Status of diamond as membrane material for X-ray lithography masks

Diamondand RelatedMaterials5 (1996)812-818

Status of diamond as membrane material for X-ray lithography masks M.F. Ravet, F. Rousseaux Laboratoire de Microstructures et Microdectronique, LZM-CNRS,

196 avenue H, Ravera, 92225 Bagneux, France

Abstract One of the key factors in X-ray lithography using synchrotron radiation is the manufacture of reliable masks on suitable substrate materials. Because of its outstanding properties, chemically vapour deposited diamond is expected to be the ideal material for the next generation of X-ray masks. The membrane and mask technology requirements for high resolution X-ray lithography and for deep X-ray lithography are described in this paper. Diamond as a substrate material is critically compared with other membrane materials and the state of the art of diamond membranes for X-ray lithography applications is reviewed. Keywords:X-ray lithography; Radiation; Masks

1. Introduction Proximity X-ray lithography (XRL) is potentially the replication technique of the future that will make fully accessible the gigabit dynamic random access memory, level of integration (0.181pm) in the field of the microelectronics industry. This technique is suitable for mass production on large areas with a high throughput using high flux soft X-ray (J=O.5-1.5 nm) synchrotron radiation sources. Today, XRL allows feature sizes as small as 150 nm in development and below 100 nm in research laboratories [1,2]. Thus, in competition with deep UV lithography, XRL remains the best candidate to meet the goal of below 100 nm dimensions in microelectronics. Recently, XRL was found to be of considerable interest in the field of micromagnetism for the manufacture of large size lattices of small dots etched in magnetic thin films [3]. Deep XRL (DXRL) using hard X-rays opened up new roads in the field of microfabrications and microsysterns with the implementation of the Lithographie, Galvanoformung, Abformung (LIGA) technique [4-61. The DXRL step (A= 0.1-0.5 nm), followed by electroplating and moulding, allows for fabrication of threedimensional microstructures with high aspect ratios of more than 200. LIGA is either a favourable alternative or complements techniques such as wet-chemical anisotropic etching, dry-etching processes or micromachining for production of sensors, micromachines, micro-optics, microreactors. Although economic aspects play a vital role for the 0925-9635/96/$15.00 0 1996ElsevierScienceS.A.All rightsreserved SSDI 0925-9635(95)00373-g

future of XRL in the mass production in microelectronics, micromagnetism and microsystems, the success of XRL and DXRL in research and development today depends on well-controlled mask manufacture. Thorough investigations of materials to be used as mask support membranes are a key factor. Among the materials that satisfy the basic requirements for membranes, mechanical, optical and thermal properties make diamond an ideal candidate for the next generation of X-ray masks owing to its excellent properties. Considerable efforts focused on optimizing the thin film diamond properties for X-ray membrane purposes and some practical evaluations of diamondbased masks have already been carried out. In this paper, the key factors for the X-ray mask technology in both XRL and DXRL are described. The state of the art on the properties of optimal diamond membranes and on the evaluation of diamond based X-ray masks is presented. A critical comparison of diamond with alternative materials is given.

2. X-Ray mask technology A mask is composed of an X-ray-transparent membrane on which the absorbing patterns to be replicated are deposited. Sputtered metals such as W and Ta, or electrodeposited Au, are generally used for such patterns. Usually, the membrane material is deposited onto a Si wafer. Then the Si is back etched in order to define the mask window.

M.F. Ravet, F. Rousseaux/Diamond and Related Materials 5 (1996) 812-818

The different groups working on XRL have optimized membrane-absorber couples for their mask technology, e.g. B-doped Si/Au [7], amorphous SiN,/Ta [ 81, polysilicon/W [ 93, polycrystalline SiC/Ta4B [ lo], amorphous SiNJAu [ll,lZ], amorphous SiC:H/W and Sic: H/Au [ 133. In all cases the absorber thickness is optimized with respect to the X-ray wavelength and the membrane thickness which is typically 1-2 pm. The patterns are written onto a resist by electron beam patterning with nanometre resolution and transferred to the 0.2-0.7 pm thick metal absorber layers. For DXRL, the common way to fabricate masks is to copy from an XRL-made master mask to a working LIGA mask via soft XRL. The accuracy of the working mask is usually better than 0.2 l.trn.In another approach, a chromium mask is rephcated using conventional UV photolithography [ 141. However, in this case, resolution and dimensional accuracy are limited to 3-5 pm. Gold patterns with thicknesses of lo-15 pm are produced by electroplating. The mask substrates for LIGA masks are made of Ti, Be, Si or diamond. Thick membranes are preferred, because they are less fragile and tolerate thick absorbers as well as the temperature increase caused by the high radiation levels.

