Linearity and enhanced sensitivity of the Shipley AZ-1350B photoresist

Abstract

The properties of the Shipley AZ-1350B positive photoresist used with the Shipley AZ-303A developer were investigated. It was found that the use of AZ-303A developer results in a significant improvement of the sensitivity and the linearity of the photoresist. The unexposed etch rate of the photoresist was 35 A + 5 A/ gec. Gratings of high efficiency have been successfully fabricated using the above combination of photoresist and developer.Current work in integrated optics requires the fabrication of relief grating structures on photoresist and the subsequent chemical or ion beam etching through the photoresist. 1 If the period of the grating is to be less than 0.4gm the Shipley AZ-1350B photoresist is commonly used. The properties of this positive acting photoresist have been examined in detail, 2 3 and it was found that the photoresist exhibits strong nonlinearity, where T is the development time in seconds, c is the exposure constant characteristic of the photoresist, r, is the rate of etching of exposed molecules, r 2 is the rate Ad ArTcE + r 2 T.In this work we determined the parameters involved in Eq. (2) for the Shipley AZ-1350B photoresist used with the AZ-303A developer.The samples used were NO. 3010 microscope slides made by Clay Adams, which, cut in half, resulted in a size of 38 mm X 25 mm X 1 mm. The samples were cleaned according to the method presented in Ref. 8, and for some experiments the back surface was painted black with 3M Nextel 101-ClO velvet coating. The AZ-1350B photoresist was then deposited in a single layer, and after 30 sec it was spun at 3600 rpm for 30 sec.The samples were baked, next, for 30 min at 1000C. The first experiment involved the determination of the etch rate of the unexposed resist as a function of development time for various solutions of AZ-303A developer with distilled water. For this purpose the samples were half immersed in the developer for the required time, rinsed with deionized water for 2 min, and then baked under vacuum at 1000C for 30 min. The step size was measured using a Sloan Dektat instrument, and the results are shown in Fig. 1. The 4:1 solution (four parts distilled water, 1 part AZ-303A developer) gave unacceptably high etch rates, and the 8:1 gave low and nonlinear ones. We chose the 6:1 dilution for all our experiments, since it exhibited a linear behavior and an acceptable etch rate of r 2 = 35 A 5 A/sec. The development time chosen for the subsequent experiment was 10 sec.For comparison, we performed the same experiment using the Shipley MF-312 developer. This developer is free from trace-metallic elements and is commonly used in the fabrication of photoresist gratings in semiconductor substrates. Figure 2 shows the unexposed etch rate as a function of development time in minutes for the manufacturer's recommended dilution of 1:1. In

Developed profile of holographically exposed photoresist gratings

“…Figure 9 shows the same evolution curves as those in Fig. 7 but for Si substrates 1n 5 4.58, a 5 3.57 µm 21 2 developed in nonlinear and linear conditions. Those developed in linear conditions were done so with and without background light.…”

“…On the other hand, the isotropy of wet development produces a narrowing of the top of the structures. 21 This effect is particularly pronounced for linear development conditions and long develop-ment times 1deep gratings2. In these cases a strong shift from the expected sinusoidal profiles is observed 3Fig.…”