Eitan Zait

CD variations correction by local transmission control of photomasks done with a novel laser-based process

As IC feature sizes become smaller and smaller, requirements for Critical Dimension (CD) variations control have become a critical issue. A new process for the control and correction of intra-field CD variations (Critical Dimension Control or CDC) was applied and its influence on defects detection and photo-masks inspection capabilities at different modes of inspection was investigated. CD Control (CDC) of the photomask is a process in which Deep UV transmittance is selectively altered by patterns of small partially scattering shading elements (Shade in ElementTm) inside the quartz. The shading elements are formed by a process of shooting an ultrafast laser beam focused inside the mask substrate, resulting in localized intra-volume breakdown inside the quartz which creates local pixels of modified index of refraction (delta n). An array of such pixels with constant density constitutes one shading element. Process patterns are predetermined according to a CD variations map which may be supplied from wafer CD SEM, Optical CD or mask aerial imaging simulation tool (AIMS). Thus by changing local photomask transmission levels, it is possible to correct for the CD variations inside the field. Attenuation level, or optical density of the shading elements depends on the laser pulse energy, distance between pixels, number of layers and the size of the shading element itself. Since photomask transmittance is being changed, qualification of the impact of the transmittance changes on the defect detection and analysis capabilities are required. In this study, the principles of patterning of scattering elements inside transparent media by focusing of ultra-short laser pulses were introduced and explained. Analysis of the effects to both mask and wafer due to the CDC process was verified by full printing process applied to wafers, and by aerial imaging simulation tool. More tests for CDC required also tests by automatic reticle inspection tool to be production-worthy for the 65nm node and beyond.

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.

In-field CD uniformity control by altering transmission distribution of the photomask, using Ultra fast pulsed laser technology

As pattern feature sizes on the wafer become smaller and smaller, requirements for CD variation control has become a critical issue. In order to correct CD uniformity on the wafer, the DUV light transmission distribution of the photomask was altered using an ultra-fast pulsed laser technology. By creating a small scattering pixel inside the quartz body of the mask, a multitude of such points creates Shading Elements inside the quartz according to a predetermined CD variations distribution map. These Shading Elements reduce the dose of scanners laser illumination onto the wafer per a local area. Thus by changing the local light intensity, inside the exposure field, to a required level during the photolithographic process the wafer CD is changed locally inside the field. This complete process of writing a multitude of Shading Elements inside the mask in order to control the light transmission and hence wafer level CD locally is called the CD Control (CDC) process. We have evaluated the tool utilizing Ultra fast laser pulses (CDC 101) for local transmission and CD controllability on the wafer. We used Binary and Att-PSM test masks and three kinds of test patterns to confirm the sensitivity of transmission and CD change by the attenuation levels of Shading Elements which is sequentially changed from 0% to 10%. We will compare the AIMS results to printed CD on wafer or simulation results, so that we can correlate the transmission change and CD change by the attenuation levels. This paper also reports the CD uniformity correction performances by using attenuation mapping method on Binary mask. We also cover how Shading Elements affect the phase and transmission on the Att-PSM.

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.

Irradiation resistance of intravolume shading elements embedded in photomasks used for CD uniformity control by local intra-field transmission attenuation

Intra-field CD variation is, besides OPC errors, a main contributor to the total CD variation budget in IC manufacturing. It is caused mainly by mask CD errors. In advanced memory device manufacturing the minimum features are close to the resolution limit resulting in large mask error enhancement factors hence large intra-field CD variations. Consequently tight CD Control (CDC) of the mask features is required, which results in increasing significantly the cost of mask and hence the litho process costs. Alternatively there is a search for such techniques (1) which will allow improving the intrafield CD control for a given moderate mask and scanner imaging performance. Currently a new technique (2) has been proposed which is based on correcting the printed CD by applying shading elements generated in the substrate bulk of the mask by ultrashort pulsed laser exposure. The blank transmittance across a feature is controlled by changing the density of light scattering pixels. The technique has been demonstrated to be very successful in correcting intra-field CD variations caused by the mask and the projection system (2). A key application criterion of this technique in device manufacturing is the stability of the absorbing pixels against DUV light irradiation being applied during mask projection in scanners. This paper describes the procedures and results of such an investigation. To do it with acceptable effort a special experimental setup has been chosen allowing an evaluation within reasonable time. A 193nm excimer laser with pulse duration of 25 ns has been used for blank irradiation. Accumulated dose equivalent to 100,000 300 mm wafer exposures has been applied to Half Tone PSM mask areas with and without CDC shadowing elements. This allows the discrimination of effects appearing in treated and untreated glass regions. Several intensities have been investigated to define an acceptable threshold intensity to avoid glass compaction or generation of color centers in the glass. The impact of the irradiation on the mask transmittance of both areas has been studied by measurements of the printed CD on wafer using a wafer scanner before and after DUV irradiation.

