Alignment method combining interference lithography with anisotropic wet etch technique for fabrication of high aspect ratio silicon gratings Yanchang Zheng,1 Keqiang Qiu,1,* Huoyao Chen,1 Yong Chen,2 Zhengkun Liu,1 Ying Liu,1 Xiangdong Xu,1 and Yilin Hong1 1
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China 2 Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China * [email protected]
Abstract: A method was developed for aligning interference fringes generated in interference lithography to the vertical {111} planes of oriented silicon wafers. The alignment error is 0.036°. This high precision method makes it possible to combine interference lithography with anisotropic wet etch technique for the fabrication of high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. With this alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The roughness (root mean square) of the sidewall is about 0.267 nm. ©2014 Optical Society of America OCIS codes: (220.1140) Alignment; (220.4000) Microstructure fabrication; (220.4241) Nanostructure fabrication; (050.1950) Diffraction gratings.
References and links 1.
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#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23592
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1. Introduction High-aspect-ratio-silicon-gratings (HARSGs) have gained a lot of interests due to their wide range of applications. In X-ray phase contrast and dark-field imaging [1–10], neutron imaging [11–13], HARSG performs as a phase grating and can be used to fabricate absorption analyzer grating. In X-ray wave front characterization [14, 15], HARSG acts as a beam splitter in the grating interferometer. In X-ray astrophysics spectroscopy [16–19], blazed transmission
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23593
HARSG becomes more and more welcome for its high broadband diffraction efficiency and high spectral resolution. Moreover, there are other applications including nanoimprint lithography [20, 21] and ultraviolet filtration [22, 23]. Bosch plasma process [18, 19, 24–26] and anisotropic wet etch using oriented wafers [16, 27–31] are the two common techniques used to fabricate HARSGs. Though the former has its own advantages, it is limited to aspect ratios of below 60 and requires complicated fabrication processes to prepare the etch mask, and has a well-known sidewall scalloping problem. While the latter can avoid those disadvantages by anisotropically etching monocrystalline silicon in potassium hydroxide (KOH) solution, it can get grating bars with extremely smooth sidewalls and ultrahigh aspect ratios up to 150. Moreover, it only needs very thin silicon nitride mask for etching process. However, grating lines need to be precisely aligned to the vertical {111} planes of oriented wafers to achieve enough etch anisotropy which determines the aspect ratio one can get. The requirement of precise grating line alignment to the vertical {111} planes limits the types of lithography used together with anisotropic wet etch technique for fabrication of HARSGs. Electron beam lithography (EBL) [27], scanning probe lithography (SPL) [28], focused ion beam lithography (FIBL) [29], and scanning-beam interference lithography (SBIL) [16, 30] have been used together with anisotropic wet etch technique. However, these types of lithography mentioned above are all scanning lithography, which need expensive precise control systems to align grating lines to the vertical {111} planes. Furthermore EBL, SPL and FIBL suffer from low throughput, high cost and limited ability in fabricating large area patterns. SBIL can fabricate large area patterns, however its tool, MIT Nanoruler [32], is unique. Optical lithography with mask-aligner [31] is also used to fabricate HARSGs with anisotropic wet etch technique, but it cannot fabricate sub-micro structures due to diffraction limit. Currently, interference lithography (IL) has been investigated as an efficient way to make periodic patterns over a large area with superior control of pattern regularity. By using IL, we can achieve grating patterns with wide period coverage from infinity to half the wavelength of light in theory, which is another advantage of IL. The process combining interference lithography with anisotropic wet etch technique is an effective way to fabricate large area HARSGs with extremely smooth sidewalls. Improved alignment method with higher precision is a quest to achieve higher etch anisotropy and smoother grating sidewall. Bruccoleri et al. developed an alignment method in the work of KOH polishing of reactiveion etched grating sidewall [33]. In their work, grating lines are aligned to the {111} planes with an error of 0.14°. In this paper, the authors introduce a higher precision method based on a different technique with an alignment error of 0.036°. Using this alignment method, 320 nm and 2 μm period HARSGs have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The RMS roughness of the sidewall is about 0.267 nm. 2. Alignment method In this section, we will describe in detail the method for aligning interference fringes to the vertical {111} planes of oriented silicon wafers. As shown in Fig. 1, the alignment process consists of three steps: (1) locating the vertical {111} plane direction using a pre-etch technique with a fan-shape pattern, (2) fabricating reference grating parallel with the located spoke closest to the vertical {111} plane, (3) aligning reference grating lines to interference fringes. Moreover, in order to reduce the alignment error caused by the bending of interference fringes generated by spherical waves, the configuration of Lloyd-mirror interferometer without collimating lens should be optimized.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23594
Fig. 1. Process of aligning the vertical {111} planes parallel to interference fringes.
