Gerhard Kalkowski

Electrostatic chucks for lithography applications

Abstract High precision electrostatic chucks with diameters up to 12 inches are being developed at IOF for electron/ion-beam lithography applications. For optimal performance, selection of the appropriate chuck dielectric is crucial. We have tested various materials, including sapphire, quartz and glass-ceramics with respect to chucking force under vacuum conditions. Differences in electrostatic force of more than an order of magnitude were observed and are attributed to Coulomb and Johnsen–Rahbek behaviour. For the former, reasonable agreement with theoretical calculations was obtained when taking the corresponding dielectric constants and a finite gap between wafer and support into account. Time constants for chucking and dechucking were determined for the latter.

Electrostatic chuck behaviour at ambient conditions

For advanced lithography applications, high-precision electrostatic chucks with diameters up to 12 inch are being developed at IOF. Although electrostatic chucking is mostly used in vacuum, the principle also works at ambient conditions. However, forces exerted to the wafer may not be the same in both cases. To quantify the influence of environmental conditions, electrostatic forces for various chuck dielectrics were measured under ambient conditions (humid air), dry nitrogen and in vacuum. The forces are found to be significantly reduced at ambient conditions as compared to the vacuum case. When flushing with dry nitrogen, the vacuum level is essentially retained. Clearly, water at the chuck-to-wafer interface reduces the chucking force. We suspect that dissociation of water occurs, entailing ionic charge transport and trapping in the dielectric. This eventually shields the wafer from the chuck electrode.

Direct Bonding of Glass Substrates

We report on investigations of direct bonding of glass materials for application as optical devices and high precision mechanical stages under vacuum conditions. Thin SiO2 (fused silica) wafers of about 1mm thickness were bonded to massive SiO2 substrates of up to 20mm thickness at diameters up to 200mm. Low Temperature Glass Bonding (LTGB) was performed under moderate vacuum, using a commercial bonding equipment of EVG (Austria). We report on our experience with suitable cleaning and low pressure plasma surface activation technologies to achieve high quality (optically transparent) bonds with a very low fraction of arial defects. Wetting angles were measured to monitor surface conditions during various steps of cleaning and low pressure plasma activation. Lowest defect levels were achieved with a combination of wet cleaning and N2-plasma processing, immediately proceeding the bonding process. Successful bonding was achieved in a moderate vacuum by exerting compressive forces of several kN to the glass stack at temperatures of about 250°Celsius. In the sandwich composite, essentially two classes of defects were discernible from interference fringes or haze: small circular defects –apparently from finite particle contamination– and slightly more extended edge related features –apparently from insufficient compressive forces– during the bonding process at these locations. The remaining bonding defects were analyzed by transmitting polarized light and measuring stress birefringence with equipment from ILIS (Germany). Particle related internal defects in the bonding area revealed a typical stress level of the order of 1 MPa. The unbounded edge regions showed no particular stress above the noise level. An exemplary 200mm diameter glass bond is documented in Figs. 1 and 2 below. Wafer (substrate) thickness was about 1mm (15mm), respectively, in this particular bond. This work was supported by DLR/Germany under contract No. 50YB0814.

Fabrication of a high power Faraday isolator by direct bonding

With increasing output power of lasers, absorption in optical components grows larger and demands on heat withdrawal become challenging. We report on the fabrication of a Faraday isolator for high power fiber laser applications (P = 1 kW) at a wavelength of 1080 nm and operation at ambient conditions. We investigate direct bonding of Terbium Gallium Garnet to sapphire disks, to benefit from the good heat spreading properties (having a 6-fold higher thermal conductivity than TGG) at high transparency of the latter. Successful bonding was achieved by extensive cleaning of the plane and smooth surfaces prior to low pressure plasma activation. The surfaces to be bonded were then contacted in a vacuum environment at elevated temperature under axial load. Our measurements show that the bonded interface has no measurable influence on transmission properties and bonded samples are stable for laser output powers of at least 260 W. As compared to a single Terbium Gallium Garnet substrate, wavefront aberrations were significantly decreased by bonding sapphire disks to Terbium Gallium Garnet.

Low temperature GRISM direct bonding

For spectroscopy in space, GRISM elements –obtained by patterning gratings on a prism surface – are gaining increasing interest. Originally developed as dispersive elements for insertion into an imaging light path without deflecting the beam, they are progressively found in sophisticated multi stage dispersion optics. We report on GRISM manufacturing by joining the individual functional elements –prisms and gratings – to suitable components. Fused silica was used as glass material and the gratings were realized by e-beam lithography und dry etching. Alignment of the grating dispersion direction to the prism angle was realized by passive adjustment. Materials adapted bonds of high transmission, stiffness and strength were obtained at temperatures of about 200°C in vacuum by hydrophilic direct bonding. Examples for bonding uncoated as well as coated fused silica surfaces are given. The results illustrate the great potential of hydrophilic glass direct bonding for manufacturing transmission optics to be used under highly demanding environmental conditions, as typical in space.

