Advanced wafer steppers must be capable of meeting production requirements for devices with 0.5 μm design rules. Such devices require a very good optical performance matched to advanced resist processing techniques. Overlay performance at this resolution is also critical, with capability needed in the region of 100 nm. This paper gives an overview of developments in wafer stepper technology that have taken place to meet these requirements. The advances of projection lenses for 5x reduction steppers is reviewed. The performance of recently developed g-line and i-line lenses with numerical apertures greater than 0.4 and fields larger then 21 mm are reported. It is shown that half micron resolution can be achieved with i-line steppers. Furthermore, the possible extension to deep UV wavelengths is discussed. The overlay accuracy improvements recently realized on production steppers (PAS 2500) are discussed. It is concluded that single machine overlay better than 0.1 μm can be expected in the near future. In combination with short wavelength lenses, this means that wafer stepper technology can be extended down to at least 0.3 μm.
In order to meet the resolution rquirements of future devices while maintaining cost effectiveness and high productivity, a low-cost machine modification was made. A newly designed bandpass filter was produced that optimizes the broadband illumination-based Micralign 500/600 projection aligner for use with i-line resist systems. The wavelength transmission of this new filter was selected to obtain the high-resolution characteristics of the UV-3 spectral range while providing the high throughput associated with the UV-4 spectral range. Performance data obtained under manufacturing conditions with a number of photoresists is presented. It shows that the practical considerations of resolution and depth of focus across the full field of the substrate were enhanced through the use of this bandpass filter. The inherently high throughput of projection systems in addition to one-micron resolution capability in a production process, make the Micralign 600 projection aligner a very cost-effective alternative to stepper systems in the selection of optical lithographic equipment.
Statistically designed screening experiments were conducted with several commercially available g-line resists to determine the most significant process factors for maximizing Depth of Focus for a new g-line lens with an NA of 0.43 and a field diameter of 21.2 mm. The experiments determined the significance of process factors such as Softbake Temperature, Resist Coat Thickness, Post Exposure Bake Temperature, Develop Temperature, Develop Time, Develop Concentration and Develop Agitation. This experimental approach lead to a much shorter process evaluation cycle. Comparative results of Depth of Focus for each of the materials tested are reported along with extended results of the optimized process critical dimension bias and linearity. First time extended results with the Zeiss 107861 lens are reported for the optimized process, which was developed for one of the resists, to achieve a Usable Depth of Focus of 2.0 μm at 0.7 μm resolution.
Grooved glass substrates for use in media directly readable with optical disks players (CD-audio,CD-ROM, WORM or erasable disks) are fabricated with conventional UV photolithography and etching techniques. The mask preparation is made by compact disk mastering equipment. After exposure and development of the photoresist, etching of the glass disk is done. The quality of the replication and of the etching process can be judged only by the undamental quality criteria (block error rate) of the compact disk. If any region of the replica is unacceptable, the disk must be rejected.
We present an analysis of the imaging of a linear grating with an extended illuminating source and of the moire effect with a second grating in the image plane. The light transmitted by the moire grating is captured by a detector with finite angular extent. The amplitude and the phase shift of the moire signal are calculated in the presence of both symmetrical and asymmetrical aberrations of the imaging lens; the influence of both the source and the detector size are taken into account. A comparison is made between the axial and transverse position of imaged features (e.g. the position of an imaged edge) and the position of the grating as predicted by the phase and the maximum modulation depth of the moire signal. The offset between the aerial image position and the detected grating position should be known when one uses the moire signal for the testing of the lens in a lithographic projection system. Numerically calculated values of this offset are given for a number of practical situations.
Alignment dispersion can only consume a small part of the overlay budget. Thus, it is important to study the causes of alignment dispersion. Two parameters which damage alignment dispersion have been studied. The first stems from the centrifugal forces acting on the photoresist in the vicinity of the alignment mark which produce an asymmetrical modulation leading to a misalignment component. The effect of the grid layout, alignment mark design, are studied. Rather than on modify the spin resist process, or remove the resist on the alignment target, a new kind of alignment mark, which is much less sensitive to this effect, is proposed. The second is the roughness of the Aluminium substrate. In order to measure the effect of the surface roughness on alignment performance, different aluminium grain sizes have been prepared and tested. The study has been performed for two alignment marks and two resist systems.
