2-d photonic crystals - periodic arrays in photoresist for further processing cd.

2. EXPERIMENTAL RESULTS

Nanostructured materials with ordered arrays of holes or rods are practical realisation of photonic band-gap concept. We present useful method for fabricating periodic arrays of arbitrary shape and size openings. This method is based on exposing thin photoresist film with two interfering laser beams. Initially experimental exposing setup with pulsed Nd-YAG laser has been built to fabricate diffraction gratings in positive photoresist films, which was a base for grating coupler device [5]. An idea of making use of similar setup as mentioned above with two-step exposure of photoresist was first proposed in 1975 [3], although no further concept of periodic arrays employment was given then. Additionally the authors applied in their exposure setup continuous 488 nm argon laser instead of pulsed Nd-YAG laser with wavelength of 355 nm and the type of photoresist was different from the one we used.

In this article two examples of obtained periodic structures, both of them produced in exposure setup, are showed. The idea of producing periodic arrays is based on two-step holographic exposure of positive photoresist. In the first step of that process one diffraction grating is produced in photoresist by exposing it with two interfering laser beams, but no development of photoresist is done at that stage. In the second step the sample was rotated through an angle Θ (inclination angle) about an axis normal to the substrate surface and once again it was exposed to fringe pattern. Exposure angle  is shown in Fig. 1 and grating period Λ depends on that parameter since:


where λ is the wavelength of a laser source used in exposure setup.



Two different shapes of openings in positive photoresist Shipley SPR 700 were a result of test exposure. First positive photoresist was spin-coated on Si-p plates with (111) orientation, where estimated thickness of photoresist is 1 μm, and then samples were exposed. In both cases two sets of one-dimensional grating with period of 1.26 μm were used. Theoretical results of two-step exposure of positive photoresist are presented in Fig. 1b. and Fig. 2b. where white forms stand for the regions, which were exposed twice and then developed. Since positive photoresist was used in the experimental setup, all double exposed regions of photoresist should be developed away during development process.

First example of produced periodic array is presented in Fig. 2a. Inclination angle Θ was equal to 75 st. and oval-shaped openings with major axis = 800 nm and minor axis = 300 nm were achieved.



Second example of fabricated periodic array is presented in Fig. 3a. Inclination angle Θ was equal to 900 and nearly circular openings with diameter of 78 st. nm were achieved. Arrays of openings were uniform at large areas of exposed photoresist and only very few holes were distorted. Another result of this test exposure were very small in size (about 200nm of diameter) edges produced in photoresist (Fig. 4a), proving great resolution capabilities of applicated positive photoresist.