Advances in micromachining (MEMS) applications such as optical components, inertial and pressure sensors, fluidic pumps and radio frequency (RF) devices are driving lithographic requirements for tighter registration, improved pattern resolution, and improved process control for pattern placement on both sides of the substrate. Consequently, there is a similar increase in demand for advanced metrology tools capable of measuring the Dual Side Alignment (DSA) performance of lithographic systems.
The requirements for an advanced DSA metrology tool include the capability of measuring points over the entire area of the substrate, and of measuring a variety of different substrates and film types and thicknesses. This paper discusses the precision and accuracy of an advanced DSA metrology system, the UltraMet 100. This system offers DSA registration measurement at greater than 90% of a wafer's surface area, providing a complete front to back side registration evaluation across a wafer. The system uses top and bottom cameras and a pattern recognition system that allow simultaneous target capture and measurement on both substrate surfaces.
Because no industry standard has been established to determine the accuracy of dual side pattern metrology, an accuracy gauge was designed for this study that allows both top and bottom cameras to simultaneously measure offsets between two targets on one substrate surface. In this paper, an accuracy gauge is measured on the UltraMet 100 and the results are compared to measurements taken on a reticle X/Y pattern placement metrology tool calibrated to a NIST traceable standard. In addition, tool performance is analyzed in terms of system repeatability and reproducibility.
Advances in micromachining (MEMS) applications such as optical components, inertial and pressure sensors, fluidic pumps and radio frequency (RF) devices are driving lithographic requirements for tighter registration, improved pattern resolution and improved process control on both sides of the substrate. Consequently, there is a similar increase in demand for advanced metrology tools capable of measuring the Dual Side Alignment (DSA) performance of the
lithography systems. There are a number of requirements for an advanced DSA metrology tool. First, the system should be capable of
measuring points over the entire area of the wafer rather than a narrow area near the lithography alignment targets. Secondly, the system should be capable of measuring a variety of different substrate types and thicknesses. Finally, it should be able to measure substrates containing opaque deposited films such as metals. In this paper, the operation and performance of a new DSA metrology tool is discussed. The UltraMet 100 offers DSA registration measurement at greater than 90% of a wafer's surface area, providing a true picture of a lithography tool’s alignment performance and registration yield across the wafer. The system architecture is discussed including the use of top and bottom cameras and the pattern recognition system. Experimental data is shown for tool repeatability and reproducibility over time.
Pellicles are used in semiconductor lithography to minimize printable defects and reduce reticle cleaning frequency. However, there are a growing number of microlithography applications, such as advanced packaging and nanotechnology, where it is not clear that pellicles always offer a significant benefit. These applications have relatively large critical dimensions and require ultra thick photoresists with extremely high exposure doses. Given that the lithography is performed in Class 100 cleanroom conditions, it is possible that the risk of defects from contamination is sufficiently low that pellicles would not be required on certain process
layer reticles. The elimination of the pellicle requirement would provide a cost reduction by saving the original pellicle cost and eliminating future pellicle replacement and repair costs. This study examines the imaging potential of defects with reticle patterns and processes typical for gold-bump and solder-bump advanced packaging lithography. The test reticle consists of 30 to 90 μm octagonal contact patterns representative of advanced packaging reticles. Programmed defects are added that represent the range of particle sizes (3 to 30 μm) normally protected by the pellicle and that are typical of advanced packaging lithography cleanrooms. The reticle is exposed using an Ultratech Saturn Spectrum 300e2 1X stepper on wafers coated with a variety of ultra thick (30 to 100 μm) positive and negative-acting photoresists commonly used in advanced packaging. The experimental results show that in many cases smaller particles continue to be yield issues for the feature size and density typical of advanced packaging processes. For the two negative photoresists studied it appears that a pellicle is not required for protection from defects smaller than 10 to 15 μm depending on the photoresist thickness. Thus the decision on pellicle usage for these materials would need to be made based on the device fabrication process and the cleanliness of a fabrication facility. For the two positive photoresists studied it appears that a pellicle is required to protect from defects down to 3 μm defects depending on the photoresist thickness. This suggests that a pellicle should always be used for these materials. Since a typical fabrication facility would use both positive and negative photoresists it may be advantageous to use pellicles on all reticles
simply to avoid confusion. The cost savings of not using a pellicle could easily be outweighed by the yield benefits of using one.
There are an increasing number of microlithography applications such as advanced packaging, nanotechnology and thin film head production that require the use of thick photoresist materials. The exposure dose requirements for these applications dramatically increase as the photoresist thickness increases. For example, some positive acting novolak photoresists require exposures in excess of 5000 mJoules/cm2 for 100 μm thick films. When a single reticle is used to pattern many wafers, a significant amount of light and heat energy is transferred from the lithography tool illumination source to the pellicle protecting the reticle image. In high volume production environments, a pellicle can be subjected to accumulated dosages exceeding 500 kJoules/cm2 within a matter of weeks.
