In this paper, the issue of intensity imbalance in an alternating phase shift mask has been studied for hole patterns with pitches 300 nm and below. A method of processing is developed, which would nullify the effects of phase errors that cause focus dependent difference in the sizes of holes belonging to opposite phases. This method uses two exposures with opposite foci. Using this method, the effect of the focus on the difference in the size of holes from opposite phases could be totally eliminated. It also changes the nature of the focus curve, bringing a significant improvement in the depth of focus without affecting exposure latitude and mask error enhancement factor. The method works quite effectively for all the via pitches, however, some constant size difference existed across focus, that is easily correctable by biasing one phase with respect to the other. It was also found that this technique could bring remarkable immunity against the lens aberrations such as defocus and astigmatism.
This paper studies the concept of and challenges in patterning trenches using hybrid phase shift mask. Our hybrid mask consists of alternating, chrome-less and 20% attenuated phase shift features on the same reticle. Using this mask, we could pattern across-pitch 120 nm trenches on 0.68-NA, KrF lithography scanner, which is equivalent to K1 of 0.33. However, many challenging issues like unequal best focus for different duty ratios of the same technique and same duty ratio of different techniques, variation in the dose requirements despite aerial image CD matching and the pitch dependent variation in the critical dimension imbalance of the zero and π phased trenches are observed. These issues, that are question marks on the viability of hybrid mask, are presented in this paper. Hybrid mask manufacturing and characterization data is also included to justify that the issues are not because of the mask manufacturing process.
One of the contributions to pattern placement/misalignment may come from the mask making process itself, in chromeless masks. This contribution will be important at 90 nm and smaller nodes. Hence it is necessary to estimate this contribution and find ways to minimize this. In this paper an effort has been made to measure this misalignment accurately. A series of box in box structures for overlay measurement, on KLA and CD SEM, were designed on the reticle. The structures had an outer box of etched chrome and an inner box with 180 degree phase. The edge of the chrome was used as the edge of the outer box. The line printed at the phase intersection was used as the edge of the inner box. Each of these structures were put in with a pre-determined value of X and Y misregistrations. The CD SEM structures were smaller in size but designed the same way as KLA structures. Such structures were put at 4 corners of the die. Overlay measurements were carried out using the optical overlay machine as well as CD SEM. An average misalignment of 11 nm and 1 nm were found in the X and Y directions respectively. When the results from each die corner was analyzed, it was found that the X misalignment had two different distributions. Also, exposure parameters such as focus and partial coherence for best misalignment measurement points were investigated. It is concluded that for obtaining accurate misalignment data, measurements should be conducted at a focus where the two opposite phase edges pattern at similar width. Also, a higher partial coherence is recommended as aberrations such a coma have more profound influence at lower partial coherence and this could contaminate the true misalignment data.
A method to fabricate a very thin channel body Fin-FET and Tri-gate MOSFET is presented. 8% Attenuated Phase-shift mask (APSM) and single phase chrome-less mask (CLM) techniques are evaluated to pattern fins in sub-50 nm regime using KrF lithography scanner with a maximum numerical aperture of 0.68. Some of the issues of single phase CLM technique with respect to fin patterning are highlighted. Dual Exposure With Shift (DEWS)’ is introduced to pattern gate lines down to 80 nm using binary mask.
Scattering bars have been an essential component of the reticle layout design to increase process yields for devices with design rules that are 0.18 um and below. These are sub-resolution features and make semi isolated and isolated features to be imaged like dense features as the illumination conditions are always decided by most dense pitch. With the use of scattering bars the depth of focus and iso-dense matching get improved. This results in better critical dimension (CD) control in the wafer fabs. Scattering bar has been helpful in extending the limit of optical lithography. This paper describes the effect of scattering bars width and separation on the printed feature size. Trench patterning is studied at different partial coherence and lens numerical aperture (NA). Also, the effects are compared for binary and 8% attenuated phase shift mask (APSM). The patterned feature size is found to be more sensitive to scattering bar parameters at small NA and low partial coherence. The CD of the feature has strong dependence on scattering bar separation than size and also influenced by the NA and sigma. An interesting phenomenon at low partial coherence is the presence of deep valley or 'V' shaped CD trend in scattering bar separation versus CD curve. CD dip is more on APSM as compare to binary mask.
