Christopher Alan Lee

Metrology for characterization of wafer thickness uniformity during 3DS-IC processing

There is a constant desire to increase substrate size in order to improve cost effectiveness of semiconductor processes. As the wafer diameter has increased from 2” to 12”, the thickness has remained largely the same, resulting in a wafer form factor with inherently low stiffness. Gravity induced deformation becomes important when using traditional metrology tools and mounting strategies to characterize a wafer with such low stiffness. While there are strategies used to try to reduce the effects of deformation, gravitational sag provides a large source of error in measurements. Furthermore, glass is becoming an important material for substrates in semiconductor applications and metrology tools developed for use for characterizing silicon are inherently less suitable for glass. Using a novel mounting strategy and a measurement technique based on optical interference provides an opportunity to improve on the methodologies utilized to characterize wafer flatness (warp, bow) and total thickness variation (TTV). Not only can the accuracy of the measurement be improved, using an interference based technique allows for full wafer characterization with spatial resolution better than 1 mm, providing substantially more complete wafer characterization.

Frequency-stepping interferometry for accurate metrology of rough components and assemblies

We describe a distance-measuring interferometer based on a novel frequency-stepping laser that is tunable over 30 nm. Conventional tunable lasers provide continuous tuning over a range of wavelengths without any mode transitions. The new frequency-stepping laser was designed to maximize frequency repeatability by exploiting the mode-hopping behavior to achieve equal frequency increments. An interferometric image is collected at consecutive laser mode frequencies making it very easy to perform Fourier transforms. The modulation frequency of the interference on each pixel is directly proportional to the optical path difference between the reference and test arms of the interferometer as well as the laser mode spacing. The inherent stability of the frequency-stepping laser results in a very accurate conversion from the modulation frequency of the pixel to its OPD. A Fourier transform is performed on each pixel to determine the height difference between the reference and measurement arms independent of its neighboring pixels. Our laser mode spacing of 36 GHz results in an unambiguous measurement range of 2.1 mm. Prior knowledge about the features of the part being measured allows us to measure over 300 mm of range with 10 nm resolution. This can be combined with conventional PMI techniques to achieve sub-nanometer resolution. This technique is applicable to both rough and smooth parts making it possible to perform metrology on individual components as well as partial assemblies that require tight tolerances.

High-resolution tool for measuring photomask flatness

As lithography wavelengths reduce, the depth of focus decreases rapidly as well, resulting in the need for flatter photomasks with specifications under 0.25 microns. With the introduction of EUV mask technology, the overlay error budget drives the flatness requirements considerably lower to just 50 nanometers or so. This paper describes a new tool that utilizes near normal incidence interferometry to perform flatness measurements on polished 6025 photomasks that are coated or uncoated. Achieving a low measurement uncertainty required a robust optical and mechanical design. Even nanometer level measurement errors due to gravity sag have to be considered. Supporting the substrate during measurement creates deformations due to gravity that must be dealt with for an accurate evaluation of the flatness. Two improvements to the recently introduced Corning Tropel UltraFlat Mask System that first minimize and then remove the remaining gravity sag errors in photomask flatness measurements will also be discussed.

Error analysis of overlay compensation methodologies and proposed functional tolerances for EUV photomask flatness

Due to the impact on image placement and overlay errors inherent in all reflective lithography systems, EUV reticles will need to adhere to flatness specifications below 10nm for 2018 production. These single value metrics are near impossible to meet using current tooling infrastructure (current state of the art reticles report P-V flatness ~60nm). In order to focus innovation on areas which lack capability for flatness compensation or correction, this paper redefines flatness metrics as being “correctable” vs. “non-correctable” based on the surface topography’s contributions to the final IP budget at wafer, as well as whether data driven corrections (write compensation or at scanner) are available for the reticle’s specific shape. To better understand and define the limitations of write compensation and scanner corrections, an error budget for processes contributing to these two methods is presented. Photomask flatness measurement tools are now targeting 6σ reproducibility <1nm (previous 3σ reproducibility ~3nm) in order to drive down error contributions and provide more accurate data for correction techniques. Taking advantage of the high order measurement capabilities of improved metrology tooling, as well as computational capabilities which enable fast measurements and analysis of sophisticated shapes, we propose a methodology for the industry to create functional tolerances focused on the flatness errors that are not correctable with compensation.

