Thin Silicon Wafers Research Articles

Neutron guidance by internal reflections in thin silicon wafers

Abstract We have performed cold neutron longitudinal transmission measurements through single crystal silicon wafers of 200 μm thickness and 50 mm length which have been coated on both sides with nickel to form microguides. Rocking curve measurements with neutrons of a wavelength of 7 A have been conducted on assemblies of straight wafers placed end-to-end for neutron pathways in silicon from 50 to 200 mm, and on curved wafers. In addition, transmission measurements have been carried out on a straight wafer as a function of wavelength. We find that the reflectivity for the internal silicon-nickel interface is 0.988±0.005.

Design of two devices for biaxial stresses and their application to silicon wafers

Two devices have been constructed for the development of biaxial stresses and the study of deformation potentials on thin layers. In the first device hydrostatic pressure was employed to develop a two-dimensional stress environment on thin silicon wafers. The deformation of the plates was found to be related to the third root of the pressure applied, in good agreement with theory. The second apparatus used a central force for the bending of the wafer, and the stresses, as detected from the frequency shift of the silicon phonon line in the Raman spectra, were confined to a small area of the plate. In both devices and analytic relation between the radius of curvature at the centre of the wafer and the stresses developed was established.

A two-wire, digital output multichannel microprobe for recording single-unit neural activity

An implantable neural microprobe has been developed to enable the correlations between electrical activity in the human central nervous system and externally applied psychophysical stimuli to be investigated. The final goal is treatment of chronic pain in a closed-loop scheme of electrical recording and stimulation in the brain. The recording probes that serve this purpose are entirely realized in a silicon technology and incorporate 10 TiW/Au recording sites each, with contact areas ranging from 100 μm 2 to 2500 μm 2, spaced at 100 μm intervals. A bipolar measurement scheme has been chosen over a unipolar one for reasons of biological noise reduction. The structure of the devices is such that strength can be combined with excellent electrical coupling to the brain tissue and that tissue damage from probe insertion is kept at a minimal level. Basically, the envisaged probe substrate structure is obtained from a thinned silicon wafer by the application of a two-electrode electrochemical etch stop in potassium hydroxide (KOH) and a subsequent reactive ion etch (RIE) of silicon in sulphur hexafluoride (SF 6). Signal transfer from the implanted microprobe to the outside is solved in a first stage by a dedicated TiW/Au on polyimide miniature flat cable. In a second stage, however, a custom CMOS integrated multiplexer circuit is designed for this purpose. The separate interface chip is to be mounted along the probe on the tip of a 1 mm diameter catheter. The circuitry performs buffering, impedance conversion, amplification, multiplexing and conversion to a digital pulse-width modulated (PWM) output current in the supply lines, which enables both powering and signal transfer to be effected via two leads only. The essence of the circuit topology and functioning will be briefly described, but the focus primarily lies on the design and fabrication of the passive probe.

Neutron focusing using microguides

Analytical techniques which are dependent solely on the neutron capture reaction rate may be advanced by neutron focusing. We describe a system composed of curved totally reflecting mirrors for the focusing of neutrons by the superposition of intensity transmitted through many thin silicon films, each of which points individually to the desired focus. The acceptance technique shows that the maximum current density is obtained with a large number of microguide sheets of minimum thickness, a short focal length, and a large radius of curvature. This analysis indicates the transmission properties of continuously curved neutron guides as a function of wavelength, and enables the calculation of an estimate of transmission efficiency. Reflection losses increase geometrically with the average number of reflections. The major losses in transmission are caused by misalignment of the layers which diffuse the focus from its theoretical maximum. We propose using 25 μm thin silicon wafers with a 100 nm coating of nickel, loaded into a cassette specially fabricated to ensure overlapping and to enable a small curvature to be maintained. The length of the system should be as short as possible (∼ few cm) to reduce both reflection losses from the nickel coating and scattering losses in the neutron transmitting medium of silicon.

