A laser-induced thermal imaging (LITI) process has been developed by 3M as a patterning method for fabricating color filters for liquid crystal displays, organic electro-luminescent devices (OLEDs), and other articles requiring precise patterning of materials with order micron scale accuracy over areas which may exceed a square meter.
For the LITI process, the material to be patterned is coated on a layered donor sheet which consists typically of a backing film, a primer layer, a light-to-heat conversion layer (LTHC), and a protective interlayer between the LTHC and the transfer material layer. The primer layer aids adhesion of the LTHC to the backing film, the LTHC absorbs imaging laser light and converts it to heat, and the interlayer serves several purposes which may include promoting structural integrity of the LTHC during the imaging process and protecting the transfer material from particle absorbers dispersed in the LTHC. During the imaging process, the donor sheet coated with transfer material is pressed against the receiving surface and one or two laser beams are scanned progressively across the donor sheet. In one mode of transfer, the LTHC absorbs the laser light and converts it to heat, the heat diffuses to the transfer layer, and the transfer layer softens and sticks to the receptor surface in the scanned areas. The donor sheet is then peeled away from the receptor and the adhered regions of transfer material pull off the donor and remain as precisely regions of material on the receptor surface.
The purpose of this talk is to introduce the LITI process and give an overview of several aspects of process characterization, modeling, and design techniques that have been investigated. The talk will therefore be given from an engineering point of view with the hopes that we will stimulate discussion that will suggest interesting problems for applied mathematical research. In this talk, we will give an introduction to the LITI process and motivation and overview for LITI process modeling. We will then discuss modeling of the imager energy deposition, computation of the average fluence pattern, and imager equalization. We will next discuss heat flow calculations and prediction of the probability of severe overheating defects given computed thermal profiles and measured defect rates. Following this, we will present methods that we have explored for predicting the width of patterned lines, an image-based metrology technique for robust measurement of patterned line width and edge roughness, and results for predicted versus measured line widths for a series of designed experiments. We will conclude with a discussion of optimal design of graded and stratified LTHC layers.