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irradiated 10 million times in approximately 6 months of continuous use [ 151. Mechanical damages can result in contraction or expansion of the membrane inducing microfractures or changes in the absorber patterns position on the mask. LIGA masks experience considerable damage under high levels of hard X-rays. Many materials were investigated for X-ray membrane applications. Properties and experimental evaluations for XRL of a number of materials including BN, SiN,, Sic, Si, B-doped Si and diamond have been compared [ 16,171. It appears that, for all materials, the properties depend strongly on the preparation conditions. In particular, the hydrogen content, which has a major influence on membrane performance, depends strongly on the material and its preparation. Because of its outstanding mechanical, optical, thermal and X-ray properties, diamond is particularly well suited for X-ray applications and is considered the ideal candidate [ 181.

4. X-ray diamond membranes and masks: results and discussion 4.1. Membrane Optimization

3. Requirements for X-ray membrane materials For XRL as well as for DXRL, the X-ray membrane must satisfy a number of criteria. l It must be transparent to X-rays (low atomic number material) to achieve a sufficient contrast. l High stiffness (high b’iaxial Young’s modulus) and tensile stress are needed. in order to avoid any mechanical in-plane and out-of-plane distortion of the pattern. l High fracture strength is especially critical for masks used for contact or very close proximity XRL and for LIGA masks supporting thick absorber structures. l Surface roughness should be compatible with the pattern resolution. Only a few nanometres are tolerable for below 100 nm resolution lithography. l Optical transparency ( 50% or more at 632.8 nm) is required for alignment procedures. Commercial X-ray steppers for XRL are blased on optical systems, which allow the mask to be :mounted and the alignment of mask and wafer to be adjusted with a high accuracy (30 nm). For LIGA membranes, optical transparency is only required for multilevel exposure procedures. l A good thermal conductivity and a thermal expansion coefficient compatible with those of the absorber and silicon substrate are nleeded, in order to prevent any thermal distortions, l Radiation hardness is necessary. The membrane material must not change its physical and mechanical properties during the exposure time. In a production line with a thr0ughpu.t of 60 wafers h-r, a mask is

Diamond membranes are fabricated by first depositing a thin diamond layer onto a silicon substrate by either hot filament chemical vapour deposition (HFCVD) or microwave plasma CVD (MWCVD). Methanehydrogen mixtures or oxygen-containing gas compositions close to the hydrogen-rich corner of the C-H-O diagram of diamond CVD [ 181 are used. High hydrogen levels in the CVD gas phase, i.e. operation in the centre of the C-H-O diagram, result in mediocre mechanical stability of the membrane and, as pointed out in Ref. 11191,are not suitable for membrane and window fabrication. Windischmann and Epps [20,21] were the first to describe the fabrication and properties of freestanding diamond membranes. Subsequent efforts were devoted to optimization of the properties of diamond membranes, primarily for XRL use [22-301. Membranes with thicknesses of l-2 pm and apertures of up to 75 mm on 4 in silicon wafers have been reported. A key issue is to find the optimum deposition for minimizing the surface roughness of the diamond film. High nuclear-densities of 10’“-lO1l nuclei cmM2 are mandatory. Suitable pretreatments and/or plasma conditions favouring secondary nucleation are one way to obtain smooth diamond X-ray membranes. In order to obtain membranes that are highly transparent in the visible region, deposition conditions that result in finegrained material with negligable scattering are required. Unfortunately, the grain boundaries of such films tend to be decorated with absorbing sp2-hybridized amorphous carbon. These impurities not only have a strong

