Microsystem Technologies

, Volume 16, Issue 8, pp 1309–1313

Fabrication of high precision X-ray mask for X-ray grating of X-ray Talbot interferometer

Authors

    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
  • Hiroshi Tsujii
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
  • Naoki Takahashi
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
  • Tadashi Hattori
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
Technical Paper

DOI: 10.1007/s00542-010-1085-x

Cite this article as:
Noda, D., Tsujii, H., Takahashi, N. et al. Microsyst Technol (2010) 16: 1309. doi:10.1007/s00542-010-1085-x

Abstract

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.

1 Introduction

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.

XTI employs two gratings and generates contrast corresponding to the differential phase shift caused by the sample. Figure 1 shows the setup for XTI. We succeeded in fabricating the diffraction gratings required for XTI using an X-ray lithography technique (Noda et al. 2007, 2008; Matsumoto et al. 2007). However, in these high aspect ratio X-ray gratings periodic errors occur. In X-ray lithography, the accuracy of the micro-structure largely depended on the accuracy of the X-ray mask. The reason for this is that the contrast between the X-ray absorber area and the blank area is directly transferred onto the structure being fabricated. Therefore, a micro-fabrication technology that allows us to fabricate untapered, high aspect ratio structures for the X-ray mask is needed. Conventional X-ray mask fabrication processes use ultraviolet (UV) lithography technique; however, if the thickness of the photoresist is a few micrometers or more, the profile of the photoresist pattern has undesirable tapering due to the diffraction of UV light, as shown in Fig. 2. With conventional processes it is extremely difficult to fabricate highly precise rectangular patterned X-ray masks. Using conventionally fabricated X-ray masks, grating structures with tapered profiles and periodic errors are obtained using X-ray lithography.
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Fig. 1

Setup of an X-ray Talbot interferometer

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Fig. 2

SEM image of tapered structure in cross section by UV lithography

On the other hand, with silicon dry etching technology it is possible to fabricate rectangular structures to a depth of a few 10 μm even when the minimum lateral dimension is less than a micron, as shown in Fig. 3. Therefore, we proposed to combine silicon dry etching technology with UV lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns. Table 1 shows the target specification for the X-ray mask in this work.
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Fig. 3

SEM image of untapered structure in cross section by silicon dry etching

Table 1

The target specification of X-ray mask

Pattern

1:1 line and space

Pitch

5.3 μm

Absorptive material

Au

Height

5 μm

Pattern area

60 mm × 60 mm

2 Conventional X-ray mask fabrication process

Figure 4 shows the fabrication process for conventional X-ray masks using UV lithography technique (Noda et al. 2008). A Ti layer is sputtered onto the Si substrate to serve as a seed layer for electroforming. Next, the wafer is spin coated with the negative photoresist SU-8, which is then patterned by UV lithography. Following this, Au, which is used as the X-ray absorptive material, is grown by electroforming. A 40 μm thick layer of SU-8 is then deposited on top to serve as a membrane support for the patterned Au, and the complete structure is attached to a frame to support the membrane. Finally, after completely removing the Si substrate in potassium hydroxide (KOH) at a temperature of 50°C, the Ti seed layer is removed by etching in buffered hydrogen fluoride (BHF). Using this process, an X-ray mask with a 5.3 μm pitch pattern and a large effective area of 60 × 60 mm2 was successfully fabricated.
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Fig. 4

Conventional fabrication process for X-ray mask

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).

Figure 5 shows the new fabrication process using Si dry etching. First, an insulating layer of silicon dioxide was formed on the Si substrate. Next, the wafer was coated and patterned with SU-8 photoresist and the microstructure for the X-ray mask patterns was formed by Si dry etching. Cr metal was used for the seed layer; this was removed from the top and sidewalls, leaving Cr layer only at the bottom of the microstructure pattern. Au was formed between the microstructure patterns by electroforming. The photoresist mask was removed and the Si microstructure was etched, leaving the Au intact. Again, we spin coating a 40 μm thick SU-8 membrane and attached a frame to support the mask. Finally, the Si substrate was completely removed in potassium hydroxide (KOH). Using this process, an X-ray mask with a 5.3 μm pitch pattern was successfully fabricated.
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Fig. 5

Proposal new fabrication process for X-ray mask

Prior to developing this new method, we carried out an experiment in which we formed rectangular silicon microstructures with a height of about 5 μm by Si dry etching using the ICP etching system. The process parameters of the ICP etching used to obtain these Si microstructures are shown in Table 2. We then used the Si microstructure as a conductive seed layer for electroforming Au. The result of this is shown in Fig. 6, which shows that the Au grows predominantly form the top and sidewalls, generating voids inside the Au film, which would allow X-rays to be transmitted X-ray (Tsujii et al. 2008). This is, of course, an undesirable result. However, it is very difficult to form a seed layer in the bottom of the groove only. With the new method presented in this paper we expect to prevent growth of Au from the sidewalls and to grow Au only from the Cr seed layer at the bottom of the grooved structure as shown in the fabrication process in Fig. 5. Figure 7 shows a SEM image of a cross-section of the X-ray mask pattern after Au electroforming. As shown in this figure, the Au layer has grown from the bottom of grooves, completely filling the microstructure. The fabricated X-ray mask is shown in Fig. 8, and has an effective area of about 60 × 60 mm2. Figure 9 shows a photograph of a top view of the X-ray mask patterns. These figures show that we successfully obtained the designed pitch of 5.3 μm and height of about 5 μm of void-free Au absorber patterns.
Table 2

Conditions of ICP etching process

Etching

 Gas

SF6

 Flow rate

20 sccm

 ICP power

600 W

 Bias

200 V

 Time

5 s

Deposition

 Gas

C4F8

 Flow rate

50 sccm

 ICP power

500 W

 Bias

0 V

 Time

9 s

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Fig. 6

SEM image of Si microstructure after Au electroforming

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Fig. 7

SEM image of X-ray absorber structure in cross section with a pitch of 5.3 μm

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Fig. 8

Photograph of X-ray mask with an effective area of 60 × 60 mm2

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Fig. 9

Photograph of X-ray mask pattern with a pitch of 5.3 μm

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).

For deep X-ray lithography, we again used the negative photoresist SU-8. A 0.3 μm thick seed layer was formed on the Si substrate, and a layer of photoresist about 30 μm thick was applied. Although SU-8 was originally designed for UV lithography, it can also be used in X-ray lithography for the production of highly precise, high aspect ratio patterns. Figure 10 shows a SEM image of the cross-section of an X-ray grating pattern. The designed pitch of 5.3 μm and height of about 30 μm were realized over the 60 × 60 mm2 area of the X-ray grating. Thus, highly precise rectangular structures for X-ray grating patterns can be fabricated by X-ray lithography. This result shows that high precision X-ray mask manufacture using Si dry etching is a viable and successful process.
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Fig. 10

SEM image of X-ray grating in cross section with a pitch of 5.3 μm and resist height of 33 μm

5 Conclusions

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.

Acknowledgments

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).

Copyright information

© Springer-Verlag 2010