Electrostatic chucking of EUVL masks: coefficients of friction

In extreme ultraviolet lithography (EUVL), the mask hangs on an electrostatic chuck and is moved laterally during exposition. For proper control of the chucked mask under corresponding inertial forces, static friction of the mask on the chuck is critical and an important input parameter for reliable theoretical modelling. To determine static and dynamic friction values, measurements were performed in vacuum on a mask blank with a test chuck, smaller than a real EUVL mask chuck, but otherwise nearly identical in its characteristics. Experimental results were obtained at various voltages for a materials combination of Low Thermal Expansion Glass (LTEM) for the pin chuck surface and a mask blank with a chromium metal backside metallisation, respectively. Dynamic friction was found to be only marginally smaller than static friction and values in the range from 0.27 to 0.33 were determined for the static friction coefficient under vacuum conditions.

Silicate and direct bonding of low thermal expansion materials

Joining of materials becomes an issue, when high stability at large temperature variation is required. Stress from thermal mismatch of auxiliary materials and corresponding distortions are often unavoidable. We describe the use of two inorganic bonding technologies for joining low thermal expansion glasses. The techniques of silicate and direct bonding were applied to join ultra-low thermal expansion glass elements of 150 mm diameter to from light-weight and high precision opto-mechanical compounds. Related bond strengths were investigated on separate reference specimen. Dimensional stability of the bonded systems during thermal cycling in vacuum was investigated by Fizeau interferometry at temperatures between 78 K and 335 K with high accuracy. The results illustrate the great potential of both bonding technologies for glass based precision engineering applications to be used under highly demanding environmental conditions, like in space.

Investigations into an electrostatic chuck design for 450mm Si wafer

We report on theoretical and experimental investigations into electrostatic chuck designs for use in future e-beam lithography on 450 mm Silicon wafers. Ultra-low thermal expansion glass (ULE) and Si infiltrated Silicon Carbide (SiSiC) designs were evaluated by finite element modeling, subject to a mass budget of 8 kg. In addition to massive chucks, light-weight designs were created by applying bore holes through the chuck body below its surface. Considerable chuck bending under gravity is observed with classical kinematic 3-point mounts. Out-of-plane distortions of about 1250 (650) nm and 400 (200) nm for the massive and light-weight designs of ULE (SiSiC), respectively, were calculated. The corresponding surface in-plane distortions for a chucked Si wafer of standard thickness 925 μm amount to about 3 (1.6) nm for the massive and 1 (0.5) nm for light-weight designs of ULE (SiSiC), respectively. By using the standard 6th order polynomial correction upon e-beam writing, these values can be reduced to ≤0.7 nm for the massive designs with both materials. Various pin-pattern configurations for an ideally flat chuck surface were adopted to determine resulting wafer bending under the influence of electrostatic forces. At a typical electrostatic pressure of about 18 kPa, a square pin pattern of pin-pitch 3.5 mm and pin-diameter 0.5 mm results in wafer in-plane distortions <0.5 nm, which is considered tolerable for obtaining the desired total overlay accuracy of <4 nm. The pin structure manufacturing process for a corresponding ULE chuck surface was experimentally tested and verified. A nearly elliptic ULE plate, slightly larger than the wafer, was structured with a Chromium hard-mask and subjected to low pressure reactive ion etching to generate the pin-pattern. A homogeneity of about 7 % was obtained for the etching process, which is fully sufficient with respect to resulting variations in electrostatic attraction.

Influence of thermal load on 450 mm Si-wafer IPD during lithographic patterning

We report on Finite Element Modeling (FEM) of the influence of heat load due to the lithographic exposure on the inplane distortion (IPD) of 450 mm Si-wafers and hence on the effect of the heat load on the achievable image placement accuracy. Based on a scenario of electron beam writing at an exposure power of 20 mW, the thermo-mechanical behavior of the chuck and the attached Si wafer is modeled and used to derive corresponding IPD values. To account for the pin structured chuck surface, an effective layer model is derived. Different materials for the wafer chuck are compared with respect to their influence on wafer IPD and thermal characteristics of the exposure process. Guidelines for the selection of the chuck material und suggestions for its cooling and corrective strategies on e-beam steering during exposure are derived.

New Joining Technologies for High Stable and Smart Optical Systems

The presentation gives an overview about joining technologies for next generation optics. New concepts for polymer-free, precise and cost efficient manufacturing and assembly processes of modern optical sub-systems are illustrated.