Development of metal etch mask by single layer lift-off for silicon nitride photonic crystals

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Abstract

We present a method for fabrication of nanoscale patterns in silicon nitride (SiN) using a hard chrome mask formed by metal liftoff with a negative ebeam resists (maN-2401). This approach enables fabrication of a robust etch mask without the need for exposing large areas of the sample by electron beam lithography. We demonstrate the ability to pattern structures in SiN with feature sizes as small as 50 nm. The fabricated structures exhibit straight sidewalls, excellent etch uniformity, and enable patterning of nanostructures with very high aspect ratios. We use this technique to fabricate two-dimensional photonic crystals in a SiN membrane. The photonic crystals are characterized and shown to have quality factors as high as 1460.

Introduction

The ability to pattern dielectric structures with high resolution electron beam lithography has enabled a broad range of photonics device applications. One important example is the fabrication of photonic crystal (PC) devices that exploit Bragg reflection in multiple dimensions to achieve strong confinement of electromagnetic fields [1], [2]. The engineering of photonic devices has mostly focused on high index materials such as silicon (Si) or Gallium Arsenide (GaAs) that typically operate at infrared wavelengths. Recently there has been great interest in extending these devices into the visible and ultraviolet (UV) wavelengths. Silicon nitride (SiN) is an ideal material for these applications due its large transparency bandwidth that spans the entire visible and part of the UV spectrum. SiN has already been used to develop low loss optical waveguides [3], [4], one-dimensional resonant tunneling structures [5], [6], [7], and two-dimensional PCs [8], [9], [10] for studying cavity quantum electrodynamic (QED). SiN also exhibits a large nonlinear coefficient [11] and is CMOS compatible, making it useful for on-chip nonlinear optical devices such as multiple-wavelength oscillators [12].

The fabrication of these devices in a SiN material system has relied on direct patterning of a polymer etch mask from ebeam resist, followed by dry etching using fluorine chemistry. However, SiN generally exhibits poor etch selectivity relative to ebeam resist under dry etching, which results in significant degradation of the mask pattern. This degradation typically results in sloped sidewalls that greatly reduce the quality (Q) factor of the device [9]. One method to improve the sidewall profile of the structures is to use a hard etch mask that does not degrade under dry etching. Hard etch masks based on silicon dioxide (SiO2) have been demonstrated in GaAs and indium phosphide (InP) systems [13], [14]. However, the etching rate ratio of SiN to SiO2 has been reported to be only 2.3 using standard fluorine chemistry [15], which significantly limits the etch depth. Metallic masks such as chrome (Cr) and nickel (Ni) would have the advantage of much higher selectivity. Such metals are usually patterned by a liftoff technique where the metal is deposited on a patterned resist mask that is subsequently removed. The majority of work on high resolution metal liftoff utilizes positive tone resists such as polymethyl methacrylate (PMMA) [16], [17]. But in many photonics applications (such as PCs) the device must be patterned on thin suspended membranes. In these cases the use of positive tone resist would require exposing the negative pattern of the device, which necessitates exposure of an extremely large area in order to support the membrane structures. This disadvantage makes the use of positive tone resists impractical for patterning of such devices.

Methods for patterning of metallic structures with negative tone resists have been reported in previous works. In particular, high resolution liftoff using Hydrogen Silses Quioxane (HSQ) negative tone resist has been previously demonstrated to pattern germanium and platinum [18]. HSQ has the advantage of very high spatial resolution, but is difficult to liftoff resulting in pattern irregularities. To overcome this problem, an HSQ/PMMA bilayer technique has been employed [19] to improve the ease of liftoff. However, the bilayer liftoff method can result is significant distortion of the pattern when transferred from resist to the metal mask by dry etching.

In this paper, we report an approach for fabrication of nanoscale patters on SiN using a hard Cr mask. The mask is deposited by single layer metal liftoff of ma-N 2401 negative tone ebeam resist. A variant of this resist, maN-2410, has previously been used to pattern gold structures with feature sizes as small as 200 nm [20], [21]. Here we use maN-2401to fabricate a robust Cr etch mask for ebeam lithography. Using this approach we demonstrate the ability to fabricate feature sizes as small as 50 nm, which are then transferred to a 200 nm thick SiN membrane. The fabricated structures exhibit straight sidewalls, superb pattern regularity, and very high aspect ratios. The fabrication technique we demonstrate is ideally suited for patterning PCs due to its high spatial resolution and the fact that we are patterning the negative tone, enabling us to pattern structures that can be undercut to form membranes. As a demonstration, we fabricate a PC cavity on a thin SiN membrane that exhibits straight sidewalls and Q factors as high as 1460.

Section snippets

Fabrication procedure

Fig. 1 illustrates our fabrication procedure for patterning of the metal Cr mask and subsequent transfer to SiN. The initial substrate is composed of a 200 nm thick SiN layer grown by low pressure chemical vapor deposition (LPCVD) on a 500 μm thick silicon wafer. After the LPCVD process, the wafer is cleaned with standard acetone, methanol, iso-propanol and de-ionized water rinse. The negative tone ebeam resist, ma-N 2401 (Micro Resist Technology), is then spin coated at 3000 rpm for 60 s and baked

Characterization of fabrication

We have carried out the fabrication procedure described in the previous section and examined the resulting structures. Fig. 2 shows several scanning electron microscope (SEM) images of the fabrication results taken at various steps in the process. Fig. 2(a–d) were taken after completion of the steps illustrated in panels (c), (d), (f) and (g) of Fig. 1. Fig. 2a shows an angle view of cylindrical negative resist patterns on a SiN layer defined by ebeam lithography (RAITH E-Line at 30 kV

Design and fabrication of photonic crystals in silicon nitride

To demonstrate that this procedure can be used to fabricate real photonic devices, we fabricated a PC optical resonator. The design of the resonator, illustrated in Fig. 4a, is based on a three holes defect L3 cavity structure [22]. The Q factor of the device was numerically optimized by sequentially shifting three groups of air holes (labeled A, B, and C in Fig. 4a) and performing Finite Difference Time Domain (FDTD) simulations. The thickness of the membrane was set to t = 200 nm while the

Conclusion

In conclusion, we have demonstrated a method to fabricate a hard Cr mask for nanofabrication using metal liftoff with negative tone ma-N 2401. The spatial resolution of this technique was demonstrated to be as small as 50 nm. The Cr mask enabled transfer of the mask pattern into 200 nm of SiN, enabling fabrication of structures with very high aspect ratios. The fabrication procedure was used to etch PC cavities into SiN that exhibited Q factors as high as 1460. Although we focused on patterning

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