydrogen diffusion for impurity/defect passivation.

These functions demand much higher uniformity of the

coating. For example, in a metallization firing by an RTPlike process, a variation in the film thickness leads to

variation in the local reflectance and the absorbed energy.

These can lead to spatial variations in the cell

temperature, producing changes in the quality of the

contact. Thus, there is a need for development of a

technique for rapid measurement of AR coating thickness

and mapping the thickness variations. Some methods

already exist for measuring thickness of dielectric films on

planar wafers. They are based on ellipsometery or

interference, and use a small beam to scan the wafer.

These methods are slow and not suitable for wafers that

have rough or textured surfaces. 

Here we describe a new technique for measurement

and mapping of AR coatings on Si solar cells. This

technique uses an existing instrument, GT- FabScan 6000,

which can perform a variety of measurements for process

monitoring in PV manufacturing. The system generates an

image of the AR thickness and presents it in a color format

using false colors. This measurement is made in less than

100 ms. Development of this application enables GTFabScan 6000 to not only measure average values of

various physical parameters of solar cells but also

generate thickness maps. These maps are valuable for

monitoring the operation of the coating deposition

machine, and control other processes such as fire-through 

metallization and hydrogen passivation. 

PRINCIPLES OF MAPPING AR COATING THICKNESS 

The new technique uses the thickness dependence of

the local reflectance from the cell/wafer, at a selected

wavelength, to determine the corresponding thickness.

Figures 1, 2, 3 and 4 illustrate the principle of this

technique. Figure 1 shows reflectance spectra (R Vs λ) of

planar, AR coated solar cells for different thicknesses

(0.05 μm to 0.1 μm) of the coating. The coating material is

assumed to be SiN:H with a refractive index of 2, and the

calculations are made using PV Optics. These plots show

that for λ > λ0 (λ0 = wavelength for minimum reflectance),

the reflectance increases as the AR coating thickness

decreases. Figure 2 is a plot of reflectance Vs (1/t) for 

Figure 1. Calculated reflectance spectra of ARcoated, planar solar cells of different AR 

thicknesses, using PV Optics. 

different wavelengths. It is seen that for a broad

wavelength range, this relationship is nearly linear.

Figures 3 and 4 show similar plots for textured wafers. In 
particular, Fig. 4 shows a linear fit for a selected

wavelength, λ = 0.8 μm. The important conclusion is 

that one can assume the local reflectance of an AR 

coated wafer is inversely proportional to the film

thickness, if the reflectance is measured at a suitable

wavelength. This feature can be deployed in a

somewhat simple way by suitably illuminating the AR

coated sample and using a camera (with a bandpass 

filter) to image the (1/t)-map, which can then be

converted into a t-map. These measurements can be

done easily using digital camera systems. 

Figure 4. Plots showing relationship between 

reflectance and (1/thickness) at different 

wavelengths for a textured, AR-coated wafer. 

Figure 2. Plots showing relationship between 

reflectance and (1/thickness) at different wavelengths 

for a planar, AR-coated, wafer. 

MEASUREMENT SYSTEM

The thickness mapping is easily accomplished in

GT-FabScan 6000 (previously known by other names

i.e., Sopori Reflectometer or PV Reflectometer). In its 

normal operation, GT-FabScan 6000 rapidly measures

reflectance spectrum, reflectance (R) Vs wavelength (λ) 

of the entire wafer or cell, and deconvolves these data 

to obtain surface roughness, texture quality, AR coating

thickness, wafer thickness, metallization parameters

(height and area of metallization), and backside 

reflectance. Because GT-FabScan 6000 can make

measurements very fast, typically in less than 100 ms, it 

can be used for monitoring various solar cell process 

steps. For film thickness mapping, the wafer is placed

in the reflectometer, where it is illuminated from all sides

and the reflectance normal to the wafer is measured,

see Figure 5. In the normal operation, the R Vs λ is 

measured by a diode array spectrometer. For the

present application of film thickness mapping, the DAS

is replaced with a CCD camera fitted with a bandpass

filter. The filtered image, in response to the illumination

by the optical source of the GT-FabScan 6000, 

represents a local intensity, which is inversely 

proportional to the AR coating thickness. The camera 

image is processed and assigned a false color scheme 

to identify thickness distribution. Figure 5 shows a

schematic of GT-FabScan 6000 with a mapping option. 

Here the light reflected normal to the wafer is passed

through a bandpass filter and imaged with the camera. 

