This paper presents an approach to Laser Illumination System (LIS) efficiency optimization for High Numerical Aperture (NA) Microlithography exposure tools. These advanced tool are immersion systems with NA>1 utilizing many Reticle Enhancement Techniques (RET). In order to provide maximum efficiency, especially for high NA systems, the following conditions have been taken into consideration: étendue law for LIS subsystems consisting of sequential optics; proper relations between transverse and longitudinal dimensions of optical path; laser light coherence management by overlap of individual exposure fields generated from different parts of the laser beam; polarization management; flexible illumination partial coherence (PC) management; illumination relay and condenser systems have to be optically matched in order to minimize: pupil ellipticity, field vigneting, and non-telecentricity. Individual importance of listed conditions and their parameters will be explained and discussed as applied to illumination systems with high NA.
It is time to revisit X-ray. By enhancing, in the Near Field, Proximity X-ray Lithography (PXL), the technique is demonstrated that extends to 15nm printed feature size with 2:1 ratio of pitch to line width. "Demagnification by bias" of clear mask features is positively used in Fresnel diffraction together with rapid, multiple exposures of sharp peaks. Pitch is kep small by multiple, stepped exposures of the intense image followed by single development. The optical field is kept compact at the mask. Since the mask-wafer gap scales as the awaure of the mask feature size, mask feature sizes and mask-wafer gaps are comparatively large. A Critical Condition has been identified which is typically used for the highest resolution. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1X PXL which is the most mature of the Next Generation Lithographies and which is now further extended. Throughput and cost are conventional.
By using the Near Field in Proximity X-ray Lithography (PXL), the technique is demonstrated that extends beyond a resolution of 25 nm print featuer size with 2:1 pitch to line width. "Demagnification by bias" of clear mask features is positively used in Fresnel diffraction together with multiple exposures of sharp peaks. Exposures are performed without lenses or mirrors between mask and wafer, and the "demagnification" is achieved in the selectable range 1X to 9X. Pitch is kept small by multiple, stepped exposures of sharp, intense, image peaks followed by single development. Low pitch nested lines are demonstrated. The optical field is kep compact at the mask. Since the mask-wafer gap scales as the square of the mask feature size, mask feature sizes and mask-wafer gaps are comparatively large. Because the features are themselves larger, the masks are more easily manufactured. Meanwhile exposure times, for development levels high on sharp peaks, are short, and there are further benefits including defect reduction. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1X PXL which is the most mature of the Next Generation Lithographies and which is now further extended. For 2D Near field patterning, temporal and spatial incoherence at the Critical Condition are used to show, not only that peculiarities in the aerial pattern, such as "ripple" and "bright spots", can be virtually eliminated, but also that there is an optimum demagnification, around 3X, in the Fresnel diffraction, where the contrast is highest. At this demagnification, patterns of various dimensions can be printed using various and appropriate demagnifications.
A high power picosecond soft x-ray source is generated by a compact, modular, diode pumped solid state laser BriteLight<SUP>TM</SUP>. Three x-ray source version are constructed from laser modules with increasing power. The power of the x-ray sources is tailored to potential applications. The building block of such a modular system is a 3 Watt x-ray power source with 1.1 keV x-ray photon energy. The laser system is very compact with dimensions of 4 ft X 3 ft X 1 fit. It is composed of a laser master oscillator, pre-amplifier and one power amplifier. A four laser amplifier system was also constructed in order to generate 12 W of x-rays for application to x-ray lithography.
The ability to produce fine features using X-ray proximity lithography is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature 'bias.' The classical approach is to attempt to minimize the bias; that is, to print features which are 1:1 images of those on the mask. However, bias can also be exploited to print features smaller than those on the mask. This demagnification-by-bias technique can be optimized with respect to mask-wafer gap and resist processing, and can provide reductions of 3X to 6X. Demagnification offers many of the same advantages as projection optical lithography in terms of critical dimension control: relaxed mask features CD. In addition, it provides a very large 'depth of focus' and wide dose latitude. In consequence proximity X-ray lithography is extendible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 0.1 nm) and large gaps (10 -25 micrometer). The method was demonstrated for demagnification values down to X3.5. To produce DRAM half- pitch fine features techniques such as multiple exposures with a single development step are proposed.