Grazing-incidence Bragg–Laue X-ray diffraction

Internal waves excited by the dynamical diffraction of X-rays from a single set of crystal planes perpendicular to the incidence surface can produce both Bragg diffracted and reflected beams and Laue diffracted and transmitted beams for crystal slabs of finite thickness. The excitation of Bragg and Laue beams for grazing-incidence diffraction geometries is investigated experimentally and theoretically. The traditional dispersion surface picture for describing allowed internal and external wave vectors is developed to illustrate how the Bragg and Laue beams arise. Experiments performed on a thin silicon wafer demonstrate the evolution of the Laue beams from the back surface into Bragg beams at the front surface of the crystal. The results are in good agreement with calculated curves obtained from the 'n-beam' diffraction theory of Colella [Acta Cryst. (1974), A30, 413- 423]. This theory is shown to be very useful for grazing-incidence excitations because of its completely general treatment of Bragg and Laue beams, and the exact numerical nature eliminates the need for approximations. Experiments also reveal how the presence of a 40 A AuPd layer on the incidence surface suppresses the Bragg-diffracted beam but not the specular and Laue beams.

Optical detection of photoacoustic pulses in thin silicon wafers

The propagation of photoacoustic pulses in thin silicon wafers is studied by a probe-beam deflection method. A comparison with transducer measurements is made, which suggests the existence of a nonpropagating plate mode. A signal-to-noise analysis is carried out, and the sensitivity of this detection scheme is analyzed for the case of high-frequency broad-band detection.

An in vitro model of erythroid egress in bone marrow

An in vitro system has been developed that mimics the passage of erythrocytes from the bone marrow to the circulation. Bone marrow egress and its proper regulation are vital physiologic processes. However, because of the inaccessibility of the marrow, it is difficult to evaluate the various factors important in controlling these processes or even to define the precise mechanism by which egress occurs. The in vitro system has been designed to evaluate the importance of different physical parameters in regulating egress. It consists of a thin silicon wafer (thickness approximately equal to 1.0 micron) cemented over the tip of a large (15.0 micron ID) micropipette. The wafer contains a single circular pore. Cells were observed under the microscope as they passed through the pore under controlled pressures. The rate and duration of passage were obtained from videorecordings of the experiment. The measured passage times agreed well with the predictions of a simple analytical model of a cell passing through a thin aperture. The experimental results confirm the conclusion reached from the analysis that the pressures needed to drive a cell through the pore are well within the physiologic range, and the time needed to complete egress is typically less than 1.0 seconds. These results support the hypothesis that erythrocyte egress may be driven by a hydrostatic pressure difference across the pore.

An In Vitro Model of Erythroid Egress in Bone Marrow

An in vitro system has been developed that mimics the passage of erythrocytes from the bone marrow to the circulation. Bone marrow egress and its proper regulation are vital physiologic processes. However, because of the inaccessibility of the marrow, it is difficult to evaluate the various factors important in controlling these processes or even to define the precise mechanism by which egress occurs. The in vitro system has been designed to evaluate the importance of different physical parameters in regulating egress. It consists of a thin silicon wafer (thickness approximately equal to 1.0 micron) cemented over the tip of a large (15.0 micron ID) micropipette. The wafer contains a single circular pore. Cells were observed under the microscope as they passed through the pore under controlled pressures. The rate and duration of passage were obtained from videorecordings of the experiment. The measured passage times agreed well with the predictions of a simple analytical model of a cell passing through a thin aperture. The experimental results confirm the conclusion reached from the analysis that the pressures needed to drive a cell through the pore are well within the physiologic range, and the time needed to complete egress is typically less than 1.0 seconds. These results support the hypothesis that erythrocyte egress may be driven by a hydrostatic pressure difference across the pore.

Optical monitoring of photoacoustic pulse propagation in silicon wafers

We demonstrate a new optical and noncontact method for direct and fast detection of photoacoustic pulse propagation in thin silicon wafers. A probe beam deflection technique is used to monitor both longitudinal and Lamb wave propagations from which information on the elastic constants and the sample orientation can be obtained. Results obtained for wafers of different orientations and doping levels are compared with previous contact measurements.

Carbon Measurement in Thin Silicon Wafers (∼400 μm) by Infrared Absorption Spectrometry

An original method is developed to determine the carbon content in industrial samples (thin silicon wafers double-side polished) through IR spectrometry. This method permits to remove interference fringes in the infrared spectra by using the properties of Brewster incidence.