This paper studies the alignment performance of dual damascene patterning using two different integration schemes. These schemes cater to two different low K materials. The via first scheme is used for CVD type material while the dual hardmask trench first is used for spin on low K material. The alignment and exposure is performed on Nikon scanner S203B using LSA (Laser Step Alignment; a scattering based system) and FIA (Field Interferometer Alignment; a contrast-based system) systems while the overlay is measured on KLA 5200XP.
Many different mark designs were evaluated for both the schemes. The effect of resist coat was found to be different on different mark types. Difference was also seen in the signal strengths in X and Y directions. Process optimisation for scanner was done by varying the slice level, signal processing algorithms, and focus. The initial alignment mark evaluations were done by studying the signals obtained from such marks and comparison of Static Random Factor (SRF) and Dynamic Random Factor (DRF) obtained from the scanner. The best marks were then applied to the lot splits and the final performance was evaluated by measuring the total overlay results. Results showed that SRF and DRF evaluation could be used for screening to find out the best marks for a certain level. It can also be concluded that dual damascene alignment is possible with trench first scheme that has small step height using the LSA window mark or the FIA narrow island mark. For both schemes the best performer was the FIA Narrow Island Mark. Hence it can be concluded that better alignment performance could be obtained by special alignment mark designs for a specific level.
Attenuated Phase Shift Masks (att PSM) have become very popular in the industry for printing contact holes. Higher transmission att PSM generally tends to give a better depth of focus and exposure latitude. However, the main drawbacks of using higher transmission masks are side lobes, printing of unnecessary patterns and resist erosion. The side lobbing is strongly dependent on the feature size, pitch, coherency of exposure radiation, illumination type and the transmission of the mask being used. Along with these factors, the other most important factor is the resist contrast. In this paper the effect of pitch, feature size, and resist sensitivity were evaluated on side lobes and rings formation for via holes designed down to 180 nm. Six different pitches were studied (1:1 to 1:5). Two different types of resists were used and the mask transmission used for the study was 8%. Simulations were carried out using PROLITH 3D version 7.1 from KLA Tencor while the experimental verifications were done at Nikon 248 nm step and scan tool. The experimental results were found in accordance with the simulation data. The effect of NA & (sigma) have also been studied on resolution, exposure latitude and depth of focus.
The current work was done jointly between Institute of Microelectronics, Singapore and Tokuyama Corporation, Japan. Here the patterning performance of four different developers was evaluated. Three of these were with surfactant and one was without surfactant. The parameters evaluated were resist thickness loss, resist contrast, process windows such as depth of focus and exposure latitudes, across wafer critical dimension (CD) uniformity, and post pattern defect density. Feature sizes of 180 nm and 150 nm were evaluated for litho process latitudes. CD uniformity was also evaluated for 180 nm and 150 nm geometries. The resist loss was found to be minimum for developer A which was without surfactant. The depth of focus and exposure latitude of smaller geometry (150nm lines) in general showed better values for developers with surfactants. Developer solutions with surfactant also gave better across wafer CD uniformity for smaller geometries (150nm). Post pattern defects were found to be least with developer C that which contained nonionic type of surfactant and cationic one. Defects were highest for developer B which contained nonionic type of surfactant. The addition of optimum surfactant to developer has potential for reducing defects to lower levels and achieves better across wafer CD uniformity for smaller geometries.