Phase-shifting masks have been applied in optical lithography and various phase-shifting techniques in X-ray Lithography (XRL) have been demonstrated. In this study, we compare different phase-shifting technologies for XRL, such as clear phase mask, attenuated phase-shifting mask and alternating aperture phase-shifting mask through computer simulation. The control of critical dimension is of primary importance as the CD shrinks to the sub-100 nm region. We have reported our design and fabrication of a more robust X-ray Phase Mask, which is capable of sub-70 nm imaging. The processing latitude of this design is investigated in terms of the X-ray source broadening, phase-shifter uniformity, mask-to-wafer gap and sidewall slope of the phase-shifter. The latitude is compared with those results from an attenuated phase shifting mask and an alternating aperture phase shifting mask.
The term Next Generation Lithography (NGL) is defined as a Departure from the Classical Concept of Replication Fidelity, thus widening the term Post Optical Lithography. It addresses the necessity to form mask features different from those to be produced in the resist. The term NGL is used to address the x- ray, e-beam projection and direct-write lithographies. It is also applicable to the advanced optical lithography employing the optical proximity correction (OPC) as a part of reticle enhancement techniques (RET) based on auxiliary features, phase shifters, complementary mask and multiple exposures. Following the basic concept of NGL and applying it to proximity x-ray lithography, we are developing a technique to form features on a wafer substantially smaller than those on the mask. This is achieved by exposure and development process and lithographic bias optimization, thus providing local demagnification-by-bias. Analysis shows that features as small as 25 nm can be formed at 10 - 15 micrometer mask/wafer gaps. This approach not only solves difficulties of 1X mask generation, but also offers an advantage of relaxed mask CD control.
A more robust XPM with improved thermal and radiation stability was designed and fabricated in CNTech. The effectiveness of this design is demonstrated and XPM fabrication processing is optimized to obtain finer control over the processing, low stress membrane and vertical sidewall in the phase-shifter materials. The XPM testing indicates that this XPM design is able to readily generating sub-100 nm feature in large volume under a much larger and more manageable gap between mask and wafer (20 - 30 micrometers ). 70 nm lines were printed with UV 5 under 25 micrometers mask-to- wafer gap. The simplicity of this design and the intrinsic multiple mask reduction ratio instead of 1:1 for conventional X-ray mask provide an easy analytical tool for this community to study lithographic performance in 50-to-70 nm region.
The usefulness of thin (< 250 micrometers ) rigid graphite plates as x-ray mask substrates for micromachining and LIGA applications has been demonstrated. Rigid graphite offers unique properties, such as moderate x-ray absorption and optimal filtration of synchrotron radiation, relatively low cost, compatibility with additive (electroplating) and subtractive (etching, micromachining) processes for absorber patterning. The surface roughness of these substrates is associated with the inherent porosity of a commercially available rigid graphite material (typical R<SUB>a</SUB> values are in the range of 1 - 2 micrometers ). The surface roughness of this rigid graphite sheet is reduced down to a 0.1 - 0.2 micrometers R<SUB>a</SUB> value by polishing. To reduce surface roughness further and make the substrate usable for fine e-beam or optical absorber imaging, additional smoothing is required. In this paper, the surface characteristics of rigid graphite sheets are analyzed and a glazing technique developed to smooth the graphite surface is described. This technique employs hard baking process of novolac-based resins. An average R<SUB>a</SUB> roughness value of approximately 5 nm was obtained after 5 coating using novolac-based AZ type resist.
Availability of production-worthy x-ray masks is of great concern to the lithographic community in anticipation of insertion of x-ray lithography as the leading contender among the next generation lithographies.
Despite growing expectations of significant progress in projection lithography using shorter wavelengths, x-ray lithography is still the most developed and production ready technology compared with the other NGL approaches. For the timely introduction of this technology into the manufacturing environment the development of fully integrated x-ray lithography systems becomes very important. Reflecting manufacturing and R and D demands, the x-ray technology integration has been pursued for goth synchrotron radiation and x-ray point source based approaches. While the synchrotron-based approach provides the high volume platform, the point source will provide the platform for low volume production and R and D efforts. SAL recognizes the needs for both, a synchrotron based stepper as well as a point source stepper and is focused on meeting those needs. This paper will present the status of integration efforts at SAL utilizing a point source system.
