Gina M. Calderon

PMMA as an X-ray resist for micro-machining application: latent image formation and thickness losses

Abstract This paper reports preliminary results of direct observation of a latent image formed in thick PMMA resist after X-ray exposure.

Multilevel 3D patterning of stacked PMMA sheets for x-ray microlithography

This paper presents a novel technique for fabricating 3D patterns in a thick layered resist and describes an alignment aide designed for the specific application of thick resist x-ray micromachining. In this technique, a PMMA layer of desired thickness is formed on a substrate by spinning or solvent bonding. The layer is exposed with X- rays to generate a latent image. A second layer of PMMA is bonded over the first layer and is exposed with an appropriate mask, generating a latent image in the second layer. This process can be repeated several times creating a 3D latent image. Simultaneous development forms a true 3D pattern in the PMMA resist.

Thick PMMA layer formation as an x-ray imaging medium for micromachining

Conventional resist application techniques are based on spinning a resist layer onto a mechanically dominating substrate. As thicker imaging layers are required, the integrity of the resist/substrate system is influenced by the resist thickness. The traditional LIGA approach is to form a PMMA resist sheet on the substrate by casting using a press. This method causes high stresses in the resist and at the resist/substrate interface. Another method consists of gluing or bonding a PMMA sheet with subsequent machining to a desired thickness. The stresses can be high enough to cause the resist to crack and/or separate from the substrate. In this paper, alternative and improved techniques are presented. One of these is a modified multiple coating spin-on method, suitable for producing PMMA resist thickness of 60-80 micrometers . The other method is based on bonding a solid PMMA sheet of desired thickness using an appropriate solvent. These techniques produce uniform PMMA layers with thicknesses ranging from 5 micrometers to 1500 micrometers and above. A mechanical cleaving test was used to estimate the resist/substrate bond strength and characterize the bonding solvents. Issues such as radiation swelling and thickness losses associated with latent image formation in PMMA are addressed.

Temperature rise in thick PMMA resists during x-ray exposure

Temperature measurements of thick PMMA resist during X-ray (1 to 5 keV) exposure are presented in this paper. Thin metal (gold) film thermal sensors were fabricated directly on the resist surface and on the resist/substrate interface using micro-lithography methods. The temperature measurements were conducted in vacuum (< 10-4 Torr) and in 1 to 25 Torr helium pressure--conditions corresponding to typically X-ray lithography exposure. The results of temperature rise measurements performed with thermal sensors and with miniature conventional thermocouples are compared.

X-ray microfabrication of multi-level structures and 3-D patterning

Abstract Preliminary results of a novel techinique for producing true three-dimensional patterns in thick resits are presented in this work.

Thin metal film thermal microsensors

Design, fabrication, and testing of thermal micro-sensors suitable for miniature and microscopic systems, for application on thin films (free standing or on substrates) as temperature sensors are presented in this paper. The sensors utilize the electrical resistivity temperature dependence of a metal. Using micro-lithography methods, several sets of gold resistors were fabricated in the form of flat 30 to 250 nm thick wires, 7 - 10 micrometers wide, and several cm long in a serpentine shape covering approximately 1.0 mm2. These sensors have demonstrated better than 0.001 degree(s) C sensitivity. The electrical resistivity and its thermal coefficient of a thin gold metal film were compared with those of bulk material. Temperature measurements on Si wafers were performed in situations corresponding to x-ray lithography exposure conditions suitable for micromachining. The temperature rise and relaxation time of a silicon wafer during x-ray exposure were measured in vacuum and different He gas pressures.