Because thick photoresist applications benefit from using 1X broadband steppers with high wafer plane irradiance, life-testing results were reviewed for broadband pellicles designed for maximum transmission at g, h and i-line wavelengths of Hg. Historically, pellicle lifetime testing was typically carried out only to approximately 500 kJoules/cm2 . While this test limit may have been sufficient for thin photoresist applications used in semiconductor applications, longer lifetime studies are required to determine pellicle durability for thick photoresist applications.
In this study, life testing was performed on multiple pellicle films designed for broadband illumination, including nitrocellulose, cellulose acetate, cellulose ester and fluoropolymer films. Spectroscopic transmission at g, h and i-line was first measured on unexposed pellicles. The pellicles were attached to test reticles and exposed to high-energy doses on an Ultratech broadband stepper, accumulating up to 3000 kJoules/cm2 . Transmission was periodically re-measured and the pellicle films were visually inspected for color change and any apparent physical damage. Results were compared to the expected optical properties for each film type, and recommendations are provided for the most appropriate fil type for high-energy applications. Of the six pellicles tested, the two fluoropolymer films showed substantially better transmission stability than the cellulose based films. Discoloration occurred on the cellulose films over the chrome to glass transition area of the test reticle field suggesting that heat in the chrome surface affects the chemical structure of the pellicle films, thereby changing their transmission properties.
The acceleration of the International Technology Roadmap for Semiconductors (ITRS) is placing significant pressure on the industry's infrastructure, particularly the lithography equipment. As recently as 1997, there was no optical solution offered past the 130 nm design node. The current roadmap has the 65 nm node (reduced from 70 nm) pulled in one year to 2007. Both 248 nm and 193 nm wavelength lithography tools will be pushed to their practical resolution limits in the near term. Very high numerical aperture (NA) 193 nm exposure tools in conjunction with resolution enhancement techniques (RET) will postpone the requirement for 157 nm lithography in manufacturing. However, ICs produced at 70 nm design rules with manufacturable k 1 values will require that 157 nm wavelength lithography tools incorporate the same RETs utilized in 248nm, and 193 nm tools. These enhancements will include Alternating Phase Shifting Masks (AltPSM) and Optical Proximity Correction (OPC) on F 2 doped quartz reticle substrates. This study investigates simulation results when AltPSM is applied to sub-100 nm test patterns in 157 nm lithography in order to maintain Critical Dimension (CD) control for both nested and isolated geometries. Aerial image simulations are performed for a range of numerical apertures, chrome regulators, gate pitches and gate widths. The relative performance for phase shifted versus binary structures is also compared. Results are demonstrated in terms of aerial image contrast and process window changes. The results clearly show that a combination of high NA and RET is necessary to achieve usable process windows for 70 nm line/space structures. In addition, it is important to consider two-dimensional proximity effects for sub-100 nm gate structures.
Microlithography applications such as advanced packaging, micromachining and thin film head (TFH) production frequently require the use of thick photoresists and large exposure doses for successful pattern transfer onto substrates. When thick negative acting photoresists are used, exposures as high as 5000mJ/cm2 may be required to maintain the pre-exposure photoresist thickness after develop. In this study, light transmission through photomasks with standard (OD3) and high-density (OD4) Cr films was measured through the ultraviolet spectrum to determine leakage thresholds and evaluate the risk of unwanted exposure with highly sensitive photoresists. Because the higher OD photomasks are the result of an increase in Cr film thickness, photomask process differences, resolution capability and Critical Dimension (CD) uniformity issues were also evaluated. The thicker Cr film could also affect pattern transfer to the wafer. Therefore, resolution and CD uniformity were compared on wafers patterned from both OD3 and OD4 Cr reticles.