The natural progression of today's semiconductor industry is toward smaller device features and tighter registration requirements. Typically, this progression results in high capital equipment investments, along with a large capacity reduction per investment dollar for most lithographic exposure processes. One major cause for the capacity loss is the industry's willingness to migrate from full-field scanning projection printers to a lower throughput field-by-field alignment step-and-repeat exposure system. Standard Microsystems Corporation (SMC) sought to achieve higher performance on its scanners without compromising throughput. The original goal at SMC was to improve Perkin-Elmer's specified Micralign 641 HT machine-to-machine registration performance from ± 0.30 micron to less than ± 0.25 micron. With this in mind, we set out to investigate the true alignment and registration limitations of a Micralign Model 600 HT Series Projection Aligner. Although SMC was apparently successful at matching two Micralign 641 HT systems to ± 0.25 micron by manually reading verniers, this technique proved to be time consuming and prone to human error. Electrical probing of wafers was considered, but the special masks and processing steps and its destructive nature were considered undesirable. For this study, an automatic optical overlay measurement system was used to optimize overlay on the SMC Micralign systems. The results were enlightening. The specified overlay of ± 0.30 micron for 98% of the data improved to better than ± 0.25 micron, 3 sigma. These results were achieved without the use of Automatic Magnification Compensation (AVM/AMC). We.also discovered that many otherwise transparent mechanical/optical anomalies, such as contamination and scan interference, could be readily identified. Experimental data is presented and the beneficial application of this technique to a production process is discussed.
For the industrial production of one micron CMOS design rule circuits, it may be advantageous in terms of cost and throughput to mix projection scanners with optical steppers. Three compatibilities have then to be completed: - prealignment, interfield, intrafield. One classical way of ensuring the prealignment compatibility is to print two global marks with the stepper. To achieve the grid compatibility, the normal method is to use die by die alignment. The intrafield capability is then achieved in adjusting the magnification and distortion of the stepper to the scanner caracteristics. Nevertheless, a specific difficulty occurs when it is wished to print the first level with the stepper. In that case, two different aproaches can be considered: - First print an extra level with the scanning projection machine to create alignment marks, and afterwards print the real first level with the stepper in die by die alignment mode. - Use blind stepping under the stage interferometric metrology control to print directly the first level. Both method have been tested and characterized. Advantages and difficulties as well as overlay results are presented. A detailled analysis and quantitative evaluation of the various error causes involved were carried out. This allows the analysis to be generalized to other machines with different specifications
This paper gives a review of previous work [1-3] describing a new method to characterize the effects of defocus on an optical lithographic process. The interaction of the aerial image with the photoresist is described mathematically in order to determine the features of the image which are important in determining lithographic performance. The slope of the log-image is determined to be an appropriate metric of aerial image quality. By calculating this log-slope as a function of defocus, rigorous definitions of both depth-of-focus (DOF) and resolution are given. The DOF, for a given feature size, can be defined as the amount of defocus for which the log-slope of the aerial image remains above some minimum value. The minimum value of the log-slope which gives acceptable process latitude is determined by the properties of the photoresist process. The primary parameter lithography model PROLITH  is used to investigate how various process parameters change the response of the lithographic system to focus. The results are compared to the log-slope defocus curve to determine the minimum acceptable log-slope for the modeled system. Finally, experimental linewidth data was collected as a function of focus and exposure using electrical linewidth measurement techniques. This data is compared with both the modeled data and the log-slope analysis.
In the years to come, on-line semi-micronic lithography will be achieved via electron beam or deep UV exposure. Each of these techniques presents advantages (flexibility of direct writing and rapidity of exposure, respectively). The best choice for the process technologies would be the complementary use of both techniques. It appears interesting to us to select a resist which is sensitive to both of these exposures. The good results (in terms of resolution and sensitivity) obtained with the Shipley SAL 601 resist under a-beam exposure and the analysis of its absorption spectrum have led us to select this particular resist. This paper presents the results we obtained by exposing a 500 nm thick layer of Shipley SAL 601 deposited over a SiO2 layer in the vaccum contact printing mode. Three types of beams have been tested; two of them from a Fusion Illuminator 100 system (centered at 220 and 260 nm, respectively), one from a KrF excimer laser (248.4 nm) and the fourth from a Karl Suss MA6 (310 nm). The Scanning Electron Micrographs of the patterns obtained with the deep UV process are explained in terms of the physical and chemical properties of the resist and of the deep UV source characteristics.
The commercial Lambda Physik excimer laser model 248 L was specially designed for deep UV projection lithography. Its output power was 2 W at a pulse repetition frequency of 200 Hz. An improved version of this laser was developed and tested thoroughly. Due to modifications of the optical system and the electrical discharge, the average power could be increased to 5 W without sacrificing other specifications. The bandwidth is better than 0.003 nm (0.5 cm-1), and the wavelength is actively stabilized to ± 0.001 nm. The pulse frequency was raised to 400 Hz to comply with the demands of todays deep UV-steppers. A newly developed module allows absolute wavelength calibration within the tuning range of 248.18 to 248.58 nm. The absolute accuracy of this calibration is ± 0.001 nm. Using this option, the laser can be tuned exactly to the wavelength of best performance of the stepper optics.
