Erez Graitzer

Detection of progressive transmission loss due to haze with Galileo mask DUV transmittance mapping based on non imaging optics

In this paper, we expand on our earlier work1,2 reporting the use of high sensitivity DUV transmission metrology as a means for detection of progressive transmission loss on mask and pellicle surfaces. We also report a use case for incoming reticle qualification based on DUV transmission uniformity. Traditional inspection systems rely on algorithms to locate discrete defects greater than a threshold size (typically > 100nm), or printing a wafer and then looking for repeating defects using wafer inspection and SEM review. These types of defect inspection do not have the ability to detect transmission degradation at the low levels where it begins to impact yield. There are numerous mechanisms for transmission degradation, including haze in its early, thin film form, electric-field induced field migration, and pellicle degradation. During the early development of haze, it behaves as a surface film which reduces 193nm transmission and requires compensation by scanner dose. The film forms in a non-uniform fashion, resulting from non-uniformity of exposure on the pattern side due to varying dose passing through the attenuating layers. As this non-uniformity evolves, there is a gradual loss of wafer critical dimension uniformity (CDU) due to a degradation of the exposure dose homogeneity. Electric-field induced migration also appears to manifest as a non-uniform transmission loss, typically presenting with a radial signature. In this paper we present evidence that a DUV transmission measurement system, GalileTM, is capable of detecting low levels of transmission loss, prior to CDU related yield loss or the appearance of printing defects. Galileo is an advanced DUV transmission metrology system which utilizes a wide-band, incoherent light source and non-imaging optics to achieve sensitivities to transmission changes of less than 0.1%. Due to its very high SNR, it has a fast MAM time of less than 1 sec per point, measuring a full field mask in as little as 30 minutes. A flexible user interface enables users to easily define measurement recipes, threshold sensitivities, and time-based tracking of transmission degradation. The system measures through pellicle under better than class 1 clean air conditions.

Correcting image placement errors using registration control (RegC) technology

The 2009 ITRS update specifies wafer overlay control as one of the major tasks for the sub 40 nm nodes. Wafer overlay is strongly dependent on mask image placement error (registration errors or Reg errors)1 in addition to CD control and defect control. The specs for registration or mask placement accuracy are twice as difficult in some of the double patterning techniques (DPT). This puts a heavy challenge on mask manufacturers (mask shops) to comply with advanced node registration specifications. Registration test masks as well as production masks were measured on a standard registration tool and the registration error was calculated and plotted. A specially developed algorithm was used to compute a correction lateral strain field that would minimize the registration error. A laser based prototype RegCTM tool was used to generate a strain field which corrected for the pre measured registration errors. Finally the post registration error map was measured. The resulting residual registration error field with and without scale and orthogonal errors removed was calculated. In this paper we present first results of registration control experiments using the prototype RegCTM tool.

Mask CD control (CDC) with ultrafast laser for improving mask CDU using AIMS as the CD metrology data source

CD uniformity control by ultrafast laser system writing inside the bulk of photomasks has previously been shown to be an effective method for local CD Control (CDC) [1], Intra-field CD variations correction has been implemented effectively in mask-shops and fabs based on CDC SEM [2, 3] and OCD as the CD data source. Using wafer CD data allows correction of all wafer field CD contributors at once, but does not allow correcting for mask CD signature alone. In case of a mask shop attempting to improve CDU of the mask regardless of a particular exposure tool, it is a better practice to use mask CD data by itself as the CD data source. We propose using an aerial imaging system AIMS 45-193i as the mask CD data source for the CDC process. In this study we created a programmed CD mask (65nm dense L/S) with relatively large CD errors. The programmed CD mask was then measured by AIMS 45-193i (AIMS45) which defined the CDU map of the programmed CD mask. The CDU data from AIMS 45-193i was then used by Pixer CDC101 to correct the CDU and bring it back to a flat almost ideal CDU. Results 1. AIMS 45-193i managed to map the full mask CDU with a resolution of 0.5 nm. 2. The CDC101 managed to correct the CDU based on the AIMS 45-193i data from Range 5nm and 3S 4nm down to Range 45-193i and CDC101 alone, without any wafer CD data, the mask CDU can be improved >70% and mask contribution to wafer CDU can be brought down to <1.0 nm 3S.