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M.F. Ravet, E RousseauxlDiamond and Related Materials 5 (1996) 812-818

influence on the transmissivity but also reduce the thermal conductivity and the mechanical strength. Consequently, a compromise that minimizes roughness, maximizes transmission, and simultaneously maintains the other favourable diamond properties has to be found. Typical growth conditions for MWCVD of membranes are substrate temperature of 800-900 “C, methane concentrations of OS-1.5%, reactor pressures of 15-30 mbar, and plasma powers of 0.7-1.2 kW. This results in linear deposition rates of 0.15-0.25 urn h-’ or deposition times of 4-8 h per membrane in a standard MWCVD reactor. Cyclic deposition processes that alternate between a high methane concentration secondary nucleation step and a low methane concentration growth step are one of several options to improve the smoothness of the films [ 29,301. For DXRL, thicker membranes are preferred. Low roughness membranes have been made by Gicquel and co-workers [30] starting from 6 urn diamond films deposited on Si wafers of 2 inch diameter using a plasma power of 600 W. To grow thick membranes in a reasonable time, higher growth rates that are possible by high power density plasmas are required. Free-standing membranes 40 mm in diameter and with thicknesses of up to 5-100 urn have been fabricated by 1.5-5 kW microwave plasmas at 80-200 mbar by Bachmann et al. [ 311. 4.1.1. Mechanical properties (i) The stress of the membrane is very important for practical performance and durability of an X-ray membrane. CVD diamond allows the stress level to be tuned by means of the deposition conditions, unlike other membrane materials (amorphous Sic (a-Sic), Sic : H, SiN : H, polysilicon), for which the compressive stress needs to be adjusted to a tensile stress by using postdeposition high temperature annealing steps. The dependence of stress on diamond purity has been intensively studied [ 30-361. The non-diamond sp2 content induces a compressive tendency of the stress. The total stress of a film results from the thermal stress, due to the mismatch of silicon and diamond thermal expansion coefficients, and the intrinsic stress related to growth defects. Both thermal and intrinsic stresses are strongly dependent on the deposition parameters. In the range of standard deposition conditions used for X-ray membranes, the thermal stress is compressive. It has been evaluated to be 240 f 35 MPa from calculations [36] and to be 570-700 MPa from in situ [35] and ex situ [30] experiments. For film thicknesses of more than 0.5 urn, the intrinsic stress of diamond films seems to be tensile. Tensile intrinsic stress and compressive thermal stress add up to a total stress of O-500 MPa for l-2 urn films. For comparison, the adjusted tensile stress of alternative membrane materials is usually less than 100 MPa. Stress determinations from X-ray diffraction

have led to the conclusion that there is no significant change in the stress of diamond on etching a 15 mm aperture into the silicon substrate [37]. (ii) The mechanical strength of diamond film can be estimated from its elastic Young’s modulus E and its fracture strength of. Correlation of mechanical properties and the film structures were studied for Sic [38] and diamond [ 39,401 membranes. The biaxial Young’s modulus E/( 1 - v), where v is the Poisson ratio, and the fracture strength of membranes are generally deduced from measurements of the membrane deflection (bulge test). Independently of the actual material, cf always increases for decreasing membrane diameters. This can be attributed to the presence of randomly distributed and mechanically weakening defects and to the fact that their probability increases with the aperture of the membrane. For Sic, E/( 1 -v) is 140-450 GPa depending on deposition techniques and stoichiometry. Typically, of lies in the range 0.1-0.3 GPa depending on hydrogen content and membrane diameter. For CVD diamond, the mechanical properties depend on the grain size and phase purity, substrate pretreatment conditions and deposition parameters (gas composition, temperature, power density or substrate bias). The Young’s modulus as well as the fracture strength deteriorate with an increase in the amorphous carbon content that mainly decorates grain boundaries. The biaxial Young’s modulus has been measured by different researchers (see Ref. [ 171 for a review and Ref. [ 391). For optimal deposition conditions 900-1000 GPa is reported. Aikawa and Baba [39] have shown that smaller grain sizes and high phase purity are required to obtain membranes with a high fracture strength. The maximal fracture strength was assessed to be 2.2 GPa for a 1 urn thick membrane with an aperture of 15 mm, grain size of 0.3 urn and high phase purity. Such biaxial Young’s modulus and fracture strength are mostly comparable with those of natural diamond (1345 and 2.8 GPa) [41]. In any case, these values are the highest among all the membrane materials. Nevertheless, like any membrane material, even diamond membranes can easily fracture under highly pointed load. 4.1.2. Roughness Among all the membrane materials, CVD diamond stands out because of its relatively high roughness due to its polycrystalline structure. Even for optimal deposition conditions, the crystallite sizes are about 30 nm and atomic force microscopy measured roughnesses (r.m.s.) are between 20 and 40 nm for l-2 urn thick membranes [25,30,42]. At only 3-5 nm, the substrate sides of the membranes are usually much smoother than the growth sides, mirroring the surface of the pretreated silicon wafer. Both grain size and roughness increase with the film thickness. For 6 pm films, 50-70 nm is