The advantage of excimer lasers over conventional UV lamps for microlithography will be discussed and the difficulties encountered will be reviewed. The first application of excimer lasers was in contact printing where the short wavelength of 193 nm of ArF is used to produce sub-half-micron resolution typically on GaAs chips. The first use of excimer lasers in the field of projection printing was in the area of reticle manufacturing. 308 nm XeC1 lasers are used in pattern generators to produce the pixels of the reticle pattern. A very promising field of applications is in the domaine of reduction wafer steppers which produce a demagnified image of the reticle on silicon wafer. Despite the numerous problems in designing a DUV stepper, the wafer stepper with excimer laser source seems to be the logical choice for the next generation of highly integrated circuit manufacturing. Finally, a promising but still distant possible application could be a 1 : 1 print using phase conjugation to circumvent the need for highly sophisticated optics between reticle and wafer.
A new method for fabricating photomasks is proposed. The mask is prepared by burying the absorbent patterns inside the transparent photoplate instead of depositing them on the surface of the photoplate. After imaging and etching trenches into the glass substrate, an absorbent material is set into them. Two different ways of filling in these holes are considered: planarisation and lift-off. Various advantages of this technique are expected, namely high resolution. This paper presents results obtained by vacuum contact printing of positive and negative novolak based photoresists exposed through buried masks.
Design principles for the illumination system of a DUV wafer stepper are derived by examining the coherence properties of a KrF excimer laser and the effect of DUV radiation of high energy density on fused silica, the material used for the optical components. Alternative designs are presented and discussed.
An illumination system suitable for an excimer laser stepper has been investigated. Unnecessary interference pattern(speckel) is reduced effectively by scanning the laser beam. We report spatial coherence of the lasers with different spectral line width, illumination system of the stepper, appearance of the interference pattern,its spacing and contrust and their relation to the illumination system and to the coherence of the laser. Then we report reduction of this pattern together with a simple method to measure its contrast.
A simple model describing the field reflected on a trapezoidal shape highly reflecting object is proposed. This model is based on the assumption of non interference between the bottom and the top of the line under test. Some results of simulation are presented and discussed.
As circuits with sub-micron structures continue to increase in complexity, previous methods of inspecting overlay and CD tolerances are proven inadequate. Therefore, sponsored by the West German Ministry for Research and Technology, Wild Leitz developed a mask measurement system that represents in terms of achievable accuracy the limit of what is technically possible today. Within this project, Siemens as one of the key customers has performed part of the accuracy tests on a prototype of the machine and is now using a final system in its mask production line with good results.
In 1987, we proposed a new concept of optical image processing well fitted to the measurements of two levels overlay and alsopattern registration. Combined on a tool, with a XY laser interferometric stage, this method allows single level absolute metrology and relative measurements between different levels of process. A method is presented to assess the range of absolute accuracy of the instrument from moderately distorted grids generated on wafer steppers, by measuring the same wafer at four flat orientations. The various errors generated by the machine, its limits and capabilities towards lens distorsions and stage errors characterization are discussed. With a precise Z axis metrology coupled to the autofocus and a flat pin recess chuck, a determination of the surface process induced deformations, on printed wafers becomes possible. Results of Z mapping repeatability are presented.
During the past ten years the highest requirements for an overall optical performance of microscope objectives have come from the semiconductor industry. The applications in the optical inspection and metrology of masks and wafers called for special lenses with long working distances and lenses with high magnification and high numerical aperture. Not only is the correction of image aberrations of importance, but among other features also the reduction of internal reflections and the autofocus capability. The performance of the latest generation of microscope objectives is demonstrated in comparison with older designs. While monochromatic image aberrations of modern lenses are negligible with regard to diffraction, the polychromatic correction is still a challenging task and defines the limitations in image quality. But even in this respect the newly developed Wild Leitz lens PL APO 100x/0.95 demonstrates that the physical limits have been reached both in design and manufacturing.
Nowadays process control has become essential for yield management in VLSI manufacturing so there is a growing need for automated in-process wafer inspection systems. In this paper we describe a fast automated system for submicrometer defect detection on patterned wafers, based on an improved image comparison algorithm. After a brief discussion of the different inspection modes and their applications, we introduce our image processing algorithm, including : subpixel spatial alignment, interimage dynamic range adaptation, multi-threshold efficient binarization, defect identification based on a specific morphological method. This algorithm provide both a significant improvement of true defect detection and a reduction of the false defect rate. Then we describe the main components of the inspection machine : the optical parts, the mechanical parts and a Fast Image Processing Unit (FIPU) based on a pipeline architecture including special purpose hardware. The FIPU allows the inspection of a 125 µm x 125 μm field in 200 ms with a defect sensitivity of 0.3 μm. Finally we report a few experimental results obtained by applying our algorithm on some real inspection problems and we compare these results with those obtained by standard inspection algorithm.
