Optical Projection Printing Research Articles

Design and Development of ArF Photoresist for XTC Lithography Process

Focus-latitude enhancement exposure (FLEX) is an approach to enhance the effective depth of focus (DOF) in optical projection printing. However, throughput is reduced with multiple exposures. Besides, the contrast of aerial images and exposure latitude (EL) are decreased by FLEX. X-tilted correction (XTC) provides a solution to improve throughput and preserve the imaging performance of FLEX. In this paper, lithographic performance of XTC exposure was studied using polymers with different loading of photo acid generators (PAG) and quenchers. Experiment results showed that polymer with middle active energy of acid-label group performed better. MEF and DOF of XTC were not sensitive to PAG loading but the diffusion length and acidity of PAG impacted MEF and DOF. Furthermore, some quenchers were added to the photoresist to control the acid diffusion rate for XTC. We can conclude that polymer in combination with proper PAG loading and quencher must be specially designed for XTC lithography process.

Linear phase ring illumination monitor

A phase shifting mask pattern consisting of an Airy pattern multiplied by a linear phase progression is introduced as a quantitative monitor for the angular distribution of illumination in optical projection printing. The proximity effect spillover from the pattern rings is in phase at the center position only for illumination from the designed off-axis illumination direction. Thus, a quantitative analysis of the intensity in a particular pupil location is available simply by measuring intensity at the center of the pattern. A theoretical analysis is presented and image simulation studies are made of an implementation based on four-phase mask making. The results show that the signal strength is high (about 1/3–1/5 of the clear field intensity per ring), the angular discrimination is good (about 0.5/ring number), and that linear phase rings are reasonably unaffected by aberrations. Mask requirements suggest this technique is most advantageous for monitoring dipoles or quadrupoles. Since the patterns print at normal exposure levels, they may also be included on product wafers to fingerprint the illumination during production.

Design and performance of a SAW ladder-type filter at 3.15 GHz using SAW mass-production technology [wireless LANs

To meet the increasing demand for high-performance filters in gigahertz radio-communication systems, we have improved the design techniques and fabrication processes for surface-acoustic wave (SAW) devices. The standard optical projection printing technique based on i-line lithography used for mass-production was optimized, thus attaining a linewidth of 0.3 /spl mu/m. As a first SAW device prototype, we designed and fabricated a ladder-type bandpass filter from LiTaO/sub 3/ substrates, at 3.15 GHz having a 3 dB-bandwidth of 128 MHz and a minimum insertion loss of 1.7 dB.

Models for characterizing phase-shift defects in optical projection printing

An algebraic model is developed for characterizing the printability, inspection, and repair of phase-shift defects in optical projection printing. Phase-shift defects are particularly difficult to characterize because of the many parameters associated with the exposure tool and with the attenuating phase shift mask (PSM) pattern. Furthermore, the parameters change during inspection of the attenuating PSM because the mask is examined under illumination conditions which differ from the exposure illumination. An algebraic model which encompasses this large set of variables is derived by considering the electric fields under the mask to be a combination of the electric fields from the feature and defect. These fields are then combined according to the mutual coherence function for the mask illumination. A notable difficulty is the relative phase shift due to defocus between large and small features. The model is shown to be valid for defects up to 0.35 /spl lambda//NA by comparison to SPLAT. Experimental verification is made for defects impacting a 6% transmitting PSM for 0.35-/spl mu/m features at i-line. The reliability of the model is illustrated by giving rules of thumb for defect printing in attenuating PSM's. >

The manufacture of rim phase shiftin reticles

Abstract RIM Phase Shifting is one of a number of Phase Shifting Techniques being considered to bring potential advantages of improved resolution and better process latitude to optical projection printing in the volume production of high density MB DRAMS with minimum feature sizes down to 0.3μm(1,2,3). This paper describes a production process developed under the ISL project capable of manufacturing rim shifted PSMs for use at g line, i line and DUV wavelengths. The formidable issues of inspection and repair for all types of PSMs are not discussed. The ISL technique uses quartz substrate Reactive Ion Etching, (R.I.E) for the phase shift, and wet chrome etching for the production of the rim. Optical and E-beam resists were used for reticle manufacture.