M.F. Ravet, F. RousseauxlDiamondand

measured, and several microns are common for thick films, more than 100 pm. Roughness values of 2-5 nm are standard for amorphous membrane materials such as Sic, SiN, or polysilicon but for diamond it becomes necessary to apply post-depositional planarization methods. Since the methods based on mechanical and mechanochemical polishing are material consuming, they are difficult to apply for the thin films used for X-ray membranes. New methods, however, using reactive ion etching of SiOz/diamond bilayers [43] or ion beam etching under grazing incidence [44] result in promising roughness values of about 10 nm. 4.1.3. Optical transparency Without any interference coating, diamond’s high refractive index of 2.38 results in reflection losses of about 15% per interface, i.e. approximately 30% for a membrane in air. In the visible range (400-900 nm) transmission is further reduced by absorbing sp’ carbon in the film and by scattering at rough surfaces [23]. Transparencies of SO%-60% at 632.8 nm for diamond membranes with thicknesses of l-2 pm are lower than for similarly thick a-SiC: H membranes (7080%). Although contrast is affected, the remaining transmittance of high quality membranes is still compatible with the requirements of alignment systems of steppers for XRL [45]. For LIGA masks, thicker membranes are preferred for mechanical reasons. So, non-transparent 300 pm thick beryllium slabs or 20 pm thick silicon membranes are commonly used for masks. However, highly optical transparent membranes are needed if multilevel alignments are required and here diamond becomes the ideal material. Unfortunately, the optical transmittance of diamond is strongly affected by the increase in roughness vs. thickness. Membranes with thicknesses higher than 10 pm are usually opaque. Therefore, two ways to make diamond-based LIGA masks are possible: either polishing thick diamond membranes or using thin transparent membranes. 4.1.4. Thermal properties The thermal conductivities of a large number of diamond films grown from different gas mixtures by MWCVD have been determined by means of the mirage technique [ 30,3 1,461. Correlation diagrams reveal that the content of sp’ carbon is the main factor governing the thermal properties of diamond films. There is a strong dependence of the thermal diffusivity on the thickness and therefore on the grain size. The thermal conductivity was found to depend strongly on the film thickness with top values of 1300-2200 W m-l K-’ only feasible for films thicker than 40 pm. Fine grain, 1-2 pm thick diamond films usually have a thermal conductivity below 0.5 W m-l K-‘, comparable with thermal conductivity of competing membrane materials.

Related Materials 5 (1996) 812-818

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Under optimum growth conditions it is, however, possible to grow diamond films with thicknesses of less than 3 pm that exhibit thermal conductivity as high as 700 W m-l K-l. Such diamond films would be advantageous over other materials in minimizing temperature-induced mask distortions for LIGA-DXRL, as well as for high resolution XRL. 4.1.5. X-ray stability Only a few researchers have studied the radiationinduced damage of X-ray membrane materials [47,48]. BN exhibits poor X-ray resistance. SiN,, Si and SiC present different radiation stabilities, depending on hydrogen content. Indeed, hydrogen may partially migrate to the surface of the membrane material on radiation, thus inducing changes in the membrane stress. Polycrystalline diamond is potentially the most radiation hard of all membrane materials with its low hydrogen content and high binding energy. The relationship between deposition conditions and the irradiation stability has been established [49]. It was found that diamond membranes having good crystallinity exhibit high X-ray irradiation durability. The optical transparency of diamond membranes is increased by 4.5%-6.1% at 633 nm after exposure and the diamond Raman band at 1332 cm-i is increased to 150%. In-plane distortions due to X-ray irradiation were measured to be extremely small (less than 20 nm). It has been shown that a diamond membrane remained undamaged after incident DXRL exposure doses of 2 MJ cmu3 in a static mode, which corresponds to 250 standard exposures [SO]. 4.2. Diamond masks 4.2.1. Processing

