Laser Interference Lithography

In 1995 van Rijn [3] proposed for the first time the use of laser interference lithography for the production of micro-and nanoengineered membranes (micro- and nanosieves). Because of the relatively simple periodic structures (orifices and slits) that are needed for the production of nanosieves and the fact that laser interference is potentially a very non-expensive patterning technique [68-72] that is also applicable on nonplanar surfaces (very large focus depth) this method will be discussed more in detail, also because there is little published material on this subject so far [73]. In 1996 in a collaboration of van Rijn [74] with G. J. Veldhuis [127], A. Driessen, P. Lambeck, and Professor C. Lodder of the University of Twente an existing laser apparatus was modified to obtain photolithographic patterns for the development of nanosieves and isolated magnetic domains [76].

Double Laser Interference Exposure Technique When two planar waves of coherent light interfere, a pattern of parallel fringes will appear. These fringes can be used for the exposure of a photosensitive layer. See Figure 23.

The depth of focus of this method is dependent on the coherence length of the light and can be on the order of meters or more, compared to (sub)micrometers for conventional optical lithography systems. As a result the demands on substrate flatness and wafer positioning are less critical.

After the first exposure the substrate is rotated over 90° and exposed to laser interference lines again. Now the gratings cross each other and after development a square array of lacquer pores (Fig. 24, left) remains. The exposure time of the photolacquer layer is a critical factor. In case the exposure time is chosen larger the middle pattern of Figure 24 will be obtained. Upon further increasing this time isolated photolacquer dots (right) remain.

Device Fabrication with Relatively Short Exposure

Time Part of an incoming plane wave is reflected by the mirror and interferes with the undisturbed part of the wave to form an interference pattern (grating) on the substrate surface. To produce the plane wave, TE polarized light of an argon laser with a wavelength Auv = 351.1 nm is spatially filtered and expanded by focusing it onto a pinhole. See Figure 25.

If the light intensity of each beam is I0, the radiance on the surface is given by

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Figure 23. Left: first exposure of the substrate with the photolacquer layer. Middle: rotation of the substrate 90°. Right: second exposure of the substrate.

Figure 24. SEM micrograph of photolacquer layer that remains after a double exposure in the laser interference setup. Left: short exposure time, middle: intermediate exposure time. Left and middle: Courtesy of J. Micromech. Microeng. Right: long exposure time. Courtesy of Aquamarijn Research.

between the beams inside the resist. We used 6 = 20° and with n = 1.7 at Avm = 351.1 nm we find A = 510 nm and A± = 105 nm. Therefore the thickness of the photoresist layer is chosen smaller than 105 nm.

The area that can be patterned using our mirror of 2.5 x 2.5 cm2 equals approximately 9 x 9 mm2 for A = 510 nm.

In Figure 26, the backside of a support (1), a single crystalline 3" (100)-silicon wafer with a thickness of 380 /m, is pre-etched to a thickness of 15 /m using optical lithography and conventional KOH etching (25%, 70 °C). On the front side of the pre-etched support (1) a layer (2) of amorphous low stress [78] silicon rich silicon nitride with thickness 0.1 /m is deposited by means of LPCVD by reaction of dichloresilane (SiH2Cl2) and ammonia (NH3) at a temperature of 850 °C. Except at the area where the microsieve pattern will be formed an etch mask layer (3) of sputtered chromium with a thickness of 30 nm is deposited.

On top of this chromium layer (3) a layer (4) of positive resist was spun and patterned using interference lithography. A 100 nm thick layer (4) of positive photoresist (Shipley S1800 series) was spun, followed by a 5 min prebake at 90 °C to evaporate the solvent. The resist was exposed to the with Ax the fringe period in the x-direction planar to the photolacquer layer:

Here Auv is the wavelength of the laser light in the medium that surrounds the substrate (usually air) and 6 is the halfangle between the two beams. The smallest period that can theoretically be obtained occurs for 6 = 90° and is equal to

The smallest period that can theoretically be obtained with our configuration is A = Auv/2 = 175 nm. The corresponding minimum pore size (with a porosity >30%) will be approximately 175/2 = 88 nm. At low porosity of course smaller pore sizesa can be made.

Since the beam is only split for a short path length near the substrate, this setup is very insensitive to mechanical instabilities and no feedback loop [77] is required to stabilize the interference pattern.

The thickness of the photosensitive layer needs to be chosen with care to avoid problems with the periodic pattern normal to the substrate surface due to interference between the incoming beam and the one reflected on the substrate surface. Its period is given by A± = Avm/2n cos 6n where n is the refractive index of the photoresist and 26n is the angle

b pnnnnnnnno.

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UV-laser

Mirror

Pinhole

UV-laser

Pinhole

1 1 il

1 '

1 1 I]

1 1

Substrate -

Substrate holder

Lens

Substrate -

Substrate holder

Figure 25. Setup for laser interference lithography. Courtesy of J. Micromech. Microeng.

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Figure 26. Schematic representation of the fabrication process of a microsieve. The numbers indicate the silicon support (1), the silicon nitride membrane (2), the chromium etch mask (3), and the photoresist layer (4).

d interference line pattern for 45 s. The intensity of the incoming light in the exposed area was measured to be 2 mW/cm2 for normal incidence (6 = 0°). After rotating the substrate over 90° the exposure was repeated. The resist was developed for 15 s in a 1:7 mixture of Shipley-Microposit 351 developer and deionized water and dried by spinning. The exposure time was chosen such that only at the crossings of the grid lines (after first and second exposure) does the resist receive enough energy to be removed completely after development. Therefore a two-dimensional pattern of pores is created in the resist. A SEM picture of the exposed (2 x 45 s) and developed resist is given at the left in Figure 26. The diameter of the pores in the photoresist depends on the duration of exposure, herewith giving a possible tool to vary the pore size at a constant pore to pore distance. However, when the exposure time is chosen too long (2 x 75 s) the pores in the resist pattern may become too large and will overlap (Fig. 26, middle).

Next the interference pattern is transferred into the silicon nitride membrane layer (2) by means of CHF3/O2 reactive ion etching at 10 mTorr and 75 W for 2 minutes forming the required perforations. Subsequently the silicon underneath the membrane layer (2) is anisotropically etched with an SF6O2 plasma at 100 mTorr and 100 W for 10 minutes with an etch rate of 2 ^m/min in order to form the macroscopic openings in the support (1). Figure 27 shows a SEM photograph of the resulting perforated membrane layer (2) showing a very regular pore pattern, the pore size being 260 nm with a pore to pore spacing of 510 nm. The pore size was very uniform over the whole 9x9 mm2 area.

Microsieves with pore sizesa down to 65 nm have been fabricated using double-exposure laser interference lithography. The pores are obtained with an inverse process, as the direct process of pore formation in photolacquer has a narrow process latitude. An array of posts is transferred into an array of pores by evaporating chromium onto the posts, followed by a lift-off in acetone. The resulting patterned chromium layer is used as an etch mask for plasma etching of the silicon nitride membrane. See Figures 28 and 29.

Figure 27. SEM micrograph of the microsieve membrane showing pores with a diameter of 260 nm in a 100 nm thick silicon nitride layer. Courtesy of Nanotechnology.

Figure 28. Left: SEM micrograph of 80 nm wide posts with rippled sidewalls caused by the vertical interference pattern. Right: SEM micrograph of a membrane after plasma etching through the pores in a chromium layer. Courtesy of Aquamarijn Research.

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