2.1 Locating vertical {111} plane direction The oriented silicon wafer has a primary flat to indicate the vertical {111} plane direction, but this is accurate to only about ± 1° according to supplier specifications. To find the actual vertical {111} plane direction, a KOH pre-etch technique is utilized with a fanshape pattern with 0.05° angular spoke spaced over the range of ± 1.95°. Figure 2 illustrates the mask layout of the fan-shape pattern.
Fig. 2. Mask layout of the fan-shape pattern. Area with grey color is dark field.
After transferring the fan-shape pattern into the silicon nitride layer by contact lithography and reactive-ion etching (RIE), the wafers are anisotropically etched in KOH solution while the undercut phenomenon appears. The two-side undercuts of silicon nitride mask will be different on top end of each spoke, as shown in Fig. 3. The widths of undercut in proximal side y1 and distal side y2 are determined by y1 = V (111)t and y2 = V ( β )t , where β is the angle between the direction of each spoke and the vertical {111} plane direction, V(β) is the etch rate of the plane at β angle, V(111) is the etch rate of the vertical {111} plane, and t is the etching time. y1 is the undercut width of the vertical {111} plane, which is a constant at time t in all spokes. And V(β) is bigger than or equal to V(111) due to the anisotropic etch of silicon in KOH. Thus y2 is bigger than or equal to y1. And the smaller the β, the closer the y2 is to y1. Therefore, we can easily compare y2 and y1 of each spoke to determine which is the closest spoke to the vertical {111} plane direction just with an optical microscope. Figure 4 shows spokes after 220 min etching in 45 wt % KOH with 0.5 wt % HT541 (a kind of surfactant) at 60 °C. The 36th spoke is closest to the vertical {111} plane direction. The locating error is within ± 0.025°.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23595
Fig. 3. Schematic of two-side undercuts of silicon nitride mask on top end of the spokes after anisotropic wet etch in KOH solution.
Fig. 4. (a)-(e) Optical microscope images of different spokes showing two-side undercuts of the silicon nitride mask. The two-side undercuts on top end of the 36th spoke have the smallest and most similar sizes of width.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23596
2.2 Fabricating reference grating parallel with the located spoke After the closest spoke is located, it will be used as an alignment reference for mask patterns in the following optical lithography to fabricate a reference grating. The reference grating mask layout and the alignment schematic are shown in Fig. 5(a). We set the closest spoke into the microchannel with mask-aligner, and the maximum alignment error is 0.01°. After optical lithography, patterns are transferred into silicon nitride layer with RIE and down to silicon layer by KOH anisotropic etching with a depth of 3.4 μm. Thus 10 μm period reference grating is fabricated in wafer shown in Fig. 5(b). Reference grating lines, which represent the vertical {111} plane direction, will be aligned with interference fringes in the following interference lithography process.
Fig. 5. (a) Reference grating mask layout and the alignment schematic in reference grating fabrication process. (b) Reference grating and fan-shape pattern in wafer.
2.3 Optimizing configuration of Lloyd-mirror interferometer A simple Lloyd-mirror interferometer is employed in interference lithography process (Fig. 6). A krypton ion laser at 413.1 nm is used as a light source. The intersecting line of the mirror and substrate assembly is passed through by the optical axis to allow half of the expanded beam to be reflected by the mirror and another half fall straight on the substrate. The mirror and substrate form a 90° dihedral angle. The period (p) of the interference fringes is determined by p = λ (2sin θ) , where θ is the half angle between the directly incoming and the mirror-reflected lights, and λ is the wavelength of light. The angle θ can be modulated by rotating the sample stage on which the mirror and substrate holder are placed.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23597
Fig. 6. Schematic of Lloyd-mirror configuration of a laser interference lithography system. r is the distance between the pinhole and the intersecting line of the mirror and substrate assembly.
To reduce the alignment error caused by the bending of interference fringes generated by spherical waves, r (distance between the pinhole and the intersecting line) should be large enough. But the larger the r, the weaker the light intensity is. Finally the r is chosen at 5000 mm. The simulation results of 320 nm period are shown in Fig. 7, the maximum value of bending is within ± 0.002° over 60 mm × 50 mm area. And for 2 μm period, the maximum value of bending is smaller.
Fig. 7. (a)-(d) Simulation results of 320 nm period interference fringes. The maximum value of bending is within ± 0.002° over 60 mm × 50 mm area.
