Showing papers in "SPIE milestone series in 2004"

Progress in digital integrated electronics

Abstract: C o m p l e x i t y o f i n t e g r a t e d c i r c u i t s h a s a p p r o x i m a t e l y d o u b l e d e v e r y y e a r s i n c e t h e i r i n t r o d u c t i o n . Cos t per func t ion has decreased severa l thousandf o l d , w h i l e s y s t e m p e r f o r m a n c e a n d r e 1 i a b i l i t y have been improved ramat ical ly. Many aspec ts o f p rocess ing and des ign techno logy have con t r i bu ted t o make the manu fac tu re o f such func t i ons as comp l e x s i n g 1 e c h i p m i c r o p r o c e s s o r s o r memory c irc u i t s e c o n o m i c a l l y f e a s i b l e . I t i s p o s s i b l e t o a n a l y z e t h e i n c r e a s e i n c o m p l e x i t y p l o t t e d i n F i g u r e I i n t o d i f f e r e n t f a c t o r s t h a t can, i n t u r n , be examined t o s e e w h a t c o n t r i b u t i o n s h a v e been i m p o r t a n t i n t h i s d e v e l o p m e n t and how t h e y m i g h t be xpected t o c o n t i n u e t o e v o l v e , The expected t rends can be recombined to see how long exponent i a l g r o w t h i n c o m p l e x i t y c a n b e e x p e c t e d t o con t inue .

Improving resolution in photolithography with a phase-shifting mask

Abstract: The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

On the diffraction theory of optical images

Abstract: The theory of image formation is formulated in terms of the coherence function in the object plane, the diffraction distribution function of the image-forming system and a function describing the structure of the object. There results a four-fold integral involving these functions, and the complex conjugate functions of the latter two. This integral is evaluated in terms of the Fourier transforms of the coherence function, the diffraction distribution function and its complex conjugate. In fact, these transforms are respectively the distribution of intensity in an 'effective source', and the complex transmission of the optical system-they are the data initially known and are generally of simple form. A generalized 'transmission factor' is found which reduces to the known results in the simple cases of perfect coherence and complete incoherence. The procedure may be varied in a manner more suited to non-periodic objects. The theory is applied to study inter alia the influence of the method of illumination on the images of simple periodic structures and of an isolated line.

Bolometer noise: nonequilibrium theory

Abstract: New theoretical results for noise in cryogenic bolometers are derived. Johnson noise is reduced by as much as 60% by electrothermal feedback from the bias supply. Phonon noise in the thermal link is reduced by as much as 30% relative to the usual equilibrium formula. Photon noise in the Rayleigh-Jeans limit is computed with attention to the attenuation of the photon correlations in the light beam. Basic results on bolometer responsivity, time constant, and thermal properties are presented in a new and convenient form. Excess 1/f and contact shot noise are also discussed.

Cramming more components onto integrated circuits

Abstract: The future of integrated electronics is the future of electronics itself. The advantages of integration will bring about a proliferation of electronics, pushing this science into many new areas. Integrated circuits will lead to such wonders as home computers—or at least terminals connected to a central computer—automatic controls for automobiles, and personal portable communications equipment. The electronic wristwatch needs only a display to be feasible today. But the biggest potential lies in the production of large systems. In telephone communications, integrated circuits in digital filters will separate channels on multiplex equipment. Integrated circuits will also switch telephone circuits and perform data processing. Computers will be more powerful, and will be organized in completely different ways. For example, memories built of integrated electronics may be distributed throughout the machine instead of being concentrated in a central unit. In addition, the improved reliability made possible by integrated circuits will allow the construction of larger processing units. Machines similar to those in existence today will be built at lower costs and with faster turnaround.

Low-temperature germanium bolometer

Abstract: A bolometer, using gallium-doped single crystal germanium as the temperature-sensitive resistive element, has been constructed and operated at 2°K with a noise equivalent power of 5×10 - 1 2 w and a time constant of 400 μsec. Sensitivities approaching the limits set by thermodynamics have been achieved, and it is shown that the background radiation limited or BLIP condition can be satisfied at 4.2°K. An approximate theory is developed which describes the performance of the device and aids in the design of bolometers with specific properties. The calculated noise equivalent power at 0.5°K, for a time constant of 10 - 2 sec, is 10 - 1 5 w. The detector is suitable for use in both infrared and microwave applications.