This paper analyzes and demonstrates the possibility of producing lithographic images at or below the 'diffraction limit' for synchrotron radiation-based x-ray proximity lithography. It is shown that at reasonable mask/wafer gaps of 15-30 micrometers , for feature sizes down to approximately 100 nm, a 30-40 nm uniform positive bias is observed. In proximity lithography, masks with clear features on a dark background demonstrate better linewidth control and more stable process optimization in terms of achieving smaller features: Sub-100 nm imaging requires positive bias for mask features: clear features have to be increased in sizes and the proper bias will depend on the mask/wafer gap. Features down to 43-46 nm have been formed in negative resists, and down to 60 nm in positive resist. The extendibility of synchrotron radiation-based x-ray proximity lithography into the sub-50 nm region at reasonable mask/wafer gaps of 20-30 micrometers was demonstrated.
In this work we characterized the temperature increase in SiHN mask membrane during e-beam writing. We observed an exponential decay with a decay length in the order of 1mm<SUP>-1</SUP>, and absolute temperature raises of 8 degrees K. This is the first time that direct measurement have been obtained. By fitting the observed data, we have extracted the thermal conductivity and emissivity of the film. These experimental values are essential in the modeling of the response of the masks.
Fresnel zone plates (ZP) have gained popularity as the optics of choice for advanced microfocusing applications. The main virtues of ZP are high resolution, high efficieny, low background, coherence preservation, and ample working distance. Zone plates are also unique because they are a normal incidence x-ray optics, which are much easier to align and use compared to other grazing incidence optics. We will report here recent progress that has drastically enhanced the performance of ZPs in 1) higher spatial resolution, 2) higher focusing efficiency, and 3) extension to higher energies. With the new developments, zone plates have proven to be one of the best microfocusing optics for monochromatic x-ray beams.
The development of the future-generation magnetic recording heads is based on availability of high resolution and high- aspect ratio lithography. A key step in the magnetic head fabrication process is the formation of high-aspect ratio trenches in photoresist that are subsequently used as a plating mask for the magnetic read-write heads. Currently, 1.2 to 1.5 micrometer wide and 10 micrometer tall trenches in the resist are formed using optical lithography. In the near future, more than 6 micrometer tall resist patterns with trenches of 0.5 micrometer or smaller will be required. A study of using X-ray lithography to generate patterns suitable for future-generation magnetic recording heads was undertaken at the Center for X-ray Lithography at UW-Madison. It was successfully demonstrated that 0.8 micrometer trenches in 15 micrometer thick resist and 0.4 micrometer trenches in 6 micrometer thick resist can be formed. The main steps in the fabrication of the high-aspect ratio resist patterns included (1) production of an initial (master) mask using e-beam lithography, (2) high-contrast replicated (final) X-ray mask manufacturing using X-ray replication process, and (3) actual patterning of thick PMMA resist using the final masks. Both X- ray masks were formed on a 2 micrometer thick silicon-nitride membranes as mask carrier. APEX-E resist 0.5 micrometer thick was used for e-beam writing, and 2 micrometer thick PMMA was utilized for the replicated mask. The absorber was electroplated gold: 0.4 micrometer thick for the master and 1.5 micrometer thick for the final mask. Details are given for 6 micrometer and 15 micrometer thick crack-free PMMA resist formation and characterization, exposure and development conditions.
The most important contributions to overlay inaccuracy are coming from well-known sources like mask pattern placement accuracy, alignment system accuracy and stage performance of the exposure tools. As the allowances for overlay budget decrease, and improvements in mask fabrication and stage performance are made, a number of previously less significant contributions, such as resolution, optical interference, and focusing accuracy of alignment system, as well as from wafer processing, have to be considered. These contributions are characterized in detail in this paper. The investigation was focused on a proven optical alignment system and overlay contribution as they apply to x-ray and optical lithography. Special emphasis was made on contributions form wafer processing.