2.4 Aligning reference grating lines to interference fringes In this part, we will describe the process for aligning reference grating lines with interference fringes. The process consists of two steps. In the first step, we make the interference fringes to be perpendicular to optical table. A glass substrate coated with photoresist, which is perpendicular to the optical table, is patterned in the Lloyd-mirror interferometer. After developing process, the developed photoresist grating is put back on the substrate holder. Then moiré fringes will appear on a screen behind the substrate. By turning the photoresist grating in roll direction, the biggest period moiré
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23598
fringes will be got. And the interference fringes are aligned to the photoresist grating lines. Then we use a He-Ne laser at 632.8 nm to irradiate the photoresist grating to generate an array of diffraction orders. The He-Ne beam is parallel to the optical table. And the reference line is the intersecting line of the incident plane and the receiving screen, which is drawn by scanning the He-Ne laser beam on the receiving screen. If all the diffraction orders fall on the reference line [Fig. 8(a)], the photoresist grating lines would be perpendicular to the optical table; If not, we should further adjust the interference fringes by turning the Lloyd-mirror in pitch direction and repeat the above procedures until all the diffraction orders fall on the reference line. Then the interference fringes are perpendicular to the optical table.
Fig. 8. Schematic of alignment system. (a) All the diffraction orders from photoresist grating fall on the reference line. (b) Not all the diffraction orders from reference grating fall on the reference line.
In the second step, the wafer is put on the substrate holder. And we use the same He-Ne laser to irradiate the reference grating to generate another array of diffraction orders. By rotating the wafer in roll direction [Fig. 8(b)], all the diffraction orders will be adjusted to fall on the same reference line on the receiving screen. Then the reference grating lines are perpendicular to the optical table. After two steps, both the interference fringes and the reference grating lines are perpendicular to the optical table, so they are parallel to each other. For this alignment process, the error consists of three parts: the alignment error (α) of the interference fringes to the photoresist grating lines, the perpendicularity error (δ1) of the photoresist grating lines to optical table, and the perpendicularity error (δ2) of the reference grating lines to optical table. The error α can be calculated by α = sin −1 ( p (2d )) , where p is the period of the interference fringes and d is the period of the moiré fringes. In our experiments, d is about 20 mm, thus α is about ± 0.001°. Both the errors δ1 and δ2 are determined by the resolving distance (L) of the diffraction orders to the reference line and the distance (D) from the incidence point on the grating to the diffraction orders on the receiving screen. In experiments, we ensure L ≤ 1 mm, D ≥ 5000 mm. The errors δ1 and δ2 can be calculated by δ1 = δ 2 = ±180° L (π D ) = ±0.011° . So the total alignment error of this part is within ± 0.023°. #219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23599
2.5 Alignment error The alignment error of each part is shown in Fig. 9. The locating error of the vertical {111} plane direction is within ± 0.025°, the fabrication error of the reference grating is within ± 0.01°, the alignment error of the reference grating to the interference fringes is within ± 0.023°, and the bending error of the interference fringes is within ± 0.002°. The total alignment error is 0.036°. Locating error within ±0.025°
Vertical {111} plane direction
Fabrication error within ±0.01°
Alignment error within ±0.023°
Reference grating
Closest spoke
Bending error within ±0.002°
Interference fringes
Total error 0.036°
Fig. 9. Alignment error of each step.
3. Fabrication of HARSG
We applied the high precision alignment method to the fabrication of HARSGs combining interference lithography with anisotropic wet etch technique. Two types of 100 mm diameter oriented silicon wafers were used in this paper. The first type was silicon-on-insulator (SOI) wafer for the fabrication of 320 nm period grating with a 5 μm device layer, 2 μm buried oxide layer, and 500 μm handle layer. The buried oxide acts as an etch stop for the anisotropic etch of the device layer. The second type was single-layer silicon wafer, which has only one 500 μm device layer for the fabrication of 2 μm period grating. A 40 nm thick silicon nitride layer was deposited on the substrate by low pressure chemical vapor deposition to serve as the anisotropic wet etch mask. After the fabrication of the fan-shape pattern and the reference grating on the wafer [Fig. 5(b)], 320 nm and 2 μm period HARSGs were fabricated in a process as shown in Fig. 10. 150 nm of antireflection coating (ARC) (WiDE15, Brewer Science Inc.) and 400 nm of photoresist (AZ MIR701, AZ Electronics Materials) were spin coated for interference lithography. The 320 nm and 2 μm period photoresist gratings were patterned and aligned to the vertical {111} planes in Lloyd-mirror interferometer using 413.1 nm wavelength laser (BeamLok 2080-KV, Spectra-Physics). The grating pattern in the photoresist was transferred into the ARC and silicon nitride with RIE. The ARC and silicon nitride were etched using O2 and CF4 plasma respectively. After removing the photoresist and ARC, the grating was anisotropically etched in KOH. KOH etching is an important process step, and the etch anisotropy can be affected by KOH concentration and temperature. We will discuss these variances in the next section. A phenomenon of nonuniform etching was observed as shown in Fig. 11(a). The reason could be the intermittent interference to the etch reaction by the trapping H2 bubbles between the grating bars, which were produced by KOH etching. So an acetylenic glycols nonionic surfactant (HT541, HanTai Chemical CO., LTD.) was added to promote H2 bubbles release, and then a significant improvement of etch uniformity was achieved [Fig. 11(b)]. Because of the high aspect ratio of 320 nm silicon grating and rinse water surface tension, drying in air led to collapse problem [Fig. 11(c)]. We instead used a liquid carbon dioxide supercritical point dryer (E3100, Quorum Technologies) after dehydration with pure alcohol. Figure 11(d) shows that the collapse problem was solved by supercritical point drying. Somewhere on the
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23600
sample the high aspect ratio grating lines run unstraight [Figs. 12(a) and 12(b)], cross supports may help to solve the problem.