Understanding Lithography Technology Issues through Simulation

Simulation has become a very effective means of understanding tradeoffs which arise in implementing innovation in optical projection printing. A number of examples associated with current concerns in image formation under investigation at Berkeley using the SPLAT, SAMPLE and TEMPEST simulators are described. The examples include diffusion effects in chemically amplified resists, imaging in thin films with high numerical aperture lenses, image quality in an exploratory extreme ultra violent (X-ray) system developed by Wood et al. [J. Vac. Sci. Technol. B 7 (1989) 1613] of AT & T, printability of out-of-focus phase-shift defects, edge effects in phase-shift masks, and scattering from underlying topography.

Monolithic two-dimensional surface-emitting strained-layer InGaAs/AlGaAs and AlInGaAs/AlGaAs diode laser arrays with over 50% differential quantum efficiencies

Monolithic two-dimensional surface-emitting arrays of strained-layer InGaAs/AlGaAs and AlInGaAs/AlGaAs diode lasers have been fabricated and operated pulsed with low-threshold current densities and differential quantum efficiencies greater than 50%. The InGaAs/AlGaAs arrays emit at 1.03 μm, while the AlInGaAs/AlGaAs arrays emit at 0.815 μm. Thus, it should be possible to fabricate monolithic arrays with comparable performance over a wide wavelength range. The individual lasers of the arrays are horizontal folded-cavity devices with two 45° internal reflectors and two top-surface facets. The design is simple to implement using optical pattern-generator masks, optical projection printing, and chlorine ion-beam-assisted etching in key fabrication steps.

Simulation of lithographic images and resist profiles

Abstract A new tool for simulation of optical projection printing is presented. This tool allows the computation of 2-dimensional aerial images and uses a simple approach for calculation of the bulk image. Resist profiles determined through simulations and experimental results are compared.

Calibrated exposure and focus test patterns for characterization of optical projection printing

Special parameter isolating test patterns have been designed, calibrated and tested for monitoring exposure and focus in optical projection printing. The test structures consist of a sequence of square areas filled with process-parameter sensitive pattern types whose dimensions systematically change area to area. For exposure monitoring sub-imageable (period < λ/(NA ∗(1+σ))) features as described by Arden and Widmann were used. For monitoring focus small opague lines and squares with a large period were used. The feature shapes and sizes were selected through two-dimensional aerial image simulation with SPLAT and tested through projection printing in an exposure-focus matrix. The exposure monitor patterns showed good sensitivity and after correcting for bias in mask making were calibrated via a sample algebraic model. The 2D focus targets were considerably more sensitive than 1D targets and both were less sensitive than predicted from aerial image behavior owing to non-vertical resist dissolution phenomena.

Optical printability of defects in two-dimensional patterns

Experiments, simulation, and algebraic analysis are used to characterize the printability of defects in various classes of features in optical projection printing. A special test mask was designed with programmed defects located adjacent to isolated lines, arrays of lines, corners, elbows, and nested elbows. These patterns were then projection printed in an exposure–focus matrix in a positive resist process on various substrates and examined in detail via scanning electron micrographs. For small defects of both polarities, the linewidth variation on isolated lines is shown to be K* (defect area) where K is 1 and 2.4 for transparent and opaque defects, respectively. For equal line and space arrays of minimum‐sized features [0.8* λ/numerical aperture (NA)], opaque defects as small as 0.3* λ/NA cause bridging while transparent defects as large as 0.5* λ/NA do not cause opens. Elbows and nested elbows are slightly more sensitive than arrays of lines due to intrafeature proximity effects. The interior corners are also sensitive to defects. Underexposure greatly increases the effects of opaque defects while defocus has little impact. The effects on aluminum substrates show similar trends to those on silicon while for thin resist on silicon, even much smaller defects print. The printability of transparent defects in positive resist is easy to model and a constant intensity threshold model is generally adequate. For opaque defects, nonvertical resist development effects typically occur and must be included in the simulation.

A study of projected optical images for typical IC mask patterns illuminated by partially coherent light

Optical projection printing using partially coherent illumination is simulated for one micrometer and half micrometer objects representative of typical mask patterns such as contact holes, rectangular bars and openings, intersections of perpendicular lines, and adjacent lines of unequal lengths. The image intensity distributions in absorptionless photoresists on nonreflective substrates are plotted as sets of constant intensity contours. For each pattern and illumination, an exposure-defocus (E-D) diagram is generated by evaluating the combined exposure and defocus tolerance yielding linewidths within ±2.5 percent of the mask linewidth. Besides comparing the image and ED margins of different object shapes and sizes, the effects of high versus low degrees of coherence, single versus dual wavelength, as well as long-wavelength high NA versus short-wavelength low NA were studied using the 1-µm rectangular opening.

