Critical Dimension Uniformity using Reticle Inspection Tool b b
Mark Wylie, bTrent Hutchinson, bGang Pan, bThomas Vavul, bJohn Miller, bAditya Dayal, Carl Hess aMike Green, aShad Hedges, aDan Chalom, aMaciej Rudzinski, aCraig Wood, aJeff McMurran a
Photronics nanoFab North America, Boise, ID, 83716 b KLA-Tencor Corporation, San Jose, CA, 95134
The Critical Dimension Uniformity (CDU) specification on photomasks continues to decrease with each successive node. The ITRS roadmap for optical masks indicates that the CDU (3 sigma) for dense lines on binary or attenuated phase shift mask is 3.4nm for the 45nm half-pitch (45HP) node and will decrease to 2.4nm for the 32HP node. The current capability of leading-edge mask shop patterning processes results in CDU variation across the photomask of a similar magnitude. Hence, we are entering a phase where the mask CDU specification is approaching the limit of the capability of the current Process of Record (POR). Mask shops have started exploring more active mechanisms to improve the CDU capability of the mask process. A typical application is feeding back the CDU data to adjust the mask writer dose to compensate for non-uniformity in the CDs, resulting in improved quality of subsequent masks. Mask makers are currently using the CD-SEM tool for this application. While the resolution of SEM data ensures its position as the industry standard and continued requirement to establish the photomask CD Mean to Target value, a dense measurement of CDs across the reticle with minimal cycle time impact would have value. In this paper, we describe the basic theory and application of a new, reticle inspection intensity-based CDU approach that has the advantage of dense sampling over larger areas on the mask. The TeraScanHR high NA reticle inspection system is used in this study; it can scan the entire reticle at relatively high throughput, and is ideally suited for collecting dense CDU data. We describe results obtained on advanced memory masks and discuss applications of CDU maps for optimizing the mask manufacturing process. A reticle inspection map of CDU is complementary to CD-SEM data. The dense data set has value for various applications, including feedback to mask writer and engineering analysis within the mask shop.
1 Introduction CD control, specifically CDU and Mean to Target (MTT) values, are critical specifications for a photomask. Nonuniformity in reticle CDs and deviation from target CD cause significant yield loss during chip manufacturing, as well as reducing the optimal process window for advanced semiconductor manufacturing. The CDU and MTT information can also be used to improve the mask manufacturing process. Mask writer global exposure dose is adjusted to compensate for MTT shift. The CDU can also be corrected on the latest pattern generator tools using a number of advanced techniques. Currently, SEM CD measurement tools are used to certify the various CD metrics for reticles prior to shipment. The measurement of the CD on a SEM is an absolute measure of CD in the reticle x-y plane that uses a single pattern for the reference. However, measuring enough sites to perform the regression analysis required to correct CDU in a feedback-loop significantly increases time at the CD SEM production step and impacts mask cycle time. Mask shops would prefer a CDU map which contains dense measurements and covers large areas on the mask. Since the mask inspection tool scans the entire reticle, the inspection system is ideally placed to generate a CDU map. This dense map can also be utilized to detect localized CD errors or hot spots that may be missed by SEM measurement due to the granular nature of the CD SEM measurement process. The output of the map can be used to optimize the mask
Photomask Technology 2009, edited by Larry S. Zurbrick, M. Warren Montgomery, Proc. of SPIE Vol. 7488, 74881O 路 漏 2009 SPIE 路 CCC code: 0277-786X/09/$18 路 doi: 10.1117/12.830148
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