Fig. 10. HARSG fabrication process.
Fig. 11. (a) and (b) SEM images showing nonuniform etching phenomenon and significant improvement of etch uniformity after adding HT541. (c) and (d) SEM images showing collapse after drying in air and non-collapse with supercritical point drying of 320 nm silicon grating.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23601
4. Results and discussion
The aspect ratio one can get is determined by etch anisotropy. To obtain high etch anisotropy, we should choose an appropriate KOH etching condition. Etch anisotropy dependency on KOH concentration and temperature has been investigated in previous works [30, 34]. There is a conclusion that higher KOH concentrations and lower temperatures result in higher etch anisotropy. To get high aspect ratio of 320 nm period silicon grating, the etching process was performed in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C). The vertical etch rate (Rv) was 1.8 μm/h and the lateral etch rate (Rl) was 9.6 nm/h. The anisotropy ratio, Rv/Rl, was 188. Figures 12(a) and 12(b) show the aspect ratio is about 100. To reduce the etching time of 2 μm period silicon grating, the process was performed at 80 °C in 50 wt % KOH with 0.5 wt % HT541. The vertical and lateral etch rate was 1.64 μm/min and 12 nm/min respectively. The anisotropy ratio was 136. Figure 12(c) shows the 2 μm period silicon grating after a 10 min etch.
Fig. 12. (a) and (b) SEM images of 320 nm period silicon grating after 160 min etch in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C). (c) SEM image of 2 μm period silicon grating after 10 min etch in 50 wt % KOH with 0.5 wt % HT541 at 80 °C. (d) SEM image of 320 nm period silicon grating after 15 min etch in 45 wt % KOH with 0.5 wt % HT541 at 60 °C.
To get the actual alignment error, we anisotropically etched the 320 nm period silicon grating using the same condition with the fan-shape pattern etching (45 wt % KOH with 0.5 wt % HT541 at 60 °C). Figure 12(d) shows the 320 nm period silicon grating after a 15 min etch. The lateral etch rate was 4.2 nm/min. Comparing this etch rate with lateral etch rates in Fig. 13(a), we can see it is bigger than the lateral etch rate of the 36th spoke while smaller than that of the 37th. And the angle between the 36th spoke and the actual vertical {111} plane direction is smaller than 0.025°. So the actual alignment error is smaller than 0.075°, with its exact value unknown. From Fig. 13(b), we can see the alignment is very important to obtain high etch anisotropy. The anisotropy reduces from 189 to 108 with only about 0.2° alignment error.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23602
Fig. 13. (a) and (b) Lateral etch rate and anisotropy at different spokes in fan-shape pattern etched in 45 wt % KOH with 0.5 wt % HT541 at 60 °C.
With the developed alignment method applied in Lloyd-mirror interference lithography, silicon gratings with a large sample area of over 50 mm × 60 mm [Fig. 14(a)] have been fabricated. Noteworthily, this alignment method can also be applied in two-expanded-beam interference lithography to fabricate larger area silicon gratings with collimating lens to reduce the alignment error caused by bending of interference fringes. The sidewall RMS roughness of the 320 nm period silicon grating etched in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C), which was measured by an atomic force microscope, is 0.267 nm over 800 nm × 800 nm area [Fig. 14(b)]. This result indicates that the sidewalls are atomically smooth.
Fig. 14. (a) Sample of 320 nm period silicon grating with area of over 50 mm × 60 mm. (b) AFM image of the sidewall of the 320 nm period silicon grating etched in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C), whose RMS roughness is 0.267 nm.
In summary, the developed high precision alignment method has the ability to fabricate large area HARSGs with extremely smooth sidewalls. This is a good news to HARSGs #219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23603
applications, especially to X-ray imaging which need larger area gratings to image bigger objects and blazed transmission gratings which require extremely smooth sidewalls working as mirrorlike reflecting surfaces. This alignment method can also be applied in wet KOH polishing to reduce the sidewall roughness after Bosch plasma etching. 5. Conclusion
We have developed a grating pattern alignment method with alignment error of 0.036°. With this high precision alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The RMS roughness of the sidewall is about 0.267 nm. By using this alignment method combining interference lithography with anisotropic wet etch technique, an effective way to fabricate large area HARSGs with extremely smooth sidewalls has been demonstrated. Acknowledgments
The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 11005111 and No. 11275201).