Nuclear magnetic resonance spectroscopy (NMR) has been sued for he identification of several products from syndiotactic- rich poly(methyl methacrylate) (PMMA) sheet exposed to x-ray and UV irradiation. Two chain-scission products and two chain-intact products were observed using 1D and 2D NMR spectroscopy. The 750 and 400 MHz 1D hydrogen (<SUP>1</SUP>H) spectrum and the 2D <SUP>1</SUP>H, <SUP>1</SUP>H-COSY, <SUP>1</SUP>HMQC<SUP>1</SUP> and HMBC<SUP>1</SUP> spectra permitted the assignment of many hydrogen and carbon NMR signals for the four products which are still polymeric. Additional signals were observed for minor stereoisomers. Methyl formate, s small molecular product, was found in both the x-ray and UV degradations. Acetaldehyde was also observed from x-ray exposure but not UV exposure. Important results from this work are well- resolved chemical 'fingerprints' for x-ray and UV exposed PMMA.
The process for replication of high aspect ratio Au patterns typically includes x-ray lithography, RIE and electroplating. In this paper study of linewidth of dense L/S patterns in a wide range of periods is undertaken through the replication process. Effects of exposure dose, mask-wafer gap, RIE and electroplating process parameters on linewidth are addressed. We found that RIE parameters are the main factor affecting the linewidth. Based on the result of this study, we propose to introduce a bias in the mask pattern to the linewidth. Based on the results of this study, we propose to introduce a bias in the mask pattern to compensate the linewidth changes occurring during subsequent replication steps. Most interestingly, a mask with a required bias can also be produced by a self-biased process. Bias adjustment has been demonstrated for 0.1/0.1 micrometers L/S features with aspect ratio of 6. To further increase aspect ratio, a wet process is developed. An aspect ratio of 9 is achieved for 0.1 micrometers Au L/S patterns by using the wet process. With this method, the linewidth fidelity during replication is substantially improved.
Crucial to any viable lithographic mask technology is the requirement that a given mask pattern be usable for the hundreds of thousands of exposures in a production environment. In a conventional approach this would be accomplished by making robust masks. A better strategy to ensure the longevity of the pattern itself, is realized by producing many defect-free copies of master masks. This approach is especially important in the case of x-ray masks, although the optical masks also have a limited usable lifetime. X-ray mask generation is accomplished today via e- beam lithography, which as a replication method has several inherent disadvantages, including low speed and high cost. X- ray replication is the best solution. In this paper, we describe the development of a mask replication method realized on a Suss x-ray stepper. The approach is based on supporting parent mask and the daughter blank in fully kinematic fixtures during replication, ensuring a minimum of distortion, excellent gap control and optimized exposure conditions. Minor modifications of the mask mounting fixtures, the replication setup, and details of processing are presented. Preliminary results of mask replication are also shown.
An exposure radiation power measurement technique utilizing thin gold film thermal sensors has been presented. The sensory system of the power meter (or calorimeter) consists of three interlaced serpentine resistors covering an area of 6 cm by 0.4 cm, functioning as a thermal sensor, a heater and a shielding electrode. The measurement principle is based on recording the change in resistance of the sensor due to heating under radiation and internal calibration. The interlaced gold sensors were fabricated using optical lithography on a 100 mm diameter silicon wafer. The power measurements have been performed at CAMD/LSU 1.3 - 1.5 GeV synchrotron source, on a 'white light' beamline (E<SUB>max</SUB> approximately 4 keV). The measurement results agree with calculations within approximately 4%. The relaxation time of the calorimeter response was 90 seconds in vacuum (10<SUP>-4</SUP>Torr) and 18 seconds in 25 Torr helium. The power from a UV lamp of an ORIEL optical exposure station was measured using an interlaced thermal sensor and a commercial calorimeter. The results agree within 2%.