Effect of dislocations on the electrical and optical properties of long-wavelength infrared HgCdTe photovoltaic detectors

Abstract: The quantitative effects of dislocations on the electrical and optical properties of long-wavelength infrared (LWIR) HgCdTe photovoltaic detectors was determined by deliberately introducing dislocations into localized regions of two high-performance arrays having cutoff wavelengths of 9.5 and 10.3 μm at T = 78 K. Results show that dislocations can have a dramatic effect on detector R 0 A product, particularly at temperatures below 78 K. For large dislocation densities, R 0 A decreases as the square of the dislocation density; the onset of the square dependence occurs at progressively lower dislocation densities as the temperature decreases. A phenomenological model was developed which describes the dependence of the detector R 0 4 product with dislocation density, based on the conductances of individual and interacting dislocations which shunt the p-n junction. Spectral response and quantum efficiency are only weakly affected, as is the diffusion component of the leakage current. The 1/f noise current was found to increase approximately linearly with dislocation density and also tracks with the magnitude of the leakage current similar to a data trendline established for undamaged HgCdTe detectors. These results can be used to understand the performance limitations of LWIR HgCdTe arrays fabricated on heteroepitaxial substrates.

On the theory of optical images, with special reference to the microscope

Abstract: (1896). XV. On the theory of optical images, with special reference to the microscope. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science: Vol. 42, No. 255, pp. 167-195.

Lithography and the future of Moore's law

Abstract: The definition of"Moore's Law" has come to refer to almost anything related to the semiconductor industry that when plotted on semi-log paper approximates a straight line. I hesitate to review it's origins and by doing so restrict it's definition. However, today I will review the history and past performance relative to predictions and show where the advances have come from. I will leave the future performance up to you. Certainly continuing on the same slope doesn't get any easier. It presents a difficult challenge to the industry. The original paper that postulated the first version of the "law" was an article I wrote for the 3 5th anniversary issue of Electronics Magazine in 1 965 . My assignment was to predict what was going to happen in the semiconductor components industry over the next ten years -- to 1975 . In 1965 the integrated circuit was only a few years old and in many cases was not very well accepted. There was still a large contingent in the user community who wanted to design their own circuits and who considered the job of the semiconductor industry to be to supply them with transistors and diodes so they could get on with their jobs. I was trying to emphasize the fact that integrated circuits really did have an important role to play.

Minimum detectable power of a pyroelectric thermal receiver

Abstract: For a ferroelectric crystal, the rate of change of polarization with temperature is large, especially close to the Curie point where a phase transition occurs. Thus, a change of polarization due to an incident radiation flux gives rise to a large pyroelectric voltage output. The "minimum detectable power" W m is calculated. For barium titanate, the value of W m thus obtained is found to be much better than other uncooled thermal receivers and comparable with a superconducting bolometer, if dielectric noise at the transition is small. In certain cases, W m is limited only by temperature noise, so that the receiver may be thought of as a true thermal receiver.

Imaging characteristics of multi-phase-shifting and halftone phase-shifting masks

Abstract: Phase-shifting masks and imaging characteristics are discussed and compared with those of conventional transmission masks. Then, new phase-shifting masks with intermediate values of phase shifting or transmittance are proposed, and their imaging characteristics are investigated. A phase-shifting mask with a 90° phase difference can ease the restrictions on pattern geometries used in phase-shifting technology but does not increase the focus latitude. It is also suggested that a halftone phase-shifting mask is suitable for printing isolated patterns and it gives wider focus latitude than conventional mask technology.

Key issues in HgCdTe-based focal plane arrays: An industry perspective

Abstract: Technological limits, not fundamental issues, are all that keep HgCdTe from completely dominating almost all infrared (IR) applications. The technological limits result from our incomplete understanding of HgCdTe materials science. This article's snapshot view, from the unique perspective of the major United States HgCdTe IR detector array companies, suggests directions that HgCdTe detector technology should take as we approach the next century. We argue that the Government, industrial users, and industrial producers of HgCdTe must support continuing scientific investigation of this material to parallel the manufacturing of second generation HgCdTe-based systems. Without this manufacturing science base, systems will develop too slowly, cost too much, and fail too often. To assist developing this critically important technology, the article highlights some of the more vexing and as yet unsolved HgCdTe materials science and engineering problems that we see as being important.