In the photolithographic process, critical dimensions (CD) of exposed features in photoresist need to be controlled to within a specified tolerance related to the nominal feature size. A portion of this tolerance budget is consumed by variations in CD on the photomask. At low k1 factor, a number of parameters in the lithography system impact linearity including lens aberrations, defocus, exposure, partial coherence, and photoresist contrast. The combined effect of these parameters is that errors in the mask CDs are not transferred to the wafer in direct proportion to the optical reduction value of the lithography system. This Mask Error Factor (MEF) becomes a significant problem as it consumes a larger than anticipated portion of the CD tolerance budget. This paper will discuss experimentally evaluated MEF using a 4X i-line stepper for a range of feature sizes from subwavelength to approximately twice the exposure wavelength. A test reticle was built with isolated lines from 200 nm to 600 nm in 12.5 nm increments at 1 X. CD measurements on the reticle were compared to corresponding CD measurements on the wafer in order to establish both linearity and MEF curves for the lithography system. MEF values were also determined across a process window for multiple feature sizes. The MEF was observed to be less than 1.4 for CDs greater than 330 nm (k1 equals 0.5) throughout the process window. However, the MEF rises rapidly to over 3 for CD values smaller than 300 nm (k1 equals 0.45) at nominal focus and exposure. Changes in exposure were not observed to have a noticeable impact on MEF while focus offsets were observed to result in significant increases in MEF. These results indicate that MEF has a much larger impact on focus latitude than on exposure latitude. As a result the process window will be compressed more in focus than in exposure.
The storage space of hard disk drives more than doubles every 18 months. In order to maintain this growth rate, thin film head (TFH) manufacturers continue to seek new technologies to increase the areal density on the magnetic media. The trimming of the track at the rowbar level known as 'pole trimming' has proven itself to be very effective at increasing the number of tracks per inch (TPI) during the inductive head generation. However, the transition to magneto-resistive (MR) head technologies with ever smaller form factors has continued to push the trackwidth (TW) requirements of the industry. Optical proximity correction (OPC) enhanced masks have been used in the semiconductor industry for controlling the shape of contacts and eliminating line shortening effects for submicron features. The TFH industry is facing a similar challenge as TWs dip below 1 micrometer. In an attempt to transition the pole trimming process technology from inductive to MR heads, the issue of magnetic performance versus pattern fidelity of the feature becomes critical. OPC masks can be used to minimize the corner rounding effects of trimmed shared magnetic poles, which are ultimately responsible for the track width. This paper evaluates OPC mask technology on rowbar level pole trimming using a 1X stepper to identify the extendibility of minimum TWs for the MR head generation. Various combinations of serifs were experimentally evaluated at different track widths. Multiple photoresists and photoresist thicknesses were selected to represent the range of processes used in the industry. The experimental results were then compared with photoresist simulation studies of the same OPC reticle features. The validation of the simulation results allowed a wider range of conditions to be studied. The results show that OPC is an effective technique for enhancing pole trimming and extending the areal density of modern head designs.
Over the past few years there has been a growing interest in using advanced image formation techniques to enhance optical lithography resolution. Techniques such as Optical Proximity Correction (OPC) and phase shifting involve changes in reticle manufacturing which increase the printability risk of small reticle defects and therefore impact wafer yields. There have been several experimental and simulation studies on the printability of sub-half micron defects using both reduction and 1X photolithography equipment. In general these studies have focused on the printability effects of line and space features. However, OPC is frequently implemented to control the size and shape of contact structures. This study was performed to gain a better understanding of the behavior of contact hole defects in a 1X lithography system using both a moderate and a high contrast photoresist. A test reticle was created with 0.72 micrometer contact holes containing edge, corner and isolated central defects in programmed sizes from 0.15 to 0.4 micrometer, and exposed on a submicron 1X stepper. Printability was determined by measurement of the normalized area of the contact (NCA). Reticle defect printability of the contact structures was modeled for each photoresist using a three-dimensional (3D) optical lithography simulation tool. The experimental NCA data was compared to modeled results to validate the simulator. Cross sectional contact simulations were then prepared to show the relative impact on the placement of the defect in the contact structure. Both the simulation and the experimental results show the relative sensitivity of the two photoresists to the printability of defects in the contact hole structure. This analysis enhances the understanding of the criticality of defect sizes in contact arrays and allows users to predict defect printability issues for new photoresists.
There has been considerable attention given to the printability of reticle defects and their impact on wafer yields. Over the last year the printability risk from small defects increased due to the wider application of optical proximity correction structures and the inclusion of more phase shifting retictles. There have been several simulation studies on the printability of sub-halfmicron defects using lens and illumination parameters of 5X reduction steppers. Since submicron 1X projection systems are being incorporated into numerous fabricant lines, there is a clear need to determine if these system show similar sensitivity to sub- halfmicron defects as reduction steppers. Earlier experimental work examined the printability of several classes of sub-halfmicron 25 micrometers defects on a submicron 1X stepper. To extend this work, a 3D optical lithography simulation tool has been employed to predict the printablity of various reticle defect scenarios. Experimental data was used to validate the 3D simulator by comparing modeling data to SEM measurements of wafers exposed with a reticle containing programmed clear pinhole and opaque pindot defects. A statistically designed simulation study was performed to quantify the critical dimension variation resulting from defects of varying size, proximity to a feature edge and variation in the pitch of the impacted line/space features. An additional statistically designed simulation was then use to predict the printability behavior of defects relative to different features sizes over a range of numerical aperture and partial coherence settings applicable to a 1X lens design. Finally, the impact of defect length and width on printability were characterized for rectangular defects over a range of sizes. Overall, this analysis enhances the understanding of the relationship between reticle defects and 1X projection optics and allows for determination of optical reticle defect specifications for cost effective lithography applications.