A LIGA based tool-set of tips for various scanning probe applications is under investigation by the LSU (mu) SET. This involves fabrication of `micro-columns' using LIGA, followed by an electrochemical sharpening process. Micro-columns ranging from 1.8 micrometers diameter and 14 micrometers tall to 165 micrometers X 165 micrometers and 1000 micrometers tall have been fabricated. In order to understand the sharpening mechanism, commercially available wires with diameters ranging from 25 - 800 micrometers were sharpened. A computer aided design tool, based on deforming finite elements, was developed to simulate the sharpening process.
This paper presents a novel technique for fabricating 3D patterns in a thick layered resist and describes an alignment aide designed for the specific application of thick resist x-ray micromachining. In this technique, a PMMA layer of desired thickness is formed on a substrate by spinning or solvent bonding. The layer is exposed with X- rays to generate a latent image. A second layer of PMMA is bonded over the first layer and is exposed with an appropriate mask, generating a latent image in the second layer. This process can be repeated several times creating a 3D latent image. Simultaneous development forms a true 3D pattern in the PMMA resist.
Temperature measurements of thick PMMA resist during X-ray (1 to 5 keV) exposure are presented in this paper. Thin metal (gold) film thermal sensors were fabricated directly on the resist surface and on the resist/substrate interface using micro-lithography methods. The temperature measurements were conducted in vacuum (< 10<SUP>-4</SUP> Torr) and in 1 to 25 Torr helium pressure--conditions corresponding to typically X-ray lithography exposure. The results of temperature rise measurements performed with thermal sensors and with miniature conventional thermocouples are compared.
Conventional resist application techniques are based on spinning a resist layer onto a mechanically dominating substrate. As thicker imaging layers are required, the integrity of the resist/substrate system is influenced by the resist thickness. The traditional LIGA approach is to form a PMMA resist sheet on the substrate by casting using a press. This method causes high stresses in the resist and at the resist/substrate interface. Another method consists of gluing or bonding a PMMA sheet with subsequent machining to a desired thickness. The stresses can be high enough to cause the resist to crack and/or separate from the substrate. In this paper, alternative and improved techniques are presented. One of these is a modified multiple coating spin-on method, suitable for producing PMMA resist thickness of 60-80 micrometers . The other method is based on bonding a solid PMMA sheet of desired thickness using an appropriate solvent. These techniques produce uniform PMMA layers with thicknesses ranging from 5 micrometers to 1500 micrometers and above. A mechanical cleaving test was used to estimate the resist/substrate bond strength and characterize the bonding solvents. Issues such as radiation swelling and thickness losses associated with latent image formation in PMMA are addressed.
In this paper we present specifics of x-ray mask fabrication suitable for high-aspect ratio microlithography in micromachining. Results of fabrication and exposures using x-ray masks with approximately 4 micrometer thick gold absorber (one level resist) are illustrated. For conventional x-ray masks commercially available substrates were used: B-doped Si membrane (2-3 micrometers thick) on a 4' Si wafer bonded to a Pyrex glass ring. A new approach -- the transfer mask technique -- is demonstrated. This technique is based on forming an absorber pattern directly onto the resist surface of the sample. The transfer mask method is suitable for any radiation (visible, UV, and x-rays) and is based on the use of a master mask (optical or x- ray) to achieve patterns with desired aspect ratio. When used in conjunction with multiple x- ray exposures and sequential developments the transfer mask method produces patterns with extremely high aspect ratio.
A new x-ray micro-lithography exposure system has been designed and built at CAMD to meet specific demands of synchrotron radiation assisted high aspect ratio micromachining. The system consists of a broad-band transmission (1.2 angstroms to 6.5 angstroms) beamline and a multi-chamber X-ray Exposure Station. The beam line accepts radiation emitted in a bending magnet of the CAMD 1.3 - 1.5 GeV synchrotron storage ring. The beam line is approximately 10 m long and terminates with 125 micrometers thick Be window which defines an X-ray beam of 50 X 10 mm<SUP>2</SUP> at the exposure plane. The beam line is configured to provide 1.0 W/cm<SUP>2</SUP> at 1.5 GeV and 100 mA storage ring operation. The Exposure Station is designed to control different exposure conditions and can handle a variety of mask/sample assemblies. The first camber of the exposure tool is designated as a radiation filter, it controls x-ray spectra using foils and inert gases, and optimizes dose delivered to a sample with a thick resist (in excess of 1 mm). The second chamber is equipped with a multi-axes scanning mechanism to provide designed orientation and exposure of the mask/sample assembly. The two chamber can be separated by a thin foil to facilitate the use of reactive atmospheres for radiation induced chemical processes during exposure. An expansion of the Exposure Station providing large area exposures (up to 300 X 300 mm<SUP>2</SUP>) is described.