Hole injection into silicon from ions in solution

It is shown that ions in solution with an ’’energy level’’ below the valence band edge of n-silicon can inject holes, whereas weaker oxidizing agents in solution cannot inject holes. The injection is detected by the collection of the holes at the far side of the thin silicon wafer using a reverse biased silicon/electrolyte Schottky barrier diode.

Migration on fine molten wires in thin silicon wafers

A series of molten Al-rich wires between 10 and 500 μ wide were translated through thin (100) and (111) silicon wafers in a thermal gradient to produce vertical p-n junctions, which are sufficiently narrow to be considered for some integrated-circuit applications. Thermal gradients were generated by heating the silicon wafers with either a scanning electron beam in vacuum or a hot tungsten filament in various atmospheres. The migration rate rapidly increased with the wafer temperature in agreement with calculations of the temperature gradient through the wafer. A reduction in the magnitude of the gradient near the colder wafer surface was explained by radiative-heat transport through the semitransparent silicon wafer. The width of the resulting doped region was finer in (100) wafers than in (111) wafers, because the shape of the liquid wire was narrower for the (100) orientation. A relationship between the linewidth, wafer temperature, and Al film geometry was derived to predict the linewidths from 30 to 140 μ in (100) wafers.

Impurity Profile Measurements of Thin Epitaxial Silicon Wafer by Multilayer Spreading Resistance Analysis

A new algorithm for spreading resistance measurement data processing is proposed. The method, originally used for solving a thermal resistance problem of a multilayered heat sink, is applied to electrical resistance problems to give fast, exact, multilayer correction factors for converting measured spreading resistances to a profile. The correction factor is calculated by times multiplication of matrices instead of the inversion of a matrix, where is an arbitrary number changing with the data points of spreading resistances (layers). The computer program, developed on the basis of this algorithm, is applied to impurity profile evaluation of thin (0.5–1.5 μm) epitaxial silicon wafers for millimeter wave IMPATT diode fabrication and affords the results agreeing with capacitance voltage profiling.

Cyclic heating of a film on a substrate

A mathematical model is presented for cyclic heat treatment of a film on a substrate, and heating curves have been constructed for a thin silicon wafer heated by an infrared source and cooled by forced convection.

Josephson tunneling through locally thinned silicon wafers

We have made and tested Josephson junctions in which tunneling takes place through a locally thinned region in a single-crystal silicon wafer. An etching technique is used to produce a square uniform thinned layer, 87.5 μm on a side and ≈400 Å thick. After removal of the oxide layer, superconducting metals are deposited on both sides. Temperature and magnetic field dependences of the critical current, as well as the magnitude of the current, conform very closely to the theory for a Josephson tunnel junction. This is the first type of Josephson junction employing a barrier which is accessible for modification prior to deposition of the superconducting electrodes.

Multilayer vapor-phase epitaxial silicon millimeter-wave IMPATT diodes

A new procedure has been developed for fabricating small-area silicon avalanche diodes directly on a plated copper heat sink. The process incorporates multilayer vapor-phase epitaxially grown silicon and a preferential electrochemical etching technique to fabricate thin uniform silicon films; 6 µ thick films on 2-cm diameter wafers have been obtained reproducibly with little difficulty. Inverted mesa diodes 30-40 µ in diameter have been formed by etching through the unmasked area of the thinned silicon wafer. Implementation of this technology has resulted in single drift region p+-n-n+diodes that generate over ¼ W CW power with 6-percent efficiency at 60 GHz.

A gauge for the precision measurement of the thickness of germanium and silicon wafers

Some of the difficulties associated with the precision measurement of the thickness of thin germanium and silicon wafers, together with several possible methods of measurement, are briefly discussed. A gauge which employs the optical-lever principle is described in which frictional restraint of the moving parts is reduced to that of a knife edge alone, thus ensuring good repeatability of reading with probe pressures of less than 3g. The gauge covers the range 0–270 microns with an overall accuracy of ±0.5 micron.