As the push for improved resolution in wafer lithography intensifies and 0.18 micrometer devices are nearing production, the potential impact of subhalf micron reticle defects has become a growing concern. There have been several studies on the printability of subhalf-micron defects on high resolution reduction photolithography equipment. These studies have been extended to 1X lithography systems and more recently to advanced sub-micron 1X steppers. Previous studies have indicated that 0.20 micrometer opaque and 0.25 micrometer clear pinhole defects were at the margins of adversely impacting 0.65 micrometer lithography on a 1X stepper. However, due to the limited number of defects at these sizes on the reticle, definitive conclusions on printability could not be drawn. An additional study, using a three dimensional (3D) optical lithography simulation program, has shown defect size, proximity to an adjacent feature, and feature pitch to be significant factors contributing to reticle defect printability. Using the simulation findings as a guide, a new reticle was designed to contain an increased number of clear pinhole and opaque defects in the 0.15 to 0.30 micrometer range located in multiple pitches of both horizontal and vertical line/space pairs. Defect printability was determined using a 1X i-line projection stepper with focus and exposure optimized for nominal critical dimensions of 0.65 micrometer. The reticle and wafer defects were measured using low voltage SEM metrology. Simulation and experimental results have shown that pitch is the most significant contributor in the printability of clear pinhole, opaque, square and aspect ratio defects. In general, the impact of defect proximity to an adjacent feature is less extreme than the effect of pitch, but is more pronounced for clear pinhole defects. This study suggests that simulation can be a useful tool to help lithographers understand the behavior of reticle defects for particular layout design parameters. Consequently, simulation can be used to develop realistic reticle defect specifications with mask vendors, and improve cost-effectiveness. Defect printability simulation can also be used to predict the effect of known defects on existing reticles to determine if these reticles should be used for manufacturing.
There have been several studies on the printability of subhalf-micron defects using reduction steppers. These studies typically involved 1X reticles with defect sizes greater than 0.3 micrometers . Because submicron 1X projection systems are being incorporated into numerous fabrication lines, there is a clear need to determine the impact of subhalf-micron defects using these systems. This paper examines defect detection and measurement capability on 1X reticles and the printability of those defects on production submicron 1X steppers. This analysis will enhance the understanding of the relationship between defect size and 1X projection optics and allows for determination of optimal defect specifications. A test reticle representative of a 64 Mb DRAM metal layer was manufactured with a programmed series of attached and isolated defects ranging from 0.15 to 0.5 micrometers . Both clear and opaque polarity defects were designed. The defects were identified and measured on two different reticle autoinspection systems. The performance of the two systems was compared to the reticle database to evaluate capture rates and efficiency. Actual reticle defect sizes were measured using low voltage SEM metrology. Defect printability was determined using a 1X i-line projection stepper with focus and exposure optimized for nominal critical dimensions (CD). The defects that printed on the wafer were measured and compared to the defects measured on the reticle. The effects of varying wafer exposure dose and focus within a 10 percent CD process window on defect printability were also evaluated. The results of the mask inspection comparison and the reticle versus wafer defect maps are compared.
Process latitude, especially depth of focus (DOF), is an ever-growing concern to semiconductor and thin film head (TFH) manufacturers. It is well known that as lithographers pursue smaller linewidth resolution through the use of larger numerical apertures (NA) and smaller exposure wavelengths, DOF continues to shrink. TFH manufacturers are faced with the additional burdens of thick resist and high aspect ratios. One successful method of regaining a portion of the lost DOF in i-line reduction lithography is the use of phase shift masks. There are a variety of phase shift mask types that typically involve a conventional chrome mask with an added layer of shifter material adjacent to specific geometries. Another type, without added shifter material, requires a second etch into the quartz substrate. Compared to conventional masks, these mask types require additional patterning and processing steps. Because of their high cost and technical limitations, phase shift masks are frequently used only for contact layers in i-line processes. Attenuated embedded phase shift (AEPS) masks have the potential to improve depth of focus for 1X g/h-line systems. Because the shifter material is incorporated in the substrate, these masks can be patterned and processed similar to conventional chrome masks. Consequently, their cost is much lower than other phase shift mask types and their applications are not limited by dense geometries.