Breakthrough in photoresist characterisation

Fast accurate photoresist film thickness measurement

Press Release

Talysurf CCI SunStar




22/07/11

Photoresist is a well known light sensitive masking material used to form a patterned coating on a surface. It is routinely used for photolithography and photoengraving. There are many applications in numerous technologies including semiconductor and printed circuit board manufacture as well as MEMS, solar PV, holography and biomedical engineering.

Correct exposure of the photoresist film is the key in controlling production costs: an incorrect exposure dose will result in an increase in the number of failed pattern parts. The exposure time can be obtained by measuring the photoresist film thickness, as a relationship exists between the developed resist film thickness and the exposure dose.



It is almost impossible to make a precise predetermination of the proper exposure because it can be difficult to produce photoresist films with uniform thickness. An ideal photoresist film should have not only the desired thickness, but also good uniformity over the surface. To meet these requirements and then to control the exposure of the photoresist films, it is necessary to measure accurately the thickness and distribution of the photoresist coating, during or after the photoresist formation process.

A number of metrology tools have been employed to measure film thickness. These include conventional methods such as spectrophotometry, ellipsometry and physical step measurement. Scanning White Light Interferometry (SWLI) is becoming a popular technique because of its high lateral resolution and speed. However one of the limitations of traditional interferometry is the thickness of the photoresist that can be measured; typically it needs to be greater than 1.5 µm to obtain accurate data. It is now possible to measure thicknesses down to 50 nm or less using the CCI HD with patented Film Thickness software. This technology offers film thickness measurement with unsurpassed accuracy, repeatability and vertical and spatial resolution.

When the thickness of a film is larger than ~1.5 µm (the actual thickness will depend slightly on refractive index), SWLI interaction with the layer results in the formation of two fringes, each pertaining to a surface interface. The thickness of the film can therefore be simply determined from the locations of the two fringe envelope maxima, assuming that the refractive index of the film is known. In addition, the surface structure of both top surface (air/film) and bottom surface (film/substrate) can also be obtained.



Talysurf CCI SunStar
Figure 1 (above) shows single pixel measurement data from a 7 µm thick film. The two interference signals can be clearly seen and analysis of the film using interferometry is trivial.
As the film gets thinner the two sets of fringes get closer until finally it is impossible to separate them.

Talysurf CCI SunStar
Figure 2 (above) shows that the fringe overlap when the sample is thinner than 1.5 µm is substantial and leads to errors in the thickness measurement. This problem will be evident in all interferometry measurements. An alternative method has to be employed. Development of the HCF (Helical Complex Field) function allows us to extract the required information. With prior knowledge of the dispersive film index n, the HCF function can be used for accurate measurement of film thicknesses within the range of ~25 nm to ~5 µm.

Talysurf CCI SunStar
Figure 3 (above) shows a single pixel fringe envelope from an interferometry measurement of a 270 nm Ta2O5 thin film coated on BK7 glass. It is clearly impossible to see the different fringes.

The table below clearly shows the difference between the two measurement techniques and given the Film thickness data had been verified by different techniques there is clearly an error in the traditional thick film approach when the film is below 1.5 µm, even though the data appears to give a believable value.





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