X-Ray masks for very deep X-Ray lithography

Microsystem Technologies 4 (1998) 70—73 ( Springer-Verlag 1998 X-Ray masks for very deep X-Ray lithography J. Klein, H. Guckel, D.P. Siddons, E.D. Johnson 70 Abstract The high aspect ratio, deep x-ray lithography and electrodeposition process [Becker et al. (1986)] can be expensive unless throughput is high enough. The use of a very high energy synchrotron has allowed the cost of exposure to be significantly reduced through simultaneous exposure of stacked photoresist [Guckel et al (1994)]. Synchrotron radiation at high photon energies has resulted the use of a large area x-ray mask. Both stacked exposures and a large area x-ray masks have significantly increased the throughput of the deep x-ray lithography and electrodeposition process. 1 Very deep X-Ray lithography The basic deep x-ray lithography and electrodeposition process creates plastic or metal prismatic structures which are up to 500 microns in height but can hold all dimensions to submicron tolerances. [Becker et al. (1986)]. These structures are used in systems for micromechanics or precision engineering. The original deep x-ray lithography and electrodeposition process uses injection molding for cost effectiveness. [Becker et al. (1986)] This is due to low throughput for a low photon energy exposures. The current average exposure cost at several synchrotrons around the United States is $100 per hour. [Guckel (1995)]. At a low photon energy synchrotron, the Synchrotron Radiation Center (SRC) at the University of Wisconsin-Madison, 5 square centimeters of common deep x-ray lithography photoresist can be exposed in one hour to a depth of 100 microns. At SRC the cost of 100 micron deep exposures is therefore $20 per hour per square centimeter. Several properties of photoresist exposure can be examined to reduce the exposure cost. If the exposure cost can be reduced, then injection molding is not required for the process to be cost effective. The common photoresist used in deep x-ray lithography is polymethylmethacrylate, PMMA. PMMA Received: 25 August 1997 /Accepted: 3 September 1997 J. Klein, H. Guckel Department of Electrical and Computer Engineering, University of Wisconsin, Madison WI 53706-1691, USA D.P. Siddons, E.D. Johnson National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973, USA Correspondence to: J. Klein is a positive photoresist requiring a reduction in molecular weight to make it soluble in a developer. A molecular weight change is obtained by bond breakage in the photoresist during exposure. The threshold molecular weight defines when a photoresist is removable in a developer and therefore specifies a threshold energy which must be absorbed from an exposure source. For PMMA this threshold is near 3000 J/cm3. For thick photoresists the upper surface will absorb more energy than the bottom. As the photon flux passes through photoresist the flux is modified by the exponential law. Equation (1) estimates the power density transmitted at a distance x into the photoresist given an input power p and an 0 absorption length L. p(x)\p e~9/L [W/cm2] (1) 0 The absorption length is a function of photon energy for each material. Figure 1 shows the absorption length for PMMA. [Soloman et al. (1988), Hubbell et al. (1996)]. The absorption length also indicates what is considered thick for a photoresist. Surface to bottom absorption ratios are 2.7 for a photoresist thickness of one absorption length. A reasonable maximum photoresist thickness exposeable is five times the spectral absorption length. The photon spectrum of a synchrotron source is not monochromatic; and therefore, the absorption into photoresist is not simple. Equation (1) must be modified to take into account the input power density at each photon energy, p (hv), 0 and the full absorption curve of the photoresist, L(hv). Equation (2) shows the result = Lp (hv) [x/L(hv) 0 e d(hv) (2) L(hv) 0 A computer program has been written to design exposures through an iterative process. This program uses the synchrotron spectrum equations and absorption curves for several materials. Figure 2 shows the filtered and unfiltered photon spectrums for the 1 GeV SRC, the 1.4 GeV Center for Advanced Micro Devices (CAMD) at Louisians State University, and the 2.6 GeV National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Filtering has been designed to remove the low energy photons because the absorption length at these low energies are small resulting in a significant amount of energy absorbed at the surface of the photoresist. This can cause thermal damage before the bottom of the photoresist reaches the threshold energy. Filtering a synchrotron source too much wastes photons since most photens pass P p(x)\ 1 Absorption length (cm) In designing an exposure for very high energy photons several issues must be considered. They include how to filter the incoming spectrum, how to design an x-ray mask for proper selectivity between exposed and unexposed regions, and how photon divergence and collimation affect linewidth control in these very thick photoresist exposures. Power absorption constraints limit the maximum power