Functional tolerancing using full surface metrology

This paper highlights two examples of the use of full surface metrology to allow for functional tolerancing of components in the areas of EUV lithography (reticle characterization) and DUV precision lens manufacturing (lens holder metrology). For both examples, the measurement of the full surface is a key enabler to understanding the critical characteristics to control and tolerance for functionality or performance. Interferometric techniques are used to provide high resolution and accurate measurements for both examples. Subsequently, this data can be used to identify the surface characteristics that contribute to the end functionality and provide a means for deterministic correction or compensation.

EUV mask flatness compensation strategies and requirements for reticle flatness, scanner optimization and E-beam writer (Conference Presentation)

High volume manufacturing with extreme ultraviolet (EUV) lithography requires mask induced overlay errors of less than 1.5nm for the N7 node. The use of electrostatic chucking and reflective optics causes the reticle backside flatness and reticle thickness to directly affect the placement of the pattern at wafer through both in-plane (IPD) and out of plane distortions (OPD). The minimization of reticle flatness alleviates some of the image placement distortion caused by the reticle’s shape, however to be within the image placement error budget, N7 EUV blanks must have flatness <16nm p-v. With the manufacturing challenges associated with generating such flat blanks, compensation may be an option for imaging improvements; such methodologies will likely be essential for EUV to meet the stringent image placement and overlay specifications needed for high volume manufacturing (HVM). Numerous compensation approaches can be utilized to minimize flatness related image placement errors including write compensation of the reticle, feed forward of reticle flatness data to the scanner corrections, and high-order empirical scanner corrections. This study investigates the benefits and limitations of each of these approaches, and seeks to better define which types of errors can be compensated and which will need further reticle flatness development in order to meet N7 and N5 specifications. Additionally, attention is given to the reticle’s shape as it relates to the limitations to the depth of focus required within the scanner systems. Utilizing an array of substrates and blanks from different vendors, we provide an assessment on which type of compensation method is most effective for addressing the various topographies for each specific reticle, and further explore for which node such schemes may be necessary. This investigation seeks to provide a guide for the industry to work towards the implementation of functional tolerances related to both the compensation scheme used in manufacturing, and the reticle’s resulting non-correctable flatness (residual).

Evaluation of photomask flatness compensation for extreme ultraviolet lithography

As the semiconductor industry continues to strive towards high volume manufacturing for EUV, flatness specifications for photomasks have decreased to below 10nm for 2018 production, however the current champion masks being produced report P-V flatness values of roughly ~50nm. Write compensation presents the promising opportunity to mitigate pattern placement errors through the use of geometrically adjusted target patterns which counteract the reticle’s flatness induced distortions and address the differences in chucking mechanisms between e-beam write and electrostatic clamping during scan. Compensation relies on high accuracy flatness data which provides the critical topographical components of the reticle to the write tool. Any errors included in the flatness data file are translated to the pattern during the write process, which has now driven flatness measurement tools to target a 6σ reproducibility <1nm. Using data collected from a 2011 Sematech study on the Alpha Demo Tool, the proposed methodology for write compensation is validated against printed wafer results. Topographic features which lack compensation capability must then be held to stringent specifications in order to limit their contributions to the final image placement error (IPE) at wafer. By understanding the capabilities and limitations of write compensation, it is then possible to shift flatness requirements towards the “non-correctable” portion of the reticle’s profile, potentially relieving polishers from having to adhere to the current single digit flatness specifications.