Process Window improvement on 45 nm technology Non Volatile Memory by CD uniformity improvement

For the next years optical lithography stays at 193nm with a numerical aperture of 1.35. Mask design becomes more complex, mask and lithography specification tighten and process control becomes more important than ever. Accurate process control is a key factor to success to maintain a high yield in chip production. One of the key parameters necessary to assure a good and reliable functionality of any integrated circuit is the Critical Dimension Uniformity (CDU). There are different contributors which impact the total wafer CDU: mask CD uniformity, scanner repeatability, resist process, lens fingerprint, wafer topography etc. In this work we focus on improvement of intra-field CDU at wafer level by improving the mask CD signature using a CDC200TM tool from Carl Zeiss SMS. The mask layout used is a line and space dark level of a 45nm node Non Volatile Memory (NVM). A prerequisite to improve intra-field CDU at wafer level is to characterize the mask CD signature precisely. For CD measurement on mask the newly developed wafer level CD metrology tool WLCD32 of Carl Zeiss SMS was used. The WLCD32 measures CD based on proven aerial imaging technology. The WLCD32 measurement data show an excellent correlation to wafer CD data. For CDU correction the CDC200TM tool is used which utilizes an ultrafast femto-second laser to write intra-volume shading elements (Shade-In ElementsTM) inside the bulk material of the mask. By adjusting the density of the shading elements, the light transmission through the mask is locally changed in a manner that improves wafer CDU when the corrected mask is printed. In the present work we will demonstrate a closed loop process of WLCD32 and CDC200TM to improve mask CD signature as one of the main contributors to intra-field wafer CDU. Furthermore we will show that the process window will be significantly enlarged by improvement of intra-field CDU. An increase of 20% in exposure latitude was observed.

Expanding The Lithography Process Window (PW) With CDC Technology

The continuous shrinking of the semiconductor device nodes requires tough specifications of CD uniformity which result in narrowing of the lithography process window. Finding methods for expanding the process window will enable to continue manufacturing at least one more generation using the existing litho equipment. The CDC technology has been described in detail in past studies beginning in 2006; however it has typically been studied from a mask shop perspective. In this paper we will demonstrate a way to improve the CD Uniformity (CDU) on a new mask, which has a CD uniformity problem that leads to shrinking of the lithography process window, by using the Carl Zeiss CD Control (CDC) Technology. The methodology used and the process window improvement verification we show are based purely on fab available techniques and do not require any input from the mask shop. A production memory product in PSC fab P1/2 showed reduced yield due to reduced process window in one line/space (L/S) layer. A close investigation in the fab showed wafer CD non-uniformity of 6.5nm Range and 3.95nm 3S in this layer due to a mask CDU problem. A CDC process to improve the CDU was applied by the Carl Zeiss CDC200 tool based on wafer CD data only. Post CDC treatment results show that CD Range was reduced to 3.8nm (42% improvement) and 3S was reduced to 1.94nm (51% improvement). Further assessment of the litho process window of this layer showed an increase of CD-DOF from 0.15um before (Pre) CDC to 0.30um after (Post) CDC and an exposure latitude increase from 14.1% Pre to 26.7% Post CDC. To summarize our findings, applying the CDC process to the problematic layers allowed to increase the PW in

CD Uniformity correction on 45 nm technology Non Volatile Memory

One of the key parameters necessary to assure a good and reliable functionality of any integrated circuit is the Critical Dimension Uniformity (CDU). There are different contributors which impact the total CDU: mask CD uniformity, scanner and lens fingerprint, resist process, wafer topography, mask error enhancement factor (MEEF) etc. In this work we focus on improvement of intra-field CDU at wafer level by improving the mask CD signature using a CDC200TM tool from Carl Zeiss SMS. The mask layout used is a line and space dark level of a 45nm node Non Volatile Memory (NVM). A prerequisite to improve intra-field CDU at wafer level is to characterize the mask CD signature precisely. For CD measurement on mask the newly developed wafer level CD metrology tool WLCD32 of Carl Zeiss SMS was used. The WLCD32 measures CD based on aerial imaging technology. The WLCD32 measurement data show an excellent correlation to wafer CD data. For CDU correction the CDC200TM tool is used. By utilizing an ultrafast femto-second laser the CDC200TM writes intra-volume shading elements (Shade-In ElementsTM) inside the bulk of the mask. By adjusting the density of the shading elements, the light transmission through the mask is locally changed in a manner that improves wafer CDU when the corrected mask is printed. In the present work we will demonstrate a closed loop process of WLCD32 and CDC200TM to improve mask CD signature as one of the main contributors to intra-field wafer CDU.

Closed loop registration control (RegC) using PROVE as the data source for the RegC process

At sub 4X nm nodes in memory and sub 3X nodes in logic devices mask registration (Reg) is becoming a significant yield limiting factor. This is especially true for Double Patterning Technologies (DPT) where mask to mask overlay on the wafer is heavily influenced by mask registration error. Getting advanced mask registration into specification is a challenge for all mask shops as the tight registration specs are driven by tight wafer overlay specs. The first step in meeting the registration spec challenge in the mask shop is to be able to measure registration with the required specifications. With PROVE® Carl Zeiss SMS has recently introduced into the market a new registration and overlay metrology system which utilizes 193nm illumination for high resolution and a six axes controlled stage. The second step in meeting the registration spec challenge is to actively correct for intrinsic registration errors on the mask. For this Carl Zeiss SMS has developed the RegC® tool. The RegC® tool is based on writing strain zones with the help of an ultrashort pulse laser in the bulk of the mask. The strain zones induce deformations in the mask which practically push the misplaced features to a new location that after removing scale and orthogonality (S/O) correctable errors reflects a smaller residual registration error. By combining the RegC® tool with data generated by PROVE® it is possible to close the loop on the registration control process in the mask shop without wafer print or mask re-write. In this paper we report the demonstration results of a closed loop process between PROVE® and the RegC® tools.

