This paper focuses on designing and fabricating high contrast alignment marks used for UV imprint lithography in situ alignment. Since the imprint resist filled in the imprint pattern will deteriorate the intensity and contrast of the Moiré fringes, it’s hard to perform alignment based on the Moiré image. Simulations based on rigorous coupled-wave analysis (RCWA) are performed to design and optimize the high contrast alignment mark in order to obtain high quality Moiré fringes. Designed high contrast mark is fabricated and tested on imprint alignment system. Experiments demonstrated that the simulation results are correct and feasible.
© 2015 Optical Society of America
With the development of the IC technology, traditional lithography encounters more and more theoretical and technical problems. Next generation lithography, such as electron beam lithography (EBL) , X-ray lithography , ion beam lithography , extreme ultraviolet lithography  and nano-imprint lithography (NIL) , are proposed. Among all these, NIL is probably the most promising technique for the advantage of high resolution and low cost. There are three techniques to realize NIL, including heating, ultra-violet curing (UV-NIL) and micro contact imprint [6,7 ]. However, since high temperature usually causes thermal deformation due to diffusion effect  and micro contact imprint has wide graphic lines, UV-NIL has the advantages to achieve higher alignment accuracy and smaller feature size.
When NIL was first proposed in 1995, the method using Moiré fringes for overlay also raised people’s concern . Two sets of gratings with slightly different periods, p1 and p2, which are illuminated by a parallel beam of light, can form Moiré fringes because of the obstruction of two geometric light beams. Moiré fringes have been used during the UV-NIL alignment process with good performance [9,10 ]. However, for in situ alignment due to the fact that imprint liquid has very similar optical properties to the mask material which is usually made of fused silica, the resulting Moiré patterns have bad contrast and low diffraction efficiency. To solve this problem, we designed and tested high contrast alignment mark which can obtain high contrast and high diffraction efficiency patterns for in situ liquid alignment. We simulated our designs and analyzed the influence by different parameters to optimize the alignment mark. Experiments were also performed to verify the findings.
2. Solutions to In-liquid UV-imprint alignment
Figure 1 illustrates the schematic drawing of optical alignment system for UV imprint lithography. The alignment scheme is based on the Moiré alignment marks. Alignment marks include a phase grating pattern on the mask and a checkerboard pattern on the wafer, which are shown in Figs. 2(a) and 2(b) . When illuminated with Littrow angle, these two overlapped patterns produce 1st order Moiré fringes, which can improve the fringes quality and yield high signal-to-noise ratio. The Littrow angle is a function of the checkerboard pitch (in the Y direction) of the substrate layer, given by Eq. (1), where is the Littrow angle, and is the period of checkerboard in Y direction.
By adopting a proper pitch size in X direction for alignment gratings on both wafer and mask, it is possible to optically amplify the mask-wafer misalignment errors. The phase of the Moiré fringes and hence the misalignment errors as small as 1nm can be measured by signal and image processing algorithms [11,12 ].
Due to the fact that imprint resist has very similar optical properties to the mask material, fused silica, when imprint resist was filled in the alignment marks the Moiré pattern would effectively become invisible. To solve this problem, we proposed a new alignment mark by coating optically dense materials inside the imprint mask. By changing the material, thickness, etching depth and grating pitch size, high contrast and high diffraction efficiency alignment patterns could be obtained. As illustrated in Fig. 3 alignment mark is demonstrated in which the dense material resides in the troughs.
To optimize the design of high contrast alignment mark, numerical simulations are performed with RCWA method according to the structures shown in Fig. 3. RCWA is an effective method to analyze and design diffractive structures. It is an exact solution of Maxwell’s equations for electromagnetic diffraction. The wavelength of the incident light is selected to be 550 nm to represent white light illumination. According to wavelength and period in Y direction, Littrow angle can be calculated by Eq. (1). To get the optimized grating structure, several parameters should be studied, including etching depth of the checkerboard grating, pitch sizes in X and Y directions, thickness and the type of coating materials. Three different materials are studied. They are metal material (Cr), dielectric material (Si3N4), and semiconductor material (a-Si). The refraction indexes of the relevant materials are listed in Table 1 .
The relationship between checkerboard grating depth and diffraction efficiency of the Moiré pattern is shown in Fig. 4 . Here the pitch sizes of checkerboard are 1.8um and 2.6um. If the etching depth is too small, its dispersion capability is limited. On the other side, if etching depth is too deep, it could trap too much light and reduce the diffraction efficiency. The best performance can be obtained at about 100nm grating depth.