Fast proximity correction with zone sampling

Abstract: Proximity effects cause integrated circuit features to be distorted when compared to the original mask pattern. These effects are becoming widely recognized as serious barriers to achieving effective half-micrometer and smaller wafer lithography processes. A promising remedy for proximity effects is to adjust mask feature shapes to compensate for predictable distortions in the wafer lithography process. Proximity effects are systematic -- the effects on a pattern repeat when printed with the same equipment under the same process conditions. Predicting proximity distortions is a non-trivial problem. The distortions are functions of the size and shape of each feature as well as the sizes, shapes, and spacings of nearby features. Imaging characteristics of the exposure tool, resist and etch characteristics, and effects from the underlying substrate all contribute to wafer feature distortions. We present a general method for computing proximity-corrected shapes. A technique we call "zone sampling" provides accurate models of proximity behaviors with optimum computational efficiency. Proximity behavior for arbitrary layout configurations is derived from two-dimensional density measurements (zone samples) computed from IC layout pattern data. Zone sampling provides a complete "behavior model" of combined, non-linear proximity effects, including optical, process, and underlying substrate mechanisms.

Si:Sb blocked impurity band detectors for infrared astronomy

Abstract: We discuss the characteristics of impurity band conduction detectors prepared from epitaxial Si:Sb. These detectors are sensitive to infrared light at wavelengths between 2.5 and 40 μm and outperform the current state-of-the-art Ge:Be photoconductors for this wavelength range by a combination of a peak responsivity R=32 A/W, low noise equivalent power (NEP) =1.1×10 - 1 5 W/Hz 1 / 2 (at a flux of 10 1 2 ph/cm 2 s), inherently low sensitivity to cosmic ray particles, and freedom from the anomalous behavior observed in photoconductors.

On the detectivity of quantum-dot infrared photodetectors

Abstract: We report on the analysis of thermally-limited operation of quantum-dot infrared photodetectors (QDIPs). A device model is developed and used to calculate the QDIP detectivity as a function of the structural parameters, temperature, and applied voltage, as well as to determine the conditions for the detectivity maximum. The QDIP detectivity is compared with that of quantum-well infrared photodetectors (QWIPs). This work clarifies why the existing QDIPs are still inferior to QWIPs and shows that a significant improvement in the QDIP performance can be accomplished by the utilization of dense QD arrays with small QDs.

A comparison of the limits to the performance of thermal and photon detector imaging arrays

Abstract: Photon detectors are fundamentally limited by generation-recombination noise arising from photon exchange with a radiating background. Thermal detectors are fundamentally limited by temperature fluctuation noise arising from radiant power exchange with a radiating background. These two classes of detectors have differing dependencies of their detectivities upon their operating temperature and that of the radiating background. The limits favor photon detectors at shorter wavelengths (LWIR) and lower operating temperatures. Thermal detectors are favored at longer wavelengths (VLWIR) and higher operating temperatures.

Automated optical proximity correction: a rules-based approach

Abstract: In this work we demonstrate the power, speed and effectiveness of an automated rules-based approach for performing optical proximity correction. The approach applies to both conventional and phase-shifting mask layouts for optical lithography. Complex imaging, substrate and process phenomena can be folded into comparatively few rules parameters. Using simple arithmetic, these parameters pre-compensate the layout for the combined proximity effects. The rules consist of edge rules and corner rules for biasing feature edges and for adding sub-resolution assist features. This paper describes an integrated solution which includes rules parameter generation and fast, hierarchical rules application. Experimental results demonstrate improved edge placements and wider process latitude than for non-corrected layouts.

Two-point resolution with partially coherent light

Abstract: The two-point resolution problem has been discussed previously using the Sparrow criterion. However, this masks the real changes of the image structure as the degree of coherence between the two points changes. The only measurable quantity is, of course, the separation between the two object points. It is found that in all but the incoherent limit the ratio of the real to measured separation is not equal to unity except for specific values of the real separation. Both theoretical and experimental results are presented.