Diamond masks for XRL have been manufactured [ 25,451 by using an optimized process (Fig. 1) developed for an X-ray mask technology based on SiC : H membranes [ 131. The process involves an electron beam patterning of a resist-coated diamond membrane, followed by electrodeposition of gold. Feature sizes as small as 60 nm for line gratings (Fig. 2(a)) and 50 nm for dot gratings have been obtained. It has been shown that a 20-30 nm roughness of optimized as-deposited diamond films is tolerable for high resolution patterns. Evaluation of such a mask on a commercial Karl-Suss X-ray stepper allows us to conclude that optical transmission of 50% at 632.8 nm for a 1.3 pm thick diamond membrane is suitable for correct alignment performances. The alignment accuracies for diamond and Sic : H masks are comparable and are included within the 50 nm alignment error specified for this type of stepper. As a consequence of the rather higher tensile stress of diamond deposits (above 200 MPa), compared with the stress of silicon carbide membranes (50-100 MPa), thin

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Ravet, F. RousseauxJDiamond and Related Materials 5 (1996) 812-818

mepodtim of 1.3 w Polycrystalllne Dlemond layer on P pretrated 6SUconwafer

6-Gold Electrodeposition

2.chemhl back-etching of the substrate

7-Removing of PMMA

3-DF?positionoftbe plating bsse (Cr 1OmdAu 2&m)

8-Ion-Etching of the plating base

4-Resist coating PMMA 0.5 ~II

5- El&ron beam patterning OfPMMA

Fig. 1. X-ray mask processing based on diamond membrane.

silicon substrate can be severely bent (lo-30 pm for a 1.3 p thick film on a 4 in, 500 pm thick wafer). Thicker silicon wafers (l-2 mm) are required in order to limit this deformation and to improve the mask flatness. LIGA masks were made by using 6 pm thick diamond membranes [ 301. Well-adhering gold structures with thicknesses of lo-15 pm were deposited without increasing the fracture tendency of the membrane (Fig. 2(b)). Such masks are currently employed to replicate micrometric patterns into 300 pm thick PMMA X-ray resists c511. 4.2.2. Distortions In-plane distortions were investigated by measuring the positions of special arrays of lines and crosses at the

beginning and at the end of the X-ray mask fabrication process. Measurements using 0.2 pm thick evaporated tungsten on a 1.5 pm diamond membrane indicate extremely low distortions (70 nm) compared with the maximum distortion of 0.3 pm found for the 0.45 pm thick tungsten embedded in a 2.5 pm thick silicon membrane [52]. These low distortions are attributed to the high value of the Young’s modulus of diamond membranes.

5. Conclusions Mechanical properties, optical transparency and high irradiation hardness make diamond a favourite membrane material for XRL. However, as for the other membrane materials, the characteristics of diamond membranes depend drastically on the deposition conditions. Pure diamond with a high crystallinity offers the best properties, i.e. high fracture strength, high thermal conductivity and radiation stability are obtained at the sacrifice of surface smoothness. Optimized fine-grain, optically transparent membranes are an acceptable compromise and X-ray masks for high resolution based on such films were shown to be operational on a commercial X-ray stepper. For the LIGA-DXRL process, diamond is recognized as the best material when optically transparent masks are required. Planarizing treatments should be developed to reduce the surface roughness and to maximize the optical transmittance to make diamond membranes fully suitable for XRL applications.

Acknowledgements The authors are grateful to the researchers and engineers of L2M involved in the X-ray lithography program: H. Launois, A.M. Haghiri-Gosnet, Y. Chen, Z.Z. Wang,

Fig. 2. (a) High resolution X-ray mask: 60 nm, 300 nm gold line grating (thickness, 250 nm) on a diamond membrane (thickness, 1.3 pm). The high resolution of the pattern is not affected by the diamond roughness (r.m.s. 20 nm). (b) LIGA mask: gold line (width, 10 pm; height, 6 pm) on a diamond membrane (thickness, 6 pm).

M.F. Raw,

F RousseauxjDiamond and Related Materials 5 (1996) 812-818

D. Decanini, A. Madouri, F. Carcenac, L. FerlazzoManin and J. Bourneix. They also thank A. Gicquel (LIMHP, Villetaneuse) and P.K. Bachmann (Philips Research Laboratory, Aachen) for their collaboration in the L2M programme on diamond-based X-ray masks and for their support for writing the present paper.

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