Diffraction efficiency of different checkerboard grating pitch sizes is shown in Fig. 5 . The diffraction efficiency decreases at first, then increases to a stable value when the x direction pitch size increases from 1um to 1.8um. The pitch size in y direction only affects incident angle and has no effect on diffraction efficiency. Therefore, the combination of 100 nm grating depth and 1.8µm pitch size in x direction has the optimized diffraction efficiency.
Different coating materials thicknesses are also simulated. Based on previous simulations, we choose the period of grating pattern on the mask as 1.85µm, and the x direction period of checkerboard pattern on the wafer as 1.8µm. The etching depth of checkerboard grating is set to be 100 nm. The peak of 1st order diffraction can be obtained when two grating marks are aligned. And the valley is reached when the gratings are shifted by a quarter of period. Numerical simulations results are shown in Fig. 6 .
As shown in Fig. 6, the peak diffraction efficiency of chromium first increases with increasing film thickness and then drops slightly. For Si3N4 the peak diffraction efficiency demonstrates a rising trend with the growth of film thickness. This is because metal materials have large extinction coefficient, its reflectivity is little changed when reaching certain film thickness. But for dielectric materials with no extinction coefficient, the reflectivity changes periodically with the film thickness. It is also noted that different situation occurs for amorphous silicon coating. Its diffraction efficiency decreases sharply after it reaches the maximum. This is because the light absorption caused by its extinction coefficient.
In Fig. 6 dot lines demonstrated the contrasts of Moiré fringes with different coating materials. Without film coatings the contrast is only 17%. Such low contrast is not sufficient to achieve high accuracy alignment. With film coatings the contrast can be improved to above 95%. In the mean time, even with high contrast, high accuracy alignment can’t be realized without high diffraction efficiency. For example, when gratings coated with 10nm Si3N4 its contrast is above 99%, but its peak diffraction efficiency is only 0.18% which is useless in practise. Based on the above simulations, both high contrast and high diffraction efficiency could be achieved by optimizing the structures of designed alignment marks.
4. High contrast mark fabrication and experimental results
To experimentally verify our findings, high contrast marks with Cr film coatings were fabricated and measured. The diagram in Fig. 7 shows the key processing steps for fabrication of checkerboard grating and phase grating. The checkerboard structures were fabricated on a 4-inch silicon wafer using e-beam lithography and plasma dry etch methods. As shown in Figs. 7(a) and 8(a) , checkerboard structure with pitch size 1.8um by 2.6um was etched into silicon by 100nm depth. High contrast grating alignment marks on the mask with 1.85µm pitch and 200nm etching depth, shown in Figs. 7(b) and 8(b) , were fabricated with lift-off process, and Cr film was deposited using physical vapour deposition method. Since the quartz substrate is insulating, a conductive film needs to be deposited before electron beam lithpgraphy. The sizes of the phase grating and checkerboard grating are both 0.5mm × 0.5mm.
As demonstrated in Fig. 1 the first order Moiré pattern is collected at Littrow angle by a 7 × extra long working distance objective lens and directed onto a CCD camera. Diffraction efficiency of the alignment grating is measured using a photo multiplier tube (PMT). The optical system is illuminated by white LED light source. To collect first order Moiré image, silicon wafer with fabricated checkerboard structure is spin coated with imprint photo-resist and placed on an X-Y stage. A 5 inch by 5 inch quartz mask with fabricated high contrast alignment mark is held by an adaptor and placed approximately close to the wafer plane using Z-actuator. The collected first order Moiré pattern is shown in Fig. 9(b) . Compared with the alignment mark without coating (in Fig. 9(a)), the Moiré pattern has been improved significantly. Experiments are performed with four different Cr film thicknesses: 10nm, 20nm, 50nm and 100nm. The measured diffraction intensities are drawn in Fig. 10 . Comparing with the simulation results, they follow the same trend and data matches with curve very well. Therefore, the experiments demonstrate that the simulation results are feasible and correct.
In summary, this paper demonstrated and optimized a high contrast alignment mark used for imprint lithography in situ liquid alignment. Simulation results found that the checkerboard grating with 100 nm etching depth and 1.8 um pitch size in x direction has the best diffraction efficiency. Through coating high refractive index materials in the trough of gratings and optimize the coating film thickness, the diffraction efficiency and contrast significantly increase. Experiments were conducted by fabricating and testing the optimized alignment marks. By comparing gratings with different Cr coating thicknesses, we verified the simulation results.
The financial support to this work by University of Science and Technology of China start up funding and Anhui Provincial Natural Science Foundation are acknowledged. Fabrications were carried out at the Nanotechnology Center of University of Science and Technology of China.
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