Fabrication of high precision X-ray mask for X-ray grating of X-ray Talbot interferometer
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- Noda, D., Tsujii, H., Takahashi, N. et al. Microsyst Technol (2010) 16: 1309. doi:10.1007/s00542-010-1085-x
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X-ray imaging is used in many applications such as medical diagnosis and non-destructive inspection, and has become an essential technologies in these areas. In one image technique, X-ray phase information is obtained using X-ray Talbot interferometer, for which X-ray diffraction gratings are required; however, the manufacture of fine, highly accurate, and high aspect ratio gratings is very difficult. X-ray lithography could be used to fabricate structures with high precision since it uses highly directive syncrotron radiation. Therefore, we decided to fabricate X-ray gratings using X-ray lithography technique. The accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask used. In our research, we combined deep silicon dry etching technology with ultraviolet lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns. We succeeded in fabricating an X-ray mask with a pitch of 5.3 μm. The thickness of the Au absorber was about 5 μm, and the effective area was 60 × 60 mm2, which is a sufficient size for phase tomography imaging. We demonstrated the utility of the Si dry etching process for making high precision X-ray masks.
Imaging techniques using X-rays have found applications in many areas such as in medicine, biology, inspection, material science, and so on. However, low absorbance of soft biological tissue makes it impossible to obtain clear X-ray images in this case. To resolve this problem, several methods which use X-ray phase information have been investigated (Momose 2005). Recently, techniques using X-ray gratings have been demonstrated (David et al. 2002; Momose et al. 2003, 2006 and Pfeiffer et al. 2006). X-ray Talbot interferometer (XTI) (Momose et al. 2003) is one of those techniques, with which an improvement in sensitivity of about 1,000 compared with that of conventional absorption contrast imaging can be obtained.
The target specification of X-ray mask
1:1 line and space
60 mm × 60 mm
2 Conventional X-ray mask fabrication process
For the X-ray mask pattern required for X-ray gratings, the photoresist structures, which are several micrometers high, are often tapered due to the diffraction of UV light, as shown in Fig. 2. Therefore, it is extremely difficult to fabricate highly precise, rectangular patterns for X-ray masks using this conventional X-ray mask fabrication method.
3 New X-ray mask fabrication process
Si dry etching technique with an inductively coupled plasma (ICP) can be used to realize rectangular structures to a depth of 10 μm down to submicron geometry. Therefore, we proposed to combine this Si dry etching technology with UV lithography in order to fabricate untapered and high precision X-ray masks (Tsujii et al. 2008).
Conditions of ICP etching process
4 X-ray lithography for X-ray grating
For the deep X-ray lithography used for the LIGA process, we used the NewSUBARU beamline 2 (BL2), which is the synchrotron radiation (SR) facility installed in our university. The BL2 has the potential for patterning large areas up to A4-size with a highly uniform pattern thickness (Utsumi et al. 2007).
This research was on the fabrication of X-ray masks to be used in the production of X-ray gratings, which are used for X-ray phase imaging using X-ray Talbot interferometer. The X-ray gratings need to have a high aspect ratio and be of high precision. Therefore, an X-ray mask with a highly precise structure is also required.
Conventional X-ray masks were fabricated using UV lithography technique. However, the resist profile had an undesirable tapered structure mainly due to the diffraction of UV light by the micrometer level patterns. On the other hand, Si dry etching technique is currently being used for fabricating deep silicon microstructures. Therefore, we proposed combining Si dry etching with UV lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns.
For the X-ray mask, a high precision rectangular Si microstructure was fabricated using an ICP etching system and a seed layer was formed in the bottom of the grooved structure. As a result, we obtained a void-free Au layer by electroforming and successfully fabricated an X-ray mask with a pitch of 5.3 μm, a Au height of about 5 μm, and a large effective area of 60 × 60 mm2. From the X-ray lithography, we obtained highly precise X-ray grating with the designed pitch of 5.3 μm and height of 33 μm. This demonstrated the utility of the Si dry etching process in the manufacture of high precision X-ray masks.
In future, we intend to fabricate high precision X-ray gratings with pitches less than 5.3 μm and areas of 100 × 100 mm2 with the aim of broadening the field of view. The results presented here suggest that XTI is a novel and simple method for phase sensitive X-ray imaging.
This research was supported by the research project “Development of Systems and Technology for Advanced Measurement and Analysis” from the Japan Science and Technology Agency (JST).