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Application: Photoresist thickness  measurement

Photoresist thickness measurement is easy with MProbe Vis system.Practically, any photoresist can be measured quickly and reliably: baked or partially baked, thin or thick. Still, the first time measurement may be confusing – there is a wide variety of photoresist types and processing conditions. This application note seeks to clarify the measurement process using examples of baked, and two partially baked photoresist samples. The photoresist measurements are done using 500nm -1000nm wavelengh range to minimize the effect of the absorption. 

 Step 1. Determine  the optical constants of the photoresist.
  • Many photoresists are already defined in the material library. Nothing needs to be done in this case
  •  Photoresist  specification sheets have information about dispersion parameters. They are listed in the form of Cauchy coefficients – it is 3 coefficients that allow to determine the refractive index (R.I.) of the material at different wavelengths of light.  New Cauchy material can be created and assigned the Cauchy coefficients of the specific photoresist
  • If Cauchy coefficients of the photoresist are not available –  use the Cauchy coeffcients of the simiar photoresist, as a starting point.
Step 2. Create the filmstack

Filmstack represents a model of the physical sample – it defines the substrate and material layers. If 3000nm of photoresist is deposited on the Si wafer – filmstack will be Si substrate/ 3000nm PR.  Here PR will be the photoresit material that was defined in the Step 1. 

Step 3. Take the measurement

Measurement is, actually, a two-step process: data acquisition and data analysis. They are transparently handled by the TFCompanion software. During the first measurement one may or may not get a perfect result – filmstack needs to be adjusted. If the thickness of the photoresist is thick enough(> 1um), one can start with the thick  film (FFT based) algorithm. Once the thickness is determined, filmstack can be fine tuned using curve fitting (Marquardt-Levenberg algorithm). At this point, approach is slightly different depending on the processing/conditions of the photoresist(fully baked, not baked, partially baked, etc) If the photoresist is fully baked and the Cauchy coefficients are correct – nothing needs to be done.   Otherwise, optical constants need to be adjusted.


The results show that one needs to be especially careful with under baked photoresist where optical costants are different from the specification.  Correct optical constants can be determined or confirmed by fitting of the model to measured data. Alternatively, optical constants can be measured independently (using spectroscopic ellipsometry, etc.).  Once optical constants are determined – FFT algorithm can be used for further measurement. In the case of thick films, the thickness results of FFT closely match the thickness measured with the curve fitting algorithm. However, there maybe small descrepancies  (<0.1%) when the photoresist has a noticeable absorption. FFT will, typically, give a slightly higher thickness value because it does not use absorption information. The extinction coefficients are very small (<0.01) in the 500-1000nm range but they effect optical measurement in the thick films. At the same time, FFT offers fast and foolproof measurement method, well suited for production environment.   


Photoresist thickness measurement n,k dispersion
Creating Photoresist material

Creating a new material to represent AZ1580 photoresist dispersion. CauchyK material type is used. Cauchy approximation defines optical constants dispersion across the wavelength range using only 3 coefficients. These coefficients were taken from the AZ1500 photoresist specification sheet. Cauchy coefficients are given for an unexposed photoresist of AZ1500 series, soft baked at optimal conditions. Individual photoresists may have different solvent concentration and baking conditions – this will require an adjustment of the Cauchy coefficients. CauchyK is an extension of the Cauchy that applies the dispersion relations to both the refractive index (n) and extinction coefficient(k). The k is currently 0 – we reserve it for the future use with underbaked photoresist where absorption is noticeably present. 

Measuring a fully baked photoresist
photoresist thickness measurement: baked photoresist fit

AZ1580 photoresist on Si wafer: fully soft baked. Fit of the model to measured data shows a very good fit

photoresist thickness measurement: baked photoresist FFT

Thick film (FFT) data analysis of the same measurement – matches perfectly well the curve-fitting result (< 0.2 nm difference in thickness) 

Measuring a partially baked photoresist 
photoresist thickness measurement curve fitting

AZ1580 photoresist on Si wafer: partially baked. There is a significant descrepancy in the fit of the model to the data. It indicates that optical constants need to be adjusted 

photoresist thickness measurement: light baked fit

Thickness and n,k dispersion of the photoresist are measured to improve the fit to the data. Thickness has increased by ~500nm (as compared to original result) and refractive index descreased. This is consistent with underbaked photoresist that has lower density

photoresist thickness measurement: light baked FFT

Using of the thick film algorithm (FFT) gives results matching the curve fit thickness on the left. The measured tthickness difference is 2nm. This is mainly due to the presence of the absorption

Measuring a lightly baked photoresist
underbaked photoresist thickness measurement curve fit

AZ1580 photoresist on Si wafer: lightly baked. There is a significant descrepancy. It look like there is a residual absorption  but , in fact, both n,k and need to be adjusted

underbaked photoresist thickness measurement FFT

Thickness and n,k of the photoresist are measured to improve the fit. The resulting thickness is significantly higher (almost 30% higher)

underbaked photoresist thickness measurement FFT

Using the thick film algorithm(FFT) to determine thickness gives a good match to the curve-fitting result on the left. There is a 3nm difference in thickness due to high absorption.

MProbe 20 thickness measurement system

Wavelength range: 400-1000nm (450-1050nm for VisX)
Wavelength resolution:<1 nm
Precision: <0.01nm or 0.01%
Accuracy: <1nm or 0.2% 
 Measurement:< 10ms (typical, depends on sample reflectivity)
Spot size: <1 mm                                                                                             Sample stage: SH200A with focusing lens and fine adjustment mechanism, sample area 200x200mm 

Read more about MProbe 20 Vis
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