Direct imprint in metal film stacks with low pressure and low temperature for optical elements applications

Abstract

b Nanoimprint lithography (NIL) has great potential as a candidate of next generation lithography for its low cost, high throughput and high resolution [l]. The imprint process is accomplished by heating a resist above its glass transition temperature (Tg) and imparting a force to transfer the image into the heated resist, Recently, a process of direct imprint in gold film at room temperature with ultra high pressure (-hundreds MPa) was reported [2]. This method is a promising way to fabricate optical elements on gold films but there is a drawback of high pressure. Therefore, reducing imprint pressure would accomplish this method for optical elements fabrications.In this paper, we improve the direct imprint process by using a sharp mold and inserting a soft pad, which is the diluted resist (Sumitomo NEB22, Tg 105 OC) between the gold film and the substrate. The diagram of the gold film stack and the imprint mold was shown in Fig. 1. The silicon mold that be used in our experiments was fabricated using electron beam lithography followed by the anisotropic reactive ion etching (HE) process as shown in Fig. 2. The bias and RF power of RIE are the critical parameters for mold profile controlling. The pattern area of the moId is 10 mm x10 mm with 350 nm period gratings. In order to understand the influence of temperature in the direct imprint process, we imprinted the film stacks with different temperatures using a testing mold. As shown in Fig. 3, there are optical microscopic (OM) images of gold film stacks after imprint process. At low imprint temperature, even the room temperature, the pattern of the testing mold transfer onto the gold film stack completely. When the imprint temperature over Tg, the resist in bottom layer overflowed the top gold film cause poor pattern quality. This result indicates that the temperature of direct imprint process must be maintained below the glass transition temperature of the bottom soft pad to avoid pattern distortion. Fig. 4 shows the scanning electron microscope (SEM) image of the cross-section of gold film stack patterned using direct imprint method. The process conditions of this image are 380 psi pressure and 80 O C heating temperature with 2 minutes duration. The imprint pressure is only 0.1 % of the previous method [I]. Besides, it is clearly observe that the surface of the gold film stack shows shallow sinusoidal variation, which is different from the shape of the sharp mold. According this result, we can obtain desired curvature of shape using different imprint pressure. The details will present in this conference.