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23604
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China 2 Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China * [email protected]
Abstract: A method was developed for aligning interference fringes generated in interference lithography to the vertical {111} planes of oriented silicon wafers. The alignment error is 0.036°. This high precision method makes it possible to combine interference lithography with anisotropic wet etch technique for the fabrication of high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. With this alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The roughness (root mean square) of the sidewall is about 0.267 nm. ©2014 Optical Society of America OCIS codes: (220.1140) Alignment; (220.4000) Microstructure fabrication; (220.4241) Nanostructure fabrication; (050.1950) Diffraction gratings.
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1. Introduction High-aspect-ratio-silicon-gratings (HARSGs) have gained a lot of interests due to their wide range of applications. In X-ray phase contrast and dark-field imaging [1–10], neutron imaging [11–13], HARSG performs as a phase grating and can be used to fabricate absorption analyzer grating. In X-ray wave front characterization [14, 15], HARSG acts as a beam splitter in the grating interferometer. In X-ray astrophysics spectroscopy [16–19], blazed transmission
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23593
HARSG becomes more and more welcome for its high broadband diffraction efficiency and high spectral resolution. Moreover, there are other applications including nanoimprint lithography [20, 21] and ultraviolet filtration [22, 23]. Bosch plasma process [18, 19, 24–26] and anisotropic wet etch using oriented wafers [16, 27–31] are the two common techniques used to fabricate HARSGs. Though the former has its own advantages, it is limited to aspect ratios of below 60 and requires complicated fabrication processes to prepare the etch mask, and has a well-known sidewall scalloping problem. While the latter can avoid those disadvantages by anisotropically etching monocrystalline silicon in potassium hydroxide (KOH) solution, it can get grating bars with extremely smooth sidewalls and ultrahigh aspect ratios up to 150. Moreover, it only needs very thin silicon nitride mask for etching process. However, grating lines need to be precisely aligned to the vertical {111} planes of oriented wafers to achieve enough etch anisotropy which determines the aspect ratio one can get. The requirement of precise grating line alignment to the vertical {111} planes limits the types of lithography used together with anisotropic wet etch technique for fabrication of HARSGs. Electron beam lithography (EBL) [27], scanning probe lithography (SPL) [28], focused ion beam lithography (FIBL) [29], and scanning-beam interference lithography (SBIL) [16, 30] have been used together with anisotropic wet etch technique. However, these types of lithography mentioned above are all scanning lithography, which need expensive precise control systems to align grating lines to the vertical {111} planes. Furthermore EBL, SPL and FIBL suffer from low throughput, high cost and limited ability in fabricating large area patterns. SBIL can fabricate large area patterns, however its tool, MIT Nanoruler [32], is unique. Optical lithography with mask-aligner [31] is also used to fabricate HARSGs with anisotropic wet etch technique, but it cannot fabricate sub-micro structures due to diffraction limit. Currently, interference lithography (IL) has been investigated as an efficient way to make periodic patterns over a large area with superior control of pattern regularity. By using IL, we can achieve grating patterns with wide period coverage from infinity to half the wavelength of light in theory, which is another advantage of IL. The process combining interference lithography with anisotropic wet etch technique is an effective way to fabricate large area HARSGs with extremely smooth sidewalls. Improved alignment method with higher precision is a quest to achieve higher etch anisotropy and smoother grating sidewall. Bruccoleri et al. developed an alignment method in the work of KOH polishing of reactiveion etched grating sidewall [33]. In their work, grating lines are aligned to the {111} planes with an error of 0.14°. In this paper, the authors introduce a higher precision method based on a different technique with an alignment error of 0.036°. Using this alignment method, 320 nm and 2 μm period HARSGs have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The RMS roughness of the sidewall is about 0.267 nm. 2. Alignment method In this section, we will describe in detail the method for aligning interference fringes to the vertical {111} planes of oriented silicon wafers. As shown in Fig. 1, the alignment process consists of three steps: (1) locating the vertical {111} plane direction using a pre-etch technique with a fan-shape pattern, (2) fabricating reference grating parallel with the located spoke closest to the vertical {111} plane, (3) aligning reference grating lines to interference fringes. Moreover, in order to reduce the alignment error caused by the bending of interference fringes generated by spherical waves, the configuration of Lloyd-mirror interferometer without collimating lens should be optimized.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23594
Fig. 1. Process of aligning the vertical {111} planes parallel to interference fringes.
2.1 Locating vertical {111} plane direction The oriented silicon wafer has a primary flat to indicate the vertical {111} plane direction, but this is accurate to only about ± 1° according to supplier specifications. To find the actual vertical {111} plane direction, a KOH pre-etch technique is utilized with a fanshape pattern with 0.05° angular spoke spaced over the range of ± 1.95°. Figure 2 illustrates the mask layout of the fan-shape pattern.