New imaging technique for 64M-DRAM

Abstract: We have established a new photolithographic technique called SHRINC ( Super High Resolution by Illum-Nation Control ) which is based on an innovative illumination system. SHRINC improves the resolution and depth-of-focus ( DOF ) by optimum arrangement of the illumination system in respect of the angle of the 1st-order of diffraction generated by the reticle pitch. The capabilities of SHRINC have been studied by computer simulation. Results from phase shift, annular illumination, and conventional illumination are compared with those of SHRINC. The results show that using SHRINC with 0.35μm line and space patterns, the DOF, defined as the distance over which the aerial image contrast exceeds 60%, is 2.5x larger than that obtained with conventional illumination, and almost the same as that with phase shift techniques. In our experiments we have obtained a critical resolution of 0.275μm and more than 2.8 μm DOF with 0.35μm L/S patterns, using an i-line stepper and SHRINC illumination. Moreover SHRINC is effective not only for simple line and space patterns, but also for complicated patterns with 0.30 or 0.35μm design rules, such as memory cell patterns or peripheral circuit patterns in the DRAM. From these results we conclude that i-line steppers with SHRINC will make possible pass production of 64M-DRAMs with single layer resist.

Fundamental physics of infrared detector materials

Abstract: The fundamental parameters of IR photon detection are discussed relevant to the meaningful comparison of a wide range of proposed IR detecting materials systems. The thermal generation rate of the IR material is seen to be the key parameter that enables this comparison. The simple materials physics of 1) intrinsic direct bandgap semiconductors; 2) extrinsic semiconductors; 3) quantum well devices, including types I, II, and III superlattices; 4) Si Schottky barriers; and 5) high temperature superconductors, will be examined with regard to the potential performance of these materials as IR detectors, utilizing the thermal generation rate as a differentiator. The possibility of room temperature photon detection over the whole IR spectral range is discussed, and comparisons made with uncooled thermal detection.

Properties of thermistor infrared detectors

Abstract: Thermistor bolometers are fast, sensitive infrared detectors with good responsivity from 1 to 15 microns. The detectors consist of a thin flake of thermistor material mounted on a thermal sink. The time constant of the detectors is determined by the type of thermal sink on which the flakes are mounted. By cementing thermistor flakes to quartz or glass thermal sinks, detectors with time constants ranging from 3 to 5 and 5 to 8 milliseconds, respectively, are obtained. Experimental results are given for quartz- and glass-backed thermistor detectors having sizes of sensitive areas ranging from 0.2 to 12.5 square millimeters. Performance criteria for these detectors are tabulated and plotted as a function of size of sensitive area. Typical frequency response and relative spectral responsivity data are given. Detector flakes have to be mounted in sealed housings for noise-free operation. Two types of housings are described.

QWIP FPAs for high-performance thermal imaging

Abstract: System properties of focal plane array (FPA) cameras based on GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) operating in the long-wavelength infrared are analyzed. Due to the limited charge storage capacity of available readout circuits, a small photoconductive gain improves the noise-equivalent temperature difference (NEΔT) of the sensor. We have demonstrated several camera systems based on conventional photoconductive QWIPs with NEAT < 10 mK and NEAT < 20 mK for FPAs with 256 x 256 and 640 x 512 pixels, respectively. A small photoconductive gain is obtained by operating the QWIP at very small electric fields ( < 3 kV/cm). Finally, we discuss the possibility of reducing the NEΔT even further by using QWIP structures in which the photoexcited carrier mean free path is controlled by the incorporation of additional epitaxial layers. We thus obtain a high detectivity at very small signal and noise currents, which is useful in particular for large arrays with reduced pixel sizes. In addition, these detectors are shot-noise limited since the recombination noise is suppressed. The performance of FPA sensors based on these detectors will be addressed.

Normal-incidence inas self-assembled quantum-dot infrared photodetectors with a high detectivity

Abstract: An InAs/AlGaAs quantum-dot infrared photodetector based on bound-to-bound intrahand transitions in undoped InAs quantum dots is reported. AlGaAs blocking layers were employed to achieve low dark current The photoresponse peaked at 6.2 μm. At 77 K and -0.7 V bias, the responsivity was 14 mA/W and the detectivtiy, D*, was 10 1 0 cm.Hz 1 / 2 /W.

Monolithic silicon bolometers

Abstract: A new type of bolometer detector for the millimeter and submillimeter spectral range is described. The bolometer is constructed of silicon using integrated circuit fabrication techniques. Ion implantation is used to give controlled resistance vs temperature properties as well as extremely low 1/f noise contacts. The devices have been tested between 4.2 and 0.3 K. The best electrical NEP measured is 4 × 10 - 1 6 W/Hz at 0.35 K between 1- and 10-Hz modulation frequency. This device had a detecting area of 0.25 cm2 and a time constant of 20 msec at a bath temperature of 0.35 K.