A laser heterodyne interferometer for measuring surface microtopography is described in this paper. A frequency difference between reference arm and measuring arm is obtained by utilizing two acousto-optic modulators in the interferometer. Phase demodulation technique is employed to improve the resolution up to 0.36 degrees, corresponding to one thousandth wavelength. Height variations of 10 nm can be resolved, while the theoretical height resolution is 10 angstroms. The simplest layout and the less optics are used in the interferometer without any polarizes and lens designed specially as other interferometers.
Laser induced chemistry appears to be an important and relatively novel field of research with potential applications, for instance in microelectronics. In this area, the most developed laser processing techniques are laser lithography and laser induced pyrolytic deposition of micron scale features. This paper will focus on the laser direct writing technique where the laser is used to induce chemical vapour deposition.
Interconnection direct writing on ICs is possible by localized laser-assisted Chemical Vapor Deposition. Recently we have developed and marketed a new laser microchemistry tool particularly designed for VLSI prototypes rewiring. By dissociating Ni(CO)4 molecules, Ni lines can be written at speeds higher than 5 gm/s under laser induced temperature as low as 400°C. At the same temperature tungsten stripes can be driven from decomposition of WF6-H2 mixtures. However the tungsten deposition rate is about two orders of magnitude lower than the nickel growth rate in the same temperature conditions. The resistivities of the deposits are in both cases around 10 μΩ.cm. Silicon dioxide layers can be promoted from dissociation of a Si2H6-N20 mixture under surface temperature around 500°C. These metal and insulator deposition basic steps have been integrated in a complete metal bridging process suitable for the last interconnection level of a VLSI circuit. This process has been firstly estimated from a functional point of view, by electrical characterizations realized on test patterns entirely drawn by laser chemistry. At least, by measuring the time necessary to perform a metal bridge, the process has been evaluated from an economical point of view.
The post process reconfiguration of 2 metal layers integrated VLSI circuits, achieved by via formation in intermetal oxide, was studied with doubled frequency YAG laser (532 nm). The creation of vias is experimented on test patterns placed within 3 circuits of different technology. Vias thus formed are created by thermal effect with mechanism of fusion and by mechanical explosion in the insulating layers. No damage was verified by electrical measurements and SEM cross section studies. For metallizations containing refractory metals, the via yields reached 100 % for patterns of 2.6 μm x 4.7 μm. The feasibility of this process has been established for recovering area of 3 μm x 3 μm. Via resistance was measured to be a few Ohms without resorting to post process annealing and the minimum breakdown current was found to be 150 mA. A via is creatable in a metallic line without damaging others lines as close as 1.5 μm. Reliability test of vias at 125°C and 5 mA showedno failure after 1000 hours. Consequently, creation of vias by laser has been shown to be achievable in a metal interconnection environment conforming to VLSI minimum design rules of 1.2 μm.
Surface modifications induced to GaSb single crystals by pulsed U.V. laser annealing (PLA) in ultra-high-vacuum have been studied. The (111) surfaces of GaSb single crystals were prepared by a mechanical and chemical etching before introducing into vacuum. The samples were subsequently exposed in vacuum to the U.V. radiation (193 nm) of an high power excimer laser. The analysis by Auger electron spectroscopy performed in situ shows that an atomically clean surface (free of native oxides and of carbon pollution) can be achieved by PLA. The effects of the Energy Density (ED) and of the repetition of the PLA are addressed. Superficial topology changes observed in situ by low resolution scanning electron microscopy, and later by optical microscopy, are described too. From our observations we conclude that She antimony oxides begin to decompose noticely when the surface is irradiated with 50 mJ/cm2 laser pulses. Their evaporation explains the onset of the sharp chemical reduction observed at higher fluences. We propose that superficial melting of the GaSb single crystal starts as soon as the laser beam ED reaches 100 mJ/cm . Beyond this fluence the superficial chemical reduction is completed not only by evaporation, but also : - first : by thermal stress induced fracture of the gallium oxide layer and its ejection due to the strong evaporation from the substrate, and, - second : by the gallium oxide dissolution into the melt. The amount of antimony segregated on ple surface becomes important when the surface melts deeply (i.e. at ED superior of 150 mJ/cm2).