Fig. 2. Mask layout of the fan-shape pattern. Area with grey color is dark field.
After transferring the fan-shape pattern into the silicon nitride layer by contact lithography and reactive-ion etching (RIE), the wafers are anisotropically etched in KOH solution while the undercut phenomenon appears. The two-side undercuts of silicon nitride mask will be different on top end of each spoke, as shown in Fig. 3. The widths of undercut in proximal side y1 and distal side y2 are determined by y1 = V (111)t and y2 = V ( β )t , where β is the angle between the direction of each spoke and the vertical {111} plane direction, V(β) is the etch rate of the plane at β angle, V(111) is the etch rate of the vertical {111} plane, and t is the etching time. y1 is the undercut width of the vertical {111} plane, which is a constant at time t in all spokes. And V(β) is bigger than or equal to V(111) due to the anisotropic etch of silicon in KOH. Thus y2 is bigger than or equal to y1. And the smaller the β, the closer the y2 is to y1. Therefore, we can easily compare y2 and y1 of each spoke to determine which is the closest spoke to the vertical {111} plane direction just with an optical microscope. Figure 4 shows spokes after 220 min etching in 45 wt % KOH with 0.5 wt % HT541 (a kind of surfactant) at 60 °C. The 36th spoke is closest to the vertical {111} plane direction. The locating error is within ± 0.025°.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23595
Fig. 3. Schematic of two-side undercuts of silicon nitride mask on top end of the spokes after anisotropic wet etch in KOH solution.
Fig. 4. (a)-(e) Optical microscope images of different spokes showing two-side undercuts of the silicon nitride mask. The two-side undercuts on top end of the 36th spoke have the smallest and most similar sizes of width.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23596
2.2 Fabricating reference grating parallel with the located spoke After the closest spoke is located, it will be used as an alignment reference for mask patterns in the following optical lithography to fabricate a reference grating. The reference grating mask layout and the alignment schematic are shown in Fig. 5(a). We set the closest spoke into the microchannel with mask-aligner, and the maximum alignment error is 0.01°. After optical lithography, patterns are transferred into silicon nitride layer with RIE and down to silicon layer by KOH anisotropic etching with a depth of 3.4 μm. Thus 10 μm period reference grating is fabricated in wafer shown in Fig. 5(b). Reference grating lines, which represent the vertical {111} plane direction, will be aligned with interference fringes in the following interference lithography process.
Fig. 5. (a) Reference grating mask layout and the alignment schematic in reference grating fabrication process. (b) Reference grating and fan-shape pattern in wafer.
2.3 Optimizing configuration of Lloyd-mirror interferometer A simple Lloyd-mirror interferometer is employed in interference lithography process (Fig. 6). A krypton ion laser at 413.1 nm is used as a light source. The intersecting line of the mirror and substrate assembly is passed through by the optical axis to allow half of the expanded beam to be reflected by the mirror and another half fall straight on the substrate. The mirror and substrate form a 90° dihedral angle. The period (p) of the interference fringes is determined by p = λ (2sin θ) , where θ is the half angle between the directly incoming and the mirror-reflected lights, and λ is the wavelength of light. The angle θ can be modulated by rotating the sample stage on which the mirror and substrate holder are placed.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23597
Fig. 6. Schematic of Lloyd-mirror configuration of a laser interference lithography system. r is the distance between the pinhole and the intersecting line of the mirror and substrate assembly.
To reduce the alignment error caused by the bending of interference fringes generated by spherical waves, r (distance between the pinhole and the intersecting line) should be large enough. But the larger the r, the weaker the light intensity is. Finally the r is chosen at 5000 mm. The simulation results of 320 nm period are shown in Fig. 7, the maximum value of bending is within ± 0.002° over 60 mm × 50 mm area. And for 2 μm period, the maximum value of bending is smaller.
Fig. 7. (a)-(d) Simulation results of 320 nm period interference fringes. The maximum value of bending is within ± 0.002° over 60 mm × 50 mm area.
2.4 Aligning reference grating lines to interference fringes In this part, we will describe the process for aligning reference grating lines with interference fringes. The process consists of two steps. In the first step, we make the interference fringes to be perpendicular to optical table. A glass substrate coated with photoresist, which is perpendicular to the optical table, is patterned in the Lloyd-mirror interferometer. After developing process, the developed photoresist grating is put back on the substrate holder. Then moiré fringes will appear on a screen behind the substrate. By turning the photoresist grating in roll direction, the biggest period moiré
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23598
fringes will be got. And the interference fringes are aligned to the photoresist grating lines. Then we use a He-Ne laser at 632.8 nm to irradiate the photoresist grating to generate an array of diffraction orders. The He-Ne beam is parallel to the optical table. And the reference line is the intersecting line of the incident plane and the receiving screen, which is drawn by scanning the He-Ne laser beam on the receiving screen. If all the diffraction orders fall on the reference line [Fig. 8(a)], the photoresist grating lines would be perpendicular to the optical table; If not, we should further adjust the interference fringes by turning the Lloyd-mirror in pitch direction and repeat the above procedures until all the diffraction orders fall on the reference line. Then the interference fringes are perpendicular to the optical table.