The operating parameters of the camera (contrast 

or Gamma, and brightness) and the bandpass filter can 

Figure 3. Calculated reflectance spectra of ARcoated, textured wafers of different AR thicknesses, 

using PV Optics. 
be selected so that the local intensity of the image of the

AR coated wafer is inversely proportional to the AR

thickness. The intensity of the image at any point in the

image plane, I (xi, yi), is described as: 

I (xi, yi) = R (xo, yo) ∗ T(λ) ∗ M 

Where M is the magnification, T(λ) is the transmittance 

of the bandpass filter, and R (xo, yo) is the local 

reflectance of the element (x,y) in the object plane. The 

bandpass filter is selected to have a linear relationship

between the transmission of the filter and film thickness. 

RESULTS/DISCUSIION 

To demonstrate that the mapping displays the

actual thickness variations, we taper-etched a TiO2

coating deposited on a textured wafer. The sample was

dipped in a dil.HF (which reduced the film thickness 

uniformly) and then slowly pulled out producing a

tapered layer. Figure 6 is a schematic of the etched

taper on the sample. Figure 7 is the thickness

distribution for this sample obtained by the new

technique. This figure shows that a continuous taper in

the film thickness, between 236 Å and 103 Å, is

generated. The color legend of uniformly distributed

colors is shown on the top side of the figure.

Figure 8 shows the thickness map of a textured Si

wafer (4.5-in x 4.5-in), coated with an AR coating of 

TiO2. The film thickness is the largest in the center of 

the wafer, about 800 Å. The color legend, at the top of

figure, identifies the thickness distribution over the

wafer. Also shown are the vertical and horizontal line

scans of thickness through the center of the wafer

identified by the cursor marker.

Figure 5. A schematic of GT-FabScan 6000

with a camera attachment for mapping. 

AR coating

(etched to

create this

profile)

Si wafer 

Figure 6. A cross sectional view of a taperetched AR coating on a textured Si wafer. 

Figure 7. A map of thickness variation of the taperetched film on a textured wafer. 
CONCLUSION

A new technique for rapid mapping of AR coating

thickness has been developed, which can easily be

retrofitted on an existing instrument—GT-FabScan. The

rapid-mapping capability makes the instrument well

suited for online process monitoring and control in PV

manufacturing. The advantages of this technique are: it

is very rapid—maps the entire wafer in one

measurement in less than 100 ms; the measurement is 

insensitive to the nature of the wafer surface (works on

planar, rough, and textured wafers); and because a

digital image is obtained, a variety of statistical

parameters can be quickly identified. Use of this

technique is expected to be very valuable in optimizing

hydrogen passivation and fire-through metallization

process. In particular, current method(s) of AR-coating 

deposition need to be evaluated for non-uniformities in

the coating thickness and its effects on cell 

performance. For example, Fig. 9 shows a thickness 

map of a TiO2 film on a textured wafer that was in the 

same batch for AR-coating deposition as the wafer in

Fig. 8. As seen, the general distribution-pattern of both

wafers is nearly the same, but details of thickness

profile are considerably different. It may no longer be

sufficient to relate AR-coating effects simply to the loss 

in photocurrent. It is expected that large variations in

the AR-coating thickness can be detrimental to

achieving high cell performance in fire-though contact 

processing. 

REFERENCES 

1. B. L. Sopori, “Principle of a new reflectometer for 

measuring dielectric film thickness on substrates of 

arbitrary characteristics,” Rev. Sci. Instrum. 59(5), 

1988, p725. 

2. Bhushan Sopori, U. S. Patent No. 6,275,295. 

3. B. L. Sopori, Y. Zhang, W. Chen, and J. Madjdpour, 

“Silicon solar cell process monitoring by PV

Reflectometer,” IEEE PVSC, Anchorage, AK, Sept. 

2000.

Figure 8. A thickness map of an AR coating on a 

4.5-in x 4.5-in, textured wafer generated in 100 ms by 

GT-Fab-Scan 6000 using the method described in this 

paper. 

Figure 9. A thickness map of an AR coating on a 4.5-

in x 4.5-in, textured wafer that was in the same batch 

for AR-coating deposition as the wafer in Fig. 8., 

showing wafer-to-wafer variation due to deposition 

system 
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4. TITLE AND SUBTITLE

Rapid Mapping of AR Coating Thickness on Si Solar Cells Using 

GT-FabScan 6000 

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B. Sopori, J. Amieva, B. Butterfield, and C. Li 

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14. ABSTRACT (Maximum 200 Words)

A new technique for rapid mapping of the thickness of an antireflection (AR) coating on a solar cell is described. A 

filtered, reflectance (intensity) image of the AR-coated wafer is generated by a CCD camera mounted on a GT-FabScan. 

This image is converted into a thickness image using a transformation relating local AR thickness to the local intensity in 

the image plane. The thickness map is generated in <100 ms. 

15. SUBJECT TERMS

PV; antireflection (AR) coating; solar cells; reflectance (intensity) image; photocurrent; thickness; refractive index; 

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