RegC: a new registration control process for photomasks after pattern generation

As the lithography roadmap unfolds on its path towards ever smaller geometries, the pattern placement (Registration) requirements are increasing dramatically. This trend is further enhanced by anticipating the impact of innovative process solutions as double patterning where mask to mask overlay on the wafer is heavily influenced by mask registration error. In previous work1 a laser based registration control (RegC) process in the mask periphery (outside the exposure field) was presented. While providing a fast and effective improvement of registration, the limitation of writing with the laser outside of the active area limits the registration improvement to ~25%. The periphery process can be applied after the pattern generation or after pellicle mounting and allows fine tuning of the mask registration. In this work we will show registration correction results where the full mask area is being processed. While processing inside the exposure field it is required to maintain the CD Uniformity (CDU) neutral .In order to maintain the CDU neutral several different laser writing steps are utilized. A special algorithm and software were developed in order to compute the process steps required for maintaining the CDU neutral from one side while correcting for mask placement errors on the other side. By applying the correction process inside the active area improvements of up to 50% of the 3S registration and values as low as 3 nm 3S after scale and ortho correction have been achieved. These registration improvements have been achieved while maintaining the CDU signature of the mask as measured by areal imaging with WLCDTM (Wafer Level CD Metrology tool from Carl Zeiss SMS).

Mask tuning for process window improvement

For the next years optical lithography stays at 193nm with a numerical aperture of 1.35. Mask design becomes more complex, mask and lithography specifications tighten. The k1 factor comes close to 0.25 which leads to a tremendously increased Mask Error Enhancement Factor (MEEF). This means that CD errors on mask are getting highly amplified on wafer. Process control becomes more important than ever. Accurate process control is a key factor to success to maintain a high yield in chip production. One key parameter to ensure a high and reliable functionality for any integrated circuit is the critical dimension uniformity (CDU). There are different contributors which impact the intra-field CD performance at wafer such as mask CD uniformity, scanner fingerprint, resist process etc. In the present work we focus on improvement of mask CD signature which is one of the main contributors to intra-field CD uniformity. The mask CD uniformity has been measured by WLCD32 which measures the CD based on proven aerial image technology. Based on this CD input the CD uniformity was corrected by CDC200TM and afterwards verified by WLCD32 measurement. The CDC200TM tool utilizes an ultrafast femto-second laser to write intra-volume shading elements (Shade-In ElementsTM) inside the bulk material of the mask. By adjusting the density of the shading elements, the light transmission through the mask is locally changed in a manner that improves wafer CDU when the corrected mask is printed. Additionally, the impact of the improved CD uniformity on the lithography process window was investigated. Goal of the work is to establish a process flow for mask CD uniformity improvement based on mask CD metrology by WLCD32 and mask CD uniformity control by CDC200TM and to verify its impact on the lithography process window. The proposed process flow will be validated by wafer prints. It was shown that the WLCD32 has an excellent correlation to wafer data and an outstanding CD repeatability. It provides a reliable input for CD uniformity correction and is the tool of choice to verify the CD uniformity improvement after CDC200TM treatment.

Correcting image placement errors using registration control (RegC) technology in the photomask periphery

The ITRS roadmap specifies wafer overlay control as one of the major tasks for the sub 40 nm nodes in addition to CD control and defect control. Wafer overlay is strongly dependent on mask image placement error (registration errors or Reg errors)1. The specifications for registration or mask placement accuracy are significantly tighter in some of the double patterning techniques (DPT). This puts a heavy challenge on mask manufacturers (mask shops) to comply with advanced node registration specifications. The conventional methods of feeding back the systematic registration error to the E-beam writer and re-writing the mask are becoming difficult, expensive and not sufficient for the advanced nodes especially for double pattering technologies. Six production masks were measured on a standard registration metrology tool and the registration errors were calculated and plotted. Specially developed algorithm along with the RegC Wizard (dedicated software) was used to compute a correction lateral strain field that would minimize the registration errors. This strain field was then implemented in the photomask bulk material using an ultra short pulse laser based system. Finally the post process registration error maps were measured and the resulting residual registration error field with and without scale and orthogonal errors removal was calculated. In this paper we present a robust process flow in the mask shop which leads up to 32% registration 3sigma improvement, bringing some out-of-spec masks into spec, utilizing the RegC® process in the photomask periphery while leaving the exposure field optically unaffected.