Intra-field lens aberrations and distortions affect the shape of contact hole patterning. This effect is more severe with defocus. In this paper, we have studied the effect of difference in horizontal and vertical diameter of a contact hole on the lithography process window for different illumination conditions using a 0.68NA step and scan system. It is found that the depth of focus (DOF) for 0.22um contact hole patterning shrinks considerably taking into account the horizontal and vertical diameter measurements as compared to the average diameter measurements alone.
BARC (Bottom Anti-Reflective Coating) is used to minimize thin film interference effects such as swing curve, standing waves and resist notching in the photolithography process. In the 'via first' dual damascene approach, BARC also acts as a protecting layer for the substrate underneath the via during the trench etch step. Conformal BARC is normally used for the patterning due to its uniform film thickness over the surface topography, which can provide good CD control. However, conformal BARC may not be able to provide sufficient substrate protection at the via bottom as BARC film remaining in the via may not be thick enough. In comparison, planarizing BARC has better via filling property which can provide sufficient protection for the via bottom substrate. In this study, four different BARCs; conformal and planarizing at two different viscosities, were evaluated for the 'via first' dual damascene copper patterning process. Low viscosity BARC was used to obtain a thin BARC coating for the partial via filling, while high viscosity BARC was used for full via filling process. We evaluated the performance of BARCs for via filling, depth of focus, exposure latitude, iso-dense feature bias and CD control. Dual damascene pattern was etched using two different etch recipes and we compared the performance of all the four BARCs for final etched pattern formation.
In this work, different overlay targets were evaluated for the Via first process with conventional USG dielectric. The etch stop layer nitride thickness was limited at 500 A as increasing this thickness will increase the RC delay which is undesirable. A series of targets were evaluated to find out the best performer. Target evaluation was done by their appearance, static repeatability, dynamic repeatability, target correlation, Tool induced shift, Overall misregistration, and residuals. Lot comparisons have also been done using selected targets. Lot average misregistrations (with the best target-2micrometers trench) of 9nm (X + 3(sigma) ), and 15nm (Y + 3(sigma) ) were obtained for the Metal 2 (M2) aligning to Via 1 (V1) level. The different Bar in Bar target structures evaluated were: a. Trench in trench : 1 micrometers and 2 micrometers trenches. b. Wall in wall : 1 micrometers Bars and 2 micrometers Bars. The trench in trench structures were found to work better than bar in bar for conventional dielectric Via first approach. The 2micrometers thick trenches gave the best results for target correlation, dynamic repeatability, and residual values. Metal 1 to metal 2 targets also gave good results and could be used. For low K dielectric and copper integration, a dual hard mask scheme was used. The dual hardmask was used to minimize the interaction of organic dielectric with organic barc and deep UV resist layer as this sometimes gives rise to poisoning issues. For the Organic Low K dielectric and Copper, where the dual hard Mask scheme was followed, the Wall in Wall target gave good contrast and the best results.
The flatness of the chuck on the stepper or scanner is critical to obtain good patterning performance especially in the sub quarter micron regime. In this study an attempt has been made to u7nderstand the flatness signature of the chuck by measuring the flatness of a super flat wafer in two different notch orientations and subtracting the signatures. If the chuck or the wafer were ideally flat then there would be no different in flatness signatures between the two orientations. However in practice difference was found as neither the chuck nor the wafer is perfectly flat. This difference could be used to obtain an understanding about the flatness signature on the scanner chuck itself. This signature could be used by equipment manufacturers as an additional method to measure chuck flatness so that only superior chucks are used for equipment that are being made for sub quarter micron lithography. The second part of this study consisted of finding out the effect of this flatness on the resulting CD on wafers. Wafers, with different flatness signatures, were exposed at different orientations and the CD variations were evaluated. All wafers showed improvements in the orientation of better flatness. For some wafers the improvements was significant but for others the result was close to the CD variation due to rework. This could be attributed to the inherent signatures on the wafers and how abrupt the change in flatness was. The wafer deformation factor was not analyzed for brevity as this would make the problem far more complex.