Low-cost 320×240 uncooled IRFPA using conventional silicon IC process

Abstract: A 320x240 uncooled infrared focal plane array (IRFPA ) with series PN junction diodes fabricated on a silicon-on-insulator (SO!) wafer has been developed. Resistive bolometers, pyroelectric detectors and thermopile detectors have been reported for large scale uncooled IRFPAs, while the detector developed uses the temperature dependence of forward-biased voltage of the diode. The diode has low 1/f noise because it is fabricated on the monocrystalline SOI film which has few defects. The diode is supported by buried silicon dioxide (BOX) film of the SOI wafer, which becomes a part of a thermal isolated structure by using bulk silicon micromachining technique. The detector contains an absorbing membrane with a high fill factor of 90% to achieve high IR absorption, and the readout circuit of the FPA contains a gate modulation integrator to suppress the noise. Low cost IRFPA can be supplied because the whole structure of the FPA is fabricated on commercial SOI wafers using a conventional silicon IC process.

Infrared detectors: an overview

Abstract: The paper presents progress in infrared (IR) detector technologies during 200 history of their development. Classification of two types of IR detectors (photon detectors and thermal detectors) is done on the basis of their principle of operation. The overview of IR systems and detectors is presented. Also recent progress in different IR technologies is described. Discussion is focused mainly on current and the most rapidly developing detectors: HgCdTe heterostructure photodiodes, quantum well AlGaAs/GaAs photoresistors, and thermal detectors. The outlook for near-future trends in IR technologies is also presented.

New phase shifting mask with self-aligned phase shifters for a quarter micron photolithography

Abstract: In order to improve remarkably the resolution of photolithography without improving the resolution of exposure systems, we propose a new simple and effective phase shifting mask technology. The new mask has self-aligned phase shifters to need no assistant patterns and no complicated designing the phase shifter patterns, which are essential to the conventional phase shifting mask. The new mask with phase shifter size of 0.5μm reduces the width of photo-intensity to 60% of that without phase shifters, while keeping high contrasts. We have succeeded in fabricating the new phase shifting mask and obtained 0.2pm line resist patterns with a high contrast resist profile even by a KrF excimer laser stepper with the resolution capability of 0.4μm. This new method is extremely attractive for enhancing optical lithography resolution in ULSI region.

Uncooled infrared focal plane array having 128 × 128 thermopile detector elements

Abstract: A 128 × 128 element thermopile infrared image sensor has been developed. This device has a monolithically integrated structure to increase fill factor. The CCD for signal charge accumulation and signal charge read-out is fabricated on the silicon surface. Over the CCD, silicon dioxide diaphragms for thermal isolation are made by using micromachining technology. On each diaphragm, 32 pairs of p-type polysilicon and n-type polysilicon thermopile are formed. The noise equivalent temperature difference obtained, by the device, is 0.5 °C with an f/1 lens. Since the materials used are the same as those for silicon IC'S, and since the whole fabrication process is carried out at the silicon IC plant, it can be said that a low cost uncooled infrared image sensor is realized by this technology.

Optical projection lithography using lenses with numerical apertures greater than unity

Abstract: We have studied the characteristics and limitations of optical projection lithography using high numerical-aperture (NA) optical microscopy lenses. With oil-immersion, NA's as large as 1.4 were realized, and linewidths asfine as 140 nm were achieved in 70 nm-thick commercial photoresist films using illumination of wavelength λ = 453 nm. Previous researchers have also obtained deep-submicron linewidths using high NA lenses. The new elements in our work are: (1) use of a photoresist-compatible oil to achieve NA = 1.4; (2) use of modern antireflection coatings (ARC) and tri-level processing to circumvent the problem of substrate back-reflection; (3) use of NA > 1 optical-projection to make masks for x-ray lithography, and the x-ray replication of such masks. Our motivation in pursuing this work was to explore the limits of high-NA optical projection, and to develop a simple, quick-turn-around method of making sub-quarter-micron-linewidth x-ray masks. Using the formula for minimum feature width W m i n = kλ/NA, we find that with an ARC and careful focusing one can work at k ∼ 0.43. Focusing and alignment are done by projecting the reticle image onto the resist with yellow light, which does not expose the resist. For exposure, the yellow filter is removed and a narrow-band blue filter (λ = 453 nm) inserted.