Fig. 8. Schematic of alignment system. (a) All the diffraction orders from photoresist grating fall on the reference line. (b) Not all the diffraction orders from reference grating fall on the reference line.
In the second step, the wafer is put on the substrate holder. And we use the same He-Ne laser to irradiate the reference grating to generate another array of diffraction orders. By rotating the wafer in roll direction [Fig. 8(b)], all the diffraction orders will be adjusted to fall on the same reference line on the receiving screen. Then the reference grating lines are perpendicular to the optical table. After two steps, both the interference fringes and the reference grating lines are perpendicular to the optical table, so they are parallel to each other. For this alignment process, the error consists of three parts: the alignment error (α) of the interference fringes to the photoresist grating lines, the perpendicularity error (δ1) of the photoresist grating lines to optical table, and the perpendicularity error (δ2) of the reference grating lines to optical table. The error α can be calculated by α = sin −1 ( p (2d )) , where p is the period of the interference fringes and d is the period of the moiré fringes. In our experiments, d is about 20 mm, thus α is about ± 0.001°. Both the errors δ1 and δ2 are determined by the resolving distance (L) of the diffraction orders to the reference line and the distance (D) from the incidence point on the grating to the diffraction orders on the receiving screen. In experiments, we ensure L ≤ 1 mm, D ≥ 5000 mm. The errors δ1 and δ2 can be calculated by δ1 = δ 2 = ±180° L (π D ) = ±0.011° . So the total alignment error of this part is within ± 0.023°. #219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23599
2.5 Alignment error The alignment error of each part is shown in Fig. 9. The locating error of the vertical {111} plane direction is within ± 0.025°, the fabrication error of the reference grating is within ± 0.01°, the alignment error of the reference grating to the interference fringes is within ± 0.023°, and the bending error of the interference fringes is within ± 0.002°. The total alignment error is 0.036°. Locating error within ±0.025°
Vertical {111} plane direction
Fabrication error within ±0.01°
Alignment error within ±0.023°
Reference grating
Closest spoke
Bending error within ±0.002°
Interference fringes
Total error 0.036°
Fig. 9. Alignment error of each step.
3. Fabrication of HARSG
We applied the high precision alignment method to the fabrication of HARSGs combining interference lithography with anisotropic wet etch technique. Two types of 100 mm diameter oriented silicon wafers were used in this paper. The first type was silicon-on-insulator (SOI) wafer for the fabrication of 320 nm period grating with a 5 μm device layer, 2 μm buried oxide layer, and 500 μm handle layer. The buried oxide acts as an etch stop for the anisotropic etch of the device layer. The second type was single-layer silicon wafer, which has only one 500 μm device layer for the fabrication of 2 μm period grating. A 40 nm thick silicon nitride layer was deposited on the substrate by low pressure chemical vapor deposition to serve as the anisotropic wet etch mask. After the fabrication of the fan-shape pattern and the reference grating on the wafer [Fig. 5(b)], 320 nm and 2 μm period HARSGs were fabricated in a process as shown in Fig. 10. 150 nm of antireflection coating (ARC) (WiDE15, Brewer Science Inc.) and 400 nm of photoresist (AZ MIR701, AZ Electronics Materials) were spin coated for interference lithography. The 320 nm and 2 μm period photoresist gratings were patterned and aligned to the vertical {111} planes in Lloyd-mirror interferometer using 413.1 nm wavelength laser (BeamLok 2080-KV, Spectra-Physics). The grating pattern in the photoresist was transferred into the ARC and silicon nitride with RIE. The ARC and silicon nitride were etched using O2 and CF4 plasma respectively. After removing the photoresist and ARC, the grating was anisotropically etched in KOH. KOH etching is an important process step, and the etch anisotropy can be affected by KOH concentration and temperature. We will discuss these variances in the next section. A phenomenon of nonuniform etching was observed as shown in Fig. 11(a). The reason could be the intermittent interference to the etch reaction by the trapping H2 bubbles between the grating bars, which were produced by KOH etching. So an acetylenic glycols nonionic surfactant (HT541, HanTai Chemical CO., LTD.) was added to promote H2 bubbles release, and then a significant improvement of etch uniformity was achieved [Fig. 11(b)]. Because of the high aspect ratio of 320 nm silicon grating and rinse water surface tension, drying in air led to collapse problem [Fig. 11(c)]. We instead used a liquid carbon dioxide supercritical point dryer (E3100, Quorum Technologies) after dehydration with pure alcohol. Figure 11(d) shows that the collapse problem was solved by supercritical point drying. Somewhere on the
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23600
sample the high aspect ratio grating lines run unstraight [Figs. 12(a) and 12(b)], cross supports may help to solve the problem.