Design, fabrication, and testing of thermal micro-sensors suitable for miniature and microscopic systems, for application on thin films (free standing or on substrates) as temperature sensors are presented in this paper. The sensors utilize the electrical resistivity temperature dependence of a metal. Using micro-lithography methods, several sets of gold resistors were fabricated in the form of flat 30 to 250 nm thick wires, 7 - 10 micrometers wide, and several cm long in a serpentine shape covering approximately 1.0 mm<SUP>2</SUP>. These sensors have demonstrated better than 0.001 degree(s) C sensitivity. The electrical resistivity and its thermal coefficient of a thin gold metal film were compared with those of bulk material. Temperature measurements on Si wafers were performed in situations corresponding to x-ray lithography exposure conditions suitable for micromachining. The temperature rise and relaxation time of a silicon wafer during x-ray exposure were measured in vacuum and different He gas pressures.
For conventional patterning (optical or x-ray microlithography and micromachining) a mask with a substrate reasonably transparent to desired radiation is used. A new technique - transfer mask or sacrificial patterning - is described in this paper. The technique is based on forming absorber pattern directly on the surface of the sample. This method is suitable for any radiation (visible light, UV, X-rays, electron and ion beams) and allows use of a conventional master masks (optical or x-ray) with low, medium, high (submicron) resolution to achieve patterns with desired aspect ratio. Multiple exposures and sequential developments can produce patterns with extremely high aspect ratio. New lithography techniques, such as in-situ development, UV and x-ray radiation assisted chemistry (etching and deposition), can be easily realized by using this transfer mask technique. Forming the transfer mask directly on the sample opens new possibilities not available with the conventional masks: exposure of samples with curved surfaces and dynamic deformation of the sample surfaces during the exposure, etc.
Image intensity profiles and resist profile calculations using the XMAS simulation program are presented for storage ring x-ray lithography proximity printing under several illumination conditions. The calculations indicate the existence of a wide process window for the simultaneous replication of several kinds of subquarter-micron features.
This course presents an overview of the DUV lithography utilizing 157-nm light. The course gives an insight of the status of this technology, its limitations, and its applicability to several generations of IC's. A brief review of the physical principles of optical lithography is presented, and components of the manufacturing system are discussed in some detail. Topics to be covered include: 157-nm lithography system requirements; lithography tool modules such as laser illumination system (including source, beam delivery module, and module forming illumination of desired type and spatial coherence), projection optics system; reticles, pellicles, nitrogen ambience and resists. The discussion addresses specificity (birefringence and absorption) of materials used in 157-nm lithography systems: Calcium Fluoride (CaF2) for optical elements and fluorine doped fused silica for reticle substrates and pellicles. Particular emphasis is made on substantial absorption of materials in 157-nm wavelength region and related manufacturing aspects such as fluorinated resists and necessity of nitrogen purge. The existing (and projected) infrastructure, in terms of tool availability is explained. Recent developments in 157-nm technology, and its printing capabilities down to 45 nm are described.
This course presents an overview of the x-ray lithography (XRL) technology. The course provides an understanding of the status of the technology, its basis and limitations, and its applicability to several generations of IC's. After a brief review of the physical principles, the components of the manufacturing system are discussed in detail. Topics to be covered include: sources (with emphasis on synchrotrons), beamlines, masks, aligners, and resists. The discussion addresses manufacturing aspects, exposure control, and sources of overlay error. Particular emphasis is on the x-ray mask and the approach to optimal mask design. The existing (and projected) infrastructure, in terms of tools availability in the US and Japan is explained. The perceived limits and challenges of X-ray technology, recent developments, and its printing capabilities down to 25 nm are described.