deliverable to a given thickness of photoresist. This constraint sets the required filtration. Contrast ratio requirements between the exposed photoresist and the unexposed photoresist set the x-ray mask absorber material and thickness. Photon energy and distance from the source set the line width control. Photon divergence is a function of photon energy where higher energy photons diverge less. Since the lower energy photons, which diverge more, become absorbed nearer to the surface of the photoresist; the result is no linewidth distortion due to photon divergence. Line with control is set by other factors which include distance from the source, photoresist alignment to the photon beam, x-ray mask to photoresist spacing, and thermal expansion. NSLS CAMD SRC 10 1 cm -1 10 100 µm -2 10 300 µm -3 10 10-4 C 10-5 O 10-6 102 103 104 Photon energy (eV) 105 Fig. 1. Absorption length for PMMA 0.12 2 X-ray masks 0.10 Power (W/mrad) NSLS source 0.08 0.06 CAMD source CAMD filtered NSLS filtered 0.04 SRC source SRC filtered 0.02 0 0 5000 10000 15000 Photon energy (eV) 20000 25000 Fig. 2. Power spectrum of SRC, CAMD, and NSLS. SRC (1.0 GeV, 1.60 T, 150 mA) has been filtered for photoresist exposure depths of up to 100 microns; CAMD (1.4 GeV, 1.59 T, 400 mA), upto to 300 microns; and NSLS (2.6 GeV, 1.22 T, 300 mA), up to 1 centimeter through the photoresist. Too litle filtration burns or melts the photoresist. Therefore there is an optimal filtration for each photoresist thickness. Figure 1 indicates the PMMA absorption length at the peak photon energy of each spectrum. The exposure time has been determined to be similar for the same photoresist area but to a depth scaled by the sources’s absorption length. Therefore higher photon energy sources expose a larger volume of photoresist in the same amount of time. The large volume exposure results in two options: (1) thicker photoresists may be exposed for the same square area and cost, or (2) multiple thin samples may be stacked and exposed reducing the cost per square area. In one hour a 1 centimeter thick, 5 cm by 1 cm exposure may be performed at NSLS. If one hundred, 100 micron thick samples are stacked and exposed in parallel, then the cost reduces to $0.20 per square centimeter per hour. An x-ray mask is made of a thick high atomic number absorbed supported by a thin low atomic number transparent substrate. At Wisconsin a common absorber material is gold and a common transparent substrate is silicon nitride. Figure 3 shows the absorption lengths for gold and silicon nitride and two common spectrum filter materials, silicon and beryllium. The material and thickness used as the transparent substrate of the mask is commonly combined with the filtration of the incoming photon spectrum. At SRC little filtration is necessary, so 1 micron of silicon nitride is used as the mask substrate and 50 to 500 microns of beryllium is used as the photon filter. The beryllium filter is not combined with the x-ray mask due to cost issues. At NSLS 500 to 1000 microns of silicon is used as a filter; therefore, a common 500 micron single-crystal silicon wafer is used as the substrate for the mask. This makes the mask more durable, easier or create, and have an area larger than a 1 micron thick silicon nitride mask. One micron of silicon nitride must be fabricated as a membrane supported on all sides by a thicker material a material. This results in a fragile mask where the exposeable area is small. Figure 4 shows a low and high energy x-ray mask. The increased area of the high energy x-ray mask can increase throughput and further reduce the per centimeter cost of synchrotron exposure. The thickness of the absorber depends on the contrast ratio of the exposure. Several contrast ratios can be defined. A working contrast ratio is defined as the ratio of the power absorbed at the bottom of the exposed region of the photoresist to the power absorbed at the surface of the unexposed region. When a photoresist is developed the exposed areas must develop all the way without attack on the unexposed areas of the photoresist. A working contrast of infinity will result in both the maximum height of the unexposed regions but will also leave perfectly vertical sidewalls of the exposed photoresist structures. A small contrast ratio will result in a reduction in structure height and a sidewall angle since the tops of the 71 10 Absorption length (cm) 1 72 10-1 Be 10-2 Si 10-3 10-4 SiN Au 10-5 10-6 102 103 104 Photon energy (eV) 105 106 Fig. 3. Absorption lengths for Au, SiN, Si, and Be Fig. 5. 