Fig. 10. HARSG fabrication process.
Fig. 11. (a) and (b) SEM images showing nonuniform etching phenomenon and significant improvement of etch uniformity after adding HT541. (c) and (d) SEM images showing collapse after drying in air and non-collapse with supercritical point drying of 320 nm silicon grating.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23601
4. Results and discussion
The aspect ratio one can get is determined by etch anisotropy. To obtain high etch anisotropy, we should choose an appropriate KOH etching condition. Etch anisotropy dependency on KOH concentration and temperature has been investigated in previous works [30, 34]. There is a conclusion that higher KOH concentrations and lower temperatures result in higher etch anisotropy. To get high aspect ratio of 320 nm period silicon grating, the etching process was performed in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C). The vertical etch rate (Rv) was 1.8 μm/h and the lateral etch rate (Rl) was 9.6 nm/h. The anisotropy ratio, Rv/Rl, was 188. Figures 12(a) and 12(b) show the aspect ratio is about 100. To reduce the etching time of 2 μm period silicon grating, the process was performed at 80 °C in 50 wt % KOH with 0.5 wt % HT541. The vertical and lateral etch rate was 1.64 μm/min and 12 nm/min respectively. The anisotropy ratio was 136. Figure 12(c) shows the 2 μm period silicon grating after a 10 min etch.
Fig. 12. (a) and (b) SEM images of 320 nm period silicon grating after 160 min etch in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C). (c) SEM image of 2 μm period silicon grating after 10 min etch in 50 wt % KOH with 0.5 wt % HT541 at 80 °C. (d) SEM image of 320 nm period silicon grating after 15 min etch in 45 wt % KOH with 0.5 wt % HT541 at 60 °C.
To get the actual alignment error, we anisotropically etched the 320 nm period silicon grating using the same condition with the fan-shape pattern etching (45 wt % KOH with 0.5 wt % HT541 at 60 °C). Figure 12(d) shows the 320 nm period silicon grating after a 15 min etch. The lateral etch rate was 4.2 nm/min. Comparing this etch rate with lateral etch rates in Fig. 13(a), we can see it is bigger than the lateral etch rate of the 36th spoke while smaller than that of the 37th. And the angle between the 36th spoke and the actual vertical {111} plane direction is smaller than 0.025°. So the actual alignment error is smaller than 0.075°, with its exact value unknown. From Fig. 13(b), we can see the alignment is very important to obtain high etch anisotropy. The anisotropy reduces from 189 to 108 with only about 0.2° alignment error.
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23602
Fig. 13. (a) and (b) Lateral etch rate and anisotropy at different spokes in fan-shape pattern etched in 45 wt % KOH with 0.5 wt % HT541 at 60 °C.
With the developed alignment method applied in Lloyd-mirror interference lithography, silicon gratings with a large sample area of over 50 mm × 60 mm [Fig. 14(a)] have been fabricated. Noteworthily, this alignment method can also be applied in two-expanded-beam interference lithography to fabricate larger area silicon gratings with collimating lens to reduce the alignment error caused by bending of interference fringes. The sidewall RMS roughness of the 320 nm period silicon grating etched in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C), which was measured by an atomic force microscope, is 0.267 nm over 800 nm × 800 nm area [Fig. 14(b)]. This result indicates that the sidewalls are atomically smooth.
Fig. 14. (a) Sample of 320 nm period silicon grating with area of over 50 mm × 60 mm. (b) AFM image of the sidewall of the 320 nm period silicon grating etched in 50 wt % KOH with 0.5 wt % HT541 at room temperature (21 °C), whose RMS roughness is 0.267 nm.
In summary, the developed high precision alignment method has the ability to fabricate large area HARSGs with extremely smooth sidewalls. This is a good news to HARSGs #219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23603
applications, especially to X-ray imaging which need larger area gratings to image bigger objects and blazed transmission gratings which require extremely smooth sidewalls working as mirrorlike reflecting surfaces. This alignment method can also be applied in wet KOH polishing to reduce the sidewall roughness after Bosch plasma etching. 5. Conclusion
We have developed a grating pattern alignment method with alignment error of 0.036°. With this high precision alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The RMS roughness of the sidewall is about 0.267 nm. By using this alignment method combining interference lithography with anisotropic wet etch technique, an effective way to fabricate large area HARSGs with extremely smooth sidewalls has been demonstrated. Acknowledgments
The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 11005111 and No. 11275201).
#219951 - $15.00 USD Received 28 Jul 2014; revised 25 Aug 2014; accepted 25 Aug 2014; published 18 Sep 2014 (C) 2014 OSA 22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.023592 | OPTICS EXPRESS 23604
