Harish M. Manohara

Photoemission study of direct photomicromachining in poly(vinylidene fluoride)

Direct pattern transfer onto poly(vinylidene fluoride) was achieved by using x-ray photons from a synchrotron radiation source. Quadrupole mass spectrometry and ultraviolet photoemission spectroscopy, combined with ab initio molecular orbital calculations, were employed to investigate the mechanism of direct photomicromachining. The mass spectrometry identified H2, F, and HF as the etched products, with no carbon containing species being detected. The changes in photoemission spectra due to photodegradation were analyzed by comparison with ab initio molecular orbital calculations. This analysis indicated that a high degree of conjugation is generated in the degraded polymer due to the loss of fluorine atoms. It is concluded that the mechanism of direct photomicromachining is ascribable to the shrinking of the irradiated polymer region due to defluorination and the generation of conjugation.

Thin crystalline functional group copolymer poly(vinylidene fluoride–trifluoroethylene) film patterning using synchrotron radiation

The photodegradation mechanism due to synchrotron radiation exposure of crystalline poly[vinylidene fluoride–trifluoroetylene, P(VDF–TrFE)] copolymer thin films has been studied with ultraviolet photoemission spectroscopy (UPS) and mass spectroscopy. Upon increasing exposure to x-ray white light (hν⩽1000u200aeV), UPS measurements reveal that substantial chemical modifications occur in P(VDF–TrFE) 5 monolayer films, including the emergence of new valence band features near the Fermi level, indicating a semimetallic photodegradeted product. The photodetached fragments of the copolymer consist mainly of H2, HF, CHF, CH2. This x-ray exposure study demonstrates that P(VDF–TrFE) films, possessing unique technologically important properties, can be directly patterned by x-ray lithographic processes.

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.

Deep X-ray lithography with a tunable wavelength shifter at CAMD

An additional X-ray lithography facility is under construction at the Center for Advanced Microstructures and Devices. It will receive radiation from a 7.5 T superconducting three-pole wavelength shifter. The critical energy of the insertion device is tunable up to a maximum value of 11.2 keV, allowing for optimization of photon spectra to resist thickness. In particular, this hard X-ray source will allow investigation of X-ray lithography at very high energies for devices with thicknesses in excess of 1 mm, and study of low-cost mass-production concepts, using simultaneously exposed stacks of resist layers.

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.

Preliminary results at the ultradeep x-ray lithography beamline at CAMD

The Center for Advanced Micro structures and Devices (CAMD) at Louisiana State University supports one of the strongest programs in synchrotron radiation micro fabrication in the USA and, in particular, in deep x-ray lithography. Synchrotron radiation emitted form CAMDs bending magnets has photon energies in the range extending from the IR to approximately 20 keV. CAMD operates at 1.3 and 1.5 GeV, providing characteristic energies of 1.66 and 2.55 keV, respectively. CAMD bending magnets provide a relatively soft x-ray spectrum that limits the maximal structure height achievable within a reasonable exposure time to approximately 500 micrometers . In order to extend the x-ray spectrum to higher photon energies, a 5 pole 7T superconducting wiggler was inserted in one of the straight sections. A beam line and exposure station designed for ultra deep x-ray lithography was constructed and connected to the wiggler. First exposures into 1 mm and 2 mm thick PMMA resist using a graphite mask with 40 micrometers thick gold absorber has been completed.

New technique for direct integration of mechanical motion with electronics on a chip

A novel device to directly integrate mechanical motion with electronics on a chip for system integration is designed and fabricated. The device is a laterally movable gate field effect transistor. Here the gate moves parallel to the substrate surface rather than perpendicular to its as in the moving gate transistors reported earlier. Lateral motion results in linear response of device drain current with gate motion. It also makes large motion possible. The device has a variety of applications in smart sensors, actuators and integrated smart systems. A simple fabrication process is developed that is compatible with fabrication of high-aspect ratio structures. The latter give distinct performance improvement. The basic principles of operation of the LMGFET is demonstrate din initial measurements. This, to our knowledge, is the first report on the operation of such a device.

Analysis and enhancement of bond strength of acrylic sheets to metallic substrates for use in LIGA-type processing

In thick photoresist applications, commercially available acrylic sheets are bonded to a substrate as an alternative to the casting and in-situ polymerization of PMMA. The factors affecting the adhesion of a thick acrylic sheet to different substrates have been studied. In case of copper and titanium substrates and bond-strength can be improved by roughening the surface through chemical oxidation which then provides a mechanical interlocking between the resist and substrate surfaces. Annealing of PMMA sheet before gluing and use of adhesion promoter such as organosilane further improves the bond strength at the resist-substrate interface. The resist adhesion to various substrates is evaluated by measuring the debonded length of the acrylic sheet during a mechanical cleaving test.

Power measurements of exposure radiation using thin metal film sensors

An exposure radiation power measurement technique utilizing thin gold film thermal sensors has been presented. The sensory system of the power meter (or calorimeter) consists of three interlaced serpentine resistors covering an area of 6 cm by 0.4 cm, functioning as a thermal sensor, a heater and a shielding electrode. The measurement principle is based on recording the change in resistance of the sensor due to heating under radiation and internal calibration. The interlaced gold sensors were fabricated using optical lithography on a 100 mm diameter silicon wafer. The power measurements have been performed at CAMD/LSU 1.3 - 1.5 GeV synchrotron source, on a white light beamline (Emax approximately 4 keV). The measurement results agree with calculations within approximately 4%. The relaxation time of the calorimeter response was 90 seconds in vacuum (10-4Torr) and 18 seconds in 25 Torr helium. The power from a UV lamp of an ORIEL optical exposure station was measured using an interlaced thermal sensor and a commercial calorimeter. The results agree within 2%.

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.