Submicron MOS process with 10:1 optical-projection printing and anisotropic dry etching

Results of an n-MOS process with minimum feature sizes in the submicron range are reported. Lithography is realised by 10:1 optical printing with step-and repeat exposure. Minimum linewidths of 0.7μm have been achieved using a high numerical aperture projection optics with 0.42NA. In order to obtain high fidelity in pattern transfer, anisotropic dry-etching techniques have been used for all levels. Results are given for the patterning of the TaSi2In+-poly stack, and an aluminium etching process with BCl3/Cl2 is discussed in detail. Reducing the gate oxide thickness to 12.5nm, transistors have been optimised forminimum short-channel effects down to 0.5μm channel length. The limiting gate and drain voltages for the submicron devices have been determined. For a supply voltage of 3 V, negligible long-term transistor degradation is extrapolated. The effect of scaling of the lateral dimension on the field isolation and contact hole resistance is investigated. Exploratory dynamic RAM cell arrays with a cell area 37 μm2 and a cell capacitanceof 36 fF have been fabricated and characterised.

Practicing the Novolac deep-UV portable conformable masking technique

Deep-UV portable conformable masking, a double-layer resist patterning technique, combines the advantages of a noncontact submicron printing technique such as refractive projection imaging or scanning electron-beam patterning with those of deep-UV conformable printing. Feasibility studies were reported earlier.1,2 Here, the processes leading to fabrication of 1-μm MOSFET devices by e-beam and step-and-repeat optical projection printing will be presented. These include processing steps to retain or remove the top resist layer, to obtain resist image profiles suitable for liftoff or for reactive ion etching (RIE), to improve adhesion, to treat the interfacial layer between the two resists, to prevent cracking, and to monitor the development of the two resist layers. Electrical testing results on pin hole density are given, followed by uncapped and capped resist images of 1-μm double-poly-Si MOSFET test sites as well as 0.8-μm thick Al lifted-off images.

Partially coherent imaging in two dimensions and the theoretical limits of projection printing in microfabrication

Optical projection printing is quantitatively studied, using the diffracted image of two bars of equal width and space and an aspect ratio of three. Hopkins' four-dimensional integral for partially coherent imaging is used for the numerical analysis. Diffraction-limited lenses with a circular pupil are used. Constant intensity profiles in an absorptionless photoresist and nonreflective substrate are evaluated. Then the tolerances ot defocus and intensity are derived using a linewidth tolerance of ±2.5 percent of the mask linewidth which varies from 0.5, 0.75, 1, 1.5, 2, to 2.5 µm. The illuminations used have σ = 0, 0.5, 0.7, 0.78, 1, 1.3, and infinity.

Automatic alignment system for optical projection printing

An automatic alignment method is described which has been applied in a projection system for pattern definition by direct step and repeat on the wafer. The method is based upon the imaging through the projection lens of relief gratings on the wafer onto gratings on the mask and photoelectric detection of the resulting Moire signal. The alignment accuracy is better than ±0.1 µm and the method can be used for all integrated circuit processing steps. The advantages of the use of phase structures and electrooptical detection are discussed and practical results are given.

Optical projection printing : J. D. Cuthbert. Solid St. Technol. p. 59 (August 1977)

Optical projection printing : J. D. Cuthbert. Solid St. Technol. p. 59 (August 1977)

Line‐Profile resist development simulation techniques

AbstractThe relative advantages and disadvantages of three different algorithms are compared for simulating the time evolution of two‐dimensional line‐edge profiles produced by a locally rate dependent surface etching phenomenon. Simulated profiles typical of optical projection printing and electron‐beam and X‐ray lithography of micron‐sized lines in resist and etching of ion‐implanted SiO2 are used as a basis of comparison. One of the algorithms is a cell‐by‐cell removal model used earlier by Neureuther and Dill. One of the newly developed algorithms employs ray tracing; it can be shown that the path followed by a point on a front between the developed and undeveloped regions can be calculated using ray‐optic equations. The other new algorithm uses a string of points initially on the surface of the exposed resist. The points on the string advance perpendicular to the local direction of the string; with time the string of points moves down into the resist, replicating the action of a developer. We compare the computing cost, convenience, and accuracy of the algorithms.