3 mm tall PMMA structures with sloping flanks due to leaky absorber and constant rate developer 3 Fabrication of x-ray masks Fig. 4. Low energy, small area, synchrotron x-ray mask on left (4 microns of gold on a 1 micron SiN membrane). High energy, large area, synchrotron x-ray mask on the on the right (50 microns of gold on a 400 micron silicon substrate) structures are in contact with the developer longer than the bottom areas. The necessary size of the contrast ratio depends on the selectivity of the developer. For a developer with infinite selectivity a contrast ratio of one will result in perfect pattern transfer from a mask with no sidewall slope. As the selectivity of the developer decreases, a larger contrast ratio becomes necessary. At SRC contrast ratios are near 20 for a 2 micron gold absorber thickness and 100 micron thick photoresist exposures. Figures 3 can be used to scale this thickness for NSLS and 1 centimeter of photoresist. The result is a 50 micron thick gold absorber. Leaky x-ray masks are also useful. The 3 millimeter tall posts of figure 5 were created with a 10 micron thick gold absorber at NSLS. The calculated working contrast is just over 4. Careful contrast and development design can be used to control this sidewall slope. Fabrication of a silicon nitride membrane x-ray mask for SRC starts with a 16 mil, S100T, silicon wafer. One micron of stress controlled LPCVD silicon nitride is applied to both sides. A rectangular opening is cut into the nitride on the back of the wafer. KOH is then used to etch the silicon wafer exposed in the rectangular opening. KOH does not attack the silicon nitride; therefore, the KOH will stop etching when the front nitride has been reached. The result is a 1 micron thick nitride membrane supported at all edges by the silicon wafer. A conductive plating base is next applied. 200 As of titanium followed by 200 As of nickel is common at Wisconsin. 4 to 10 microns of PMMA is then spun on which is patterned with a 230 nm deep ultra-violet light source. Low stress gold is electrodeposited into the patterned PMMA. Figure 6 shows the process sequence. The total size of the x-ray exposure is set by the size of the silicon nitride membrane etched into the silicon wafer. The maximum size depends on the mechanical properties of the silicon nitride. Very large exposure areas may be made from several smaller nitride membranes separated by the silicon wafer. No patterns can be placed on the silicon runner areas between the membranes. The end result is a fragile mask with poor use of available area. The current high energy x-ray mask is made in a two step process. First a deep UV source is used to make a 4 micron thick PMMA mold for gold electroplating on a thin membrane of stress controlled silicon nitride. The gold pattern is then replicated by a low energy synchrotron into 50 microns of PMMA on a 400 micron thick silicon substrate. Figure 7 shows the process sequence. The pattern may be stepped over the PMMA covered substrate if the silicon nitride membranes are Stress controlled silicon nitride (1 µm) <100> silicon wafer (400 µm) Conductive plating base (400 Å) PMMA (50 µm) <100> silicon wafer (400 µm) KOH etch to front side Synchrotron exposure Conductive plating base (400 Å) PMMA spin coat (4-10 µm) DUV expose & develop 73 Develop & electrodeposit gold (50 µm) Electrodeposit gold Remove PMMA Remove PMMA Fig. 6. Low photon energy (3000 eV) x-ray mask fabrication process using 1 micron of silicon nitride as a substrate and 4 to 10 microns of gold as an absorber. Fig. 7. High photon energy (20,000 eV) x-ray mask fabrication process using 400 micron of silicon as a substrate and 50 microns of gold as an absorber. smaller than the substrate size. 50 microns of gold is electroplated and is used as the absorber in the high energy synchrotron exposure while the 400 micron silicon substrate is used for beam filtration to avoid damage to the subsequently exposed photoresist. galvanoforming, and plastic moulding (LIGA process) Microelectronic Engineering 4: 35—56 Guckel H; Skrobis KJ; Klein J; Christenson TR: (1994) Micromechanics via x-ray assisted processing. J. Vac. Sci. Tech. A 12(4): 2559—2564 Guckel H: (1995) Deep x-ray lithography efforts and progress in the United States. Proc. First International Micromachine Symposium, Tokyo, Japan, 1995. Soloman E; Hubbell J; Scofield J: (1988) X-ray attenuation cross sections for energies 100 ev to 100 keV and elements z\1 to z\92. Atomic Data and Nuclear Data Tables 38(1): 1—197. Hubbell J; Seltzer S: (1996) Tables of x-ray mass attenuation coefficients and mass energy-absorption coefficients 1 keV to 20 MeV for elements Z\1 to 92 and 48 additional substances of dosimetric interest. NISTIR 5632 4 Conclusions The use of high energy photon sources allows two photoresist options: (1) thicker phoresists may be exposed, or (2) multiple thin photoresists may be exposed in parallel in the same amount of time. Parallel exposure can significantly reduce the cost of synchrotron exposure. This allows cost effectiveness of the deep x-ray lithography and electrodeposition process without injection molding. The use of high energy photon sources requires greater filtration which can be incorporated into the x-ray mask. This allows the mass to have a larger area and be more durable. Both parallel processing and larger areas further reduce the cost of exposure. Higher energy photon sources require thicker absorbers. The thicknesses of these absorbers have been determined with a new computer program. Comparison of exposures at NSLS and SRC have verified these calculations. References Becker EW; Ehrfeld W; Hagmann P; Maner A; Münchmeyer D: (1986) Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography,