Search

We report on several years of experience with the design, fabrication and use of silicon nitride thin windows. The thickness used has ranged from 20 nm to several micrometers. The window area has ranged from an edge dimension of a few micrometers to a few centimeters. Using IC technology, these windows can be mass-produced in an efficient manner. Their tensile stress can be adjusted to range from 100 MPa to 1 GPa, and they can be patterned to contain holes and slots.

Multielement microelectrode arrays have been developed for electrochemical sensors. The array sensors are fabricated on a single insulated silicon substrate (standard 2 in. wafer) using photolithographic methods. In the prototype devices, five different electrode materials have been included: Pt, Au, V, Ir, and carbon. One goal of this work was to quantitate the improvement in information content for voltammetric measurements with a matrix of different electrode materials relative to measurements made with a single electrode material. Additionally, the authors have developed fabrication methods for multielement microelectrode array detectors and evaluated their voltammetric characteristics. Ultimately, they are working toward the development of inexpensive and reproducible methods of producing these arrays so that they could be used in a disposable fashion. In order to accomplish the present goals, voltammetric data were obtained for a group of four explosive and four nonexplosive (but structurally similar) compounds in dimethyl sulfoxide using the microsensor array. The information content of these measurements was then evaluated by using the probabilistic model developed by Shannon. It was found that the information content was improved by using the multielement approach, with the average information content increasing by 25% relative to the case where a single platinum microelectrode array wasmore » used. Although their use thus far has been limited to laboratory applications, these disposable arrays could also be used in natural environments.« less

A silicon microgripper with a large gripping force, a relatively rigid structural body, and flexibility in functional design is presented. The actuation is generated by Ni-Ti-Cu shape memory alloy (SMA) films and the stress induced can deflect each side of the microgripper up to 55 {mu}m for a total gripping motion of 110 {mu}m. When fully open, the force exerted by the film corresponds to a 40 mN gripping force on the tip of the gripper.

We report new results on silicon grism and immersion grating development using photolithography and anisotropic chemical etching techniques, which include process recipe finding, prototype grism fabrication, lab performance evaluation and initial scientific observations. The very high refractive index of silicon (n=3.4) enables much higher dispersion power for silicon-based gratings than conventional gratings, e.g. a silicon immersion grating can offer a factor of 3.4 times the dispersion of a conventional immersion grating. Good transmission in the infrared (IR) allows silicon-based gratings to operate in the broad IR wavelength regions (~1- 10 micrometers and far-IR), which make them attractive for both ground and space-based spectroscopic observations. Coarser gratings can be fabricated with these new techniques rather than conventional techniques, allowing observations at very high dispersion orders for larger simultaneous wavelength coverage. We have found new etching techniques for fabricating high quality silicon grisms with low wavefront distortion, low scattered light and high efficiency. Particularly, a new etching process using tetramethyl ammonium hydroxide (TMAH) is significantly simplifying the fabrication process on large, thick silicon substrates, while providing comparable grating quality to our traditional potassium hydroxide (KOH) process. This technique is being used for fabricating inch size silicon grisms for several IR instruments and is planned to be used for fabricating ~ 4 inch size silicon immersion gratings later. We have obtained complete K band spectra of a total of 6 T Tauri and Ae/Be stars and their close companions at a spectral resolution of R ~ 5000 using a silicon echelle grism with a 5 mm pupil diameter at the Lick 3m telescope. These results represent the first scientific observations conducted by the high-resolution silicon grisms, and demonstrate the extremely high dispersing power of silicon- based gratings. The future of silicon-based grating applications in ground and space-based IR instruments is promising. Silicon immersion gratings will make very high-resolution spectroscopy (R>100,000) feasible with compact instruments for implementation on large telescopes. Silicon grisms will offer an efficient way to implement low-cost medium to high resolution IR spectroscopy (R~ 1000-50000) through the conversion of existing cameras into spectrometers by locating a grism in the instrument's pupil location.

An index matching fluid has been used to minimize the effect of interference fringes which develop when contact printing diffraction gratings on silicon wafers. These fringes are the result of interference effects when there is a small but uneven gap between the photomask and resist surface. They are especially troublesome when printing and etching large area, coarse diffraction gratings on the surface of silicon wafers and silicon disks.

Transmission ultrasound is not in widespread use, partially because of the time and expense of acquiring the data. We are addressing this problem with an optically parallel ultrasound sensor. The core of the sensor is a thin silicon nitride membrane patterned with gold to create 'acoustic pixels' over a large area. Each acoustic pixel vibrates at the frequency of the acoustic excitation. The thin membrane, supported by short walls over an optical substrate, with one side immersed in the ultrasound medium and the supported side exposed to air, flexes when an ultrasound pressure wave encounters it. This flexing causes the air gap between the optical substrate and the membrane to change. The change in the air gap modulates the reflection of an optical beam by frustrated total internal reflection. By strobing the optical beam, the deflection of the membrane can be detected and measured at any point through the acoustic period. Acquiring a sequence of images allow us to extract the relative pressure phase and amplitude. Proof of principle experiments have shown that we can build this sensor, and we are currently using a small aperture version to examine simple test objects.

High resolution spectroscopy experiments with visible adaptive optics (AO) telescopes at Starfire Optical Range and Mt. Wilson have demonstrated that spectral resolution can be routinely improved by a factor of - 10 over the seeing-limited case with no extra light losses at visible wavelengths. With large CCDs now available, a very wide wavelength range can be covered in a single exposure. In the near future, most large ground-based telescopes will be equipped with powerful A0 systems. Most of these systems are aimed primarily at diffraction-limited operation in the near IR. An exciting new opportunity will thus open up for high resolution IR spectroscopy. Immersion echelle gratings with much coarser grooves being developed by us at LLNL will play a critical role in achieving high spectral resolution with a compact and low cost IR cryogenically cooled spectrograph and simultaneous large wavelength coverage on relatively small IR detectors. We have constructed a new A0 optimized spectrograph at Steward Observatory to provide R = 200,000 in the optical, which is being commissioned at the Starfire Optical Range 3.5m telescope. We have completed the optical design of the LLNL IR Immersion Spectrograph (LISPEC) to take advantage of improved silicon etching technology. Key words: adaptive optics, spectroscopy, high resolution, immersion gratings

Silicon diffraction gratings fabricated by photolithography and anisotropic chemical etching offer unique advantages over conventional ruled gratings. Previous work showed a high amount of scatter from etched gratings and several waves of wavefront distortion due to an overetch problem. We recently began to study ways to improve the optical performance of etched silicon gratings. Defect etching was used to verify that our starting material was of high quality and that it could be polished to the required optical flatness without producing subsurface damage. We compared the effect of different etch mask materials and etching variables on the scatter and wavefront aberration of test gratings. Gratings masked with silicon nitride achieved sufficiently low wavefront distortion to be useful in high resolution spectroscopy. The scatter was improved by a factor of tow over the grating fabricated several years earlier, but is still a factor of 3 worse thana ruled echelle. A better lithography mask should reduce the scatter. The major issues in fabricating large etched gratings for astronomy are achieving good uniformity across the surface and the compatibility of thick silicon disks with standard semiconductor processing hardware.

We report on results obtained from experiments using specially prepared carbon substrates and treatment of the data by means of recently introduced theory. Medium Z grids with known parameters have been coated on top of pyrolytic carbon substrates to achieve well defined absorption geometries. The various copper grids exhibit satisfactory performance in terms of mechanical stability, homogeneity and uniformity of the coating. A detailed study of the measurement results shows that there is a more rapid increase of the associated C-K{alpha} countrate from the coated samples compared to the pure elements and is attributed to the contribution of secondary enhancement effects, including those resulting from photoelectrons generated after the primary ionization. A variety of multilayer analyzers has also been evaluated during these experiments. Only a certain combination of muitilayer component materials have been found to be appropriate for use as dispersing elements due to the reflectivity and spatial resolution requirements of our long wavelength spectrometer. Another experimental factor is the low intensity of available tube photons which is due to the selection of the target material and absorption effects in the target as well as the tube window. 12 refs., 7 figs., 2 tabs.

Over the past several years we have developed a variety of techniques for probing plasmas with x-ray lasers. These have included direct high resolution plasma imaging to quantify laser produced plasma uniformities and moire deflectometry to measure electron density profiles in one-dimension. Although these techniques have been valuable a need existed for direct two dimensional measurements of electron densities in large high density plasmas. For this reason we have worked on developing an xuv interferometer compatible with the harsh environment of laser produced plasmas. This paper describes our design and presents some results showing excellent fringe visibility using the neon-like yttrium x-ray laser operating at 155 angstrom. The coherence properties of this x-ray laser source were measured using interferometry and are also discussed.

We have developed and used for the first time a soft x-ray interferometer to probe a large laser-produced plasma with micron spatial resolutions. A neonlike yttrium x-ray laser operating at 155 A was combined with a multilayer coated Mach-Zehnder interferometer to obtain electron density profiles in a plasma produced by laser irradiation of a CH target. The measured electron density profile has been compared to hydrodynamic simulations and shows good agreement near the ablation surface but some discrepancy exists at lower densities.

recent advances in silicon micromachining techniques allow the fabrication of very coarse infrared echelle gratings. When used in immersion mode the dispersion is increased proportionally to the refractive index. This permits a very significant reduction in the overall size of a spectrometer while maintaining the same resolution. We have fabricated a right triangular prism from silicon with a grating etched into the face of the hypotenuse. The grating covers an area of 32 mm by 64 mm and has a 97.5 micrometers periodicity with a blaze angle of 63.4 degree(s). The groove surfaces are very smooth with a roughness of a few nm. Random defects in the silicon are the dominant source of grating scatter. We measure a grating ghost intensity of 1.2%. The diffraction peak is quite narrow, slightly larger than the Airy disc diameter at F/12. However, due to wavefront aberrations, perhaps 15-20% of the diffracted power is in the peak with the rest distributed in a diameter roughly five times the airy disc.

The average power performance capability of semiconductor diode laser arrays has improved dramatically over the past several years. These performance improvements, combined with cost reductions pursued by LLNL and others in the fabrication and packaging of diode lasers, have continued to reduce the price per average watt of laser diode radiation. A low cost technique developed and demonstrated at LLNL for optically conditioning the output radiation of diode laser arrays has enabled a new and scalable average power diode-end-pumping architecture that can be simply implemented in diode pumped solid state laser systems (DPSSLs). This development allows the high average power DPSSL designer to look beyond the Nd ion for the first time. Along with high average power DPSSLs which are appropriate for material processing applications, low and intermediate average power DPSSLs are now realizable at low enough costs to be attractive for use in many medical, electronic, and lithographic applications.

Detailed performance results and fabrication techniques for an efficient and low thermal impedance laser diode array heatsink are presented. High duty factor or even CW operation of fully filled laser diode arrays is enabled at high average power. Low thermal impedance is achieved using a liquid coolant and laminar flow through microchannels. The microchannels are fabricated in silicon using a photolithographic pattern definition procedure followed by anisotropic chemical etching. A modular rack-and-stack architecture is adopted for the heatsink design allowing arbitrarily large two-dimensional arrays to be fabricated and easily maintained. The excellent thermal control of the microchannel cooled heatsinks is ideally suited to pump array requirements for high average power crystalline lasers because of the stringent temperature demands that result from coupling the diode light to several nanometers wide absorption features characteristic of lasing ions in crystals.

We have developed a soft X-ray interferometer capable of probing large high-density plasmas with micron spatial resolutions. A neon-like yttrium X-ray laser operating at 155 /spl Aring/ was combined with the Mach-Zehnder interferometer to obtain electron density profiles of a laser-produced exploding selenium foil plasma. The plasma was produced with one Nova laser beam using a 120-/spl mu/m line focus, the same conditions used to create a selenium X-ray laser. The interferogram of the selenium plasma was obtained from an end-on perspective.

The development of multilayer mirror technology capable of operating in the range of 3-30 nm and the construction of thin membranes with excellent uniformity and strength have made it possible to design and implement a Mach-Zehnder interferometer operating at 15.5 nm. We have tested this interferometer by using a soft x-ray laser as a source, and we show its use in probing high-density plasmas.

Metal-oxide-semiconductor devices have been evaluated as low-energy (250 eV to 50 keV) x-ray dosimeters. They can be used to measure dosages as low as 1 rad (SiO2) to as high as 105 rad (SiO2). Their small size and basic simplicity make it possible to form arrays of dosimeters for x-ray imaging. When compared to thermoluminescent dosimeters (TLD's), photographic film, and thermopiles, MOSFET dosimeters offer distinct advantages in terms of their small size, their sensitivity to photon energies below 10 keV, and their adaptability to an electrical readout system.

To develop a fast x-ray detector with a memory, we have performed experiments with metal-oxide-semiconductor capacitor (MOS-C) devices. For timeresolved recording of the x-ray pulse, a linear array of detectors is uniformly irradiated while the elements are sequentially biased at ?t time intervals; significant positive charge trapping occurs in each element of the array only during the period ?t when the high field bias is applied across the dielectric. Detection of 10-ns resolution has been recorded. Time-resolution is presently limited by the bandwidth limitation of the bias strobing method.

Metal-oxide-semiconductor (MOS) devices have been used for detecting low-energy (2 to 20 keV) x rays for incident fluences of 10-4 to 10-1 cal/cm2 (or dosage in the device of 103 to 106 rads) in a bent-crystal spectrometer application. An attractive feature of the detector is that the three functions of 1) detection, 2) memory, and 3) readout are integrated into one semiconductor chip.

A thin-film thermopile matrix is vacuum deposited on a glass substrate. Nickel and copper thin films are used for the two junction materials. A total of 400 junctions are deposited over an area 2.2 cm by 2.2 cm, corresponding to a junction density of 82 junctions/cm2. Half of these junctions are hot junctions and the other half are reference junctions. The reference junctions are shielded from the incoming pulsed radiation by a 0.05-cm-thick gold shield. All junction pairs are series connected so that the thermal emf's add. An output voltage of 0.1 mV per junction pair is obtained when the device is illuminated with a 1.5-msec pulse from a l-kV xenon flash tube. The output voltage from an individually wired, Chromel-constantan thermopile is 0.2 mV per junction pair when the thermopile is illuminated with an identical pulse.