Method for measuring reticles with pellicles mounted

As lithography wavelengths reduce, the depth of focus decreases rapidly as well, resulting in the need for flatter photomasks with specifications under 0.25 microns. As the industry begins to extend 193 nm technology to smaller line widths, and with the significantly tighter flatness requirements for 157 nm and EUV masks, it will become necessary to inspect finished masks for flatness. By measuring the mask flatness after the pellicle has been mounted, the deformations caused by stress relief due to exposure of the reticle and mounting the pellicle can be taken into account reducing the overall uncertainty of the lithography process. This paper describes measurement techniques applied to a tool that utilizes near normal incidence interferometry to perform concurrent flatness and thickness measurements on finished reticles, as well as other techniques to bring the reticle flatness measurements more in line with the exposure tools. The flatness of the reticle is a critical aspect of the lithography system performance. Without sufficient control over flatness, the features on the image plane become distorted, resulting in image placement errors. As the line widths become finer, and especially in the case where wavelengths are being extended beyond their original nodes, control of the reticle flatness becomes more critical. In many cases, the last time the reticle flatness is measured, is after the photoresist is applied, or even as far back as the blank state. The influence of later changes such as film deposition, exposure, and pellicle mounting may only be accounted for through tightening the flatness spec on the blank itself. By measuring the flatness later in the process, it is possible to have greater control over the individual process steps, and to meet the flatness requirements for the lithographic process without excessively tight tolerances upstream. In order to interferometrically measure the reticle flatness with the pellicle mounted, it is necessary to either measure the surface through the pellicle, or through the back surface of the photomask. In either situation there will be interference patterns generated between the reference surface, the pellicle, the reticle surface, and the back surface of the photomask, as well as interference between each of these surfaces and each other. In the case of hard pellicles for 157 nm lithography, an extra surface is introduced. The total number of potential interferograms for a reticle with a soft pellicle would be 6, and for a hard pellicle 10. Retrieving the information about the surface of interest then becomes very complicated. The Corning Tropel UtraFlat is a near normal incidence interferometer with some unique measurement advantages over traditional normal incidence interferometers. By illuminating the surface at approximately 45° angle of incidence, it is possible to eliminate extraneous fringe patterns optically by limiting the spatial coherence of the laser source. Limiting the spatial coherence optically eliminates fringe patterns from surfaces far apart due to the relative shear distances of these interfered beams, while maintaining signal from the closer surfaces. In this manner it is possible to restrict the measurement data to the back surface flatness, or the backside flatness plus the thickness variation of the mask in a controlled manner. Utilizing this same technique from the pellicle side, it is possible to measure pellicle flatness and thickness variation in its final, mounted state, a key factor in overall system performance for 157 nm lithography. Figure 1 shows an example of an interferogram from a photomask blank measured from the backside with flatness and thickness variation fringes. Figure 2 shows the isolated backside flatness as well as the front side flatness measured by combining the thickness variation and backside flatness from a reticle with a pellicle mounted. Figure 3 shows the change in form over a 130 x 130 mm quality area caused by mounting the pellicle to the reticle. An alternative technique for tracking late process changes is to store the blank front and back surface measurements before coating, exposure, and pellicle mounting, and by measuring the back surface flatness through each of these steps, it is a simple matter to track the changes in the form from each step directly in the back surface, and apply this shape change to the front surface measurement. This technique has advantages later in the process since a finished reticle may have significant variation in reflectivity due to exposure, resulting in high frequency contrast variation on the thickness variation fringes, which may complicate the previously discussed methods. By increasing the control over late process flatness changes, it should be possible to improve reticle performance more directly than simply tightening the requirements on the blanks. Most importantly, it is the finished reticle flatness that the lithography process requires, and not simply the blank flatness. By measuring the finished reticle the performance is known, not simply implied, ensuring that the real lithographic needs are met, which are becoming more and more stringent as line widths and wavelengths reduce.