Photomask clear defects repair using ultrafast laser technology

The applicability of ultrafast laser 3D machining of transparent objects for photomask clear defects repair is investigated. The technology is based on patterning 3D shading elements inside quartz body of the photomask at the vicinity of clear defects in chrome layer, which effectively blocks the light for the duration of the photolithography process. Shading elements consist of an array of breakdown points in quartz, produced as a result of laser-induced breakdown and arranged in accordance with the size and location of the defects. Thresholds of bulk breakdown and chrome removal at laser irradiation from the back side of the photomask and their dependence on the pulse energy and height of focal point under chrome layer were obtained. Optical density of the shading element depends on the laser pulse energy, distance between breakdown points, the number of layers and the size of the shading element itself. To increase optical density multi layer shading elements were created. Ultrafast laser technology and a tool for photomask clear defects repair are described.

The Effect of Intra-field CD Uniformity Control (CDC) on Mask Birefringence

Mask and Wafer CD Uniformity (CDU) improvement by utilizing an ultrafast laser system for writing shading elements inside the bulk of Quartz (Qz) Photomasks has previously been shown to be an effective and practical application (1). The CD Control ( CDC) Process is working in production environments for 90 and 65 nm design rule processes which utilize KrF and ArF scanners. Advanced design rule nodes at 45 and 32 nm will utilize high and hyper NA immersion lithography, which require highly polarized light and immersion technology. Maintaining a high degree of polarization requires low birefringence (BF) of the optical path and specifically of the mask. Current mask blanks contribute between 5 to 20 nm of BF which is too high for polarized systems. This lead to the recent introduction of special low BF blanks which provide <1nm BF per mask. The CDC Process which introduces an optical element inside the quartz (Qz) mask performs a local change of the bulk Qz morphology which causes a local change in refractive index of the Qz and may induce some local BF. The induced BF, if too high, may potentially cause depolarization of the highly polarized light of hyper NA scanners. Depolarizing the light by a high degree has the potential to degrade the image contrast in the litho process The current study examined the effect of the CDC Process on the mask BF at 193 nm by writing controlled attenuation shading elements inside special low BF Qz blanks and measuring the BF induced by the CDC Process. Results: It was found that BF induced by the CDC Process is so small that its effect on loss of CDU is negligible compared to the gain in CDU. This will allow mask and IC manufactures to take advantage of Pixers CDC Process in hyper NA litho processes at 45 and 32 nm nodes.

Mask CD control (CDC) using AIMS as the CD metrology data source

It has been previously demonstrated that wafer CD uniformity can be improved via an ultrafast laser system. The system provides local CD Control (CDC) by writing inside the bulk of photomasks. Intra-field CD variation correction has been implemented effectively in mask-shops and fabs based on CD-SEM and Scatterometry (Optical CD or 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. For mask shops attempting to improve CDU of the mask regardless of the exposure tool, it is a better practice to use only mask CD data as the CD data source. In this study, we investigate the use of an aerial imaging system AIMSTM45-193i (AIMS45) as the mask CD data source for the CDC process. In order to determine the predictive value of the AIMS45 as input to the CDC process, we have created a programmed CD mask with both 45nm and 65nm node L/S and hole patterns. The programmed CD mask has CD errors of up to 20nm in 2.5nm steps (4X). The programmed CD mask was measured by AIMS45, defining the CDU map of the programmed CD mask. The CDU data was then used by Pixer CDC200TM to correct the CDU and bring it back to a flat, almost ideal CDU. In order to confirm that real CDU improvement on wafer had been achieved, the mask was printed before and after CDC on an immersion scanner at IMEC and results of pre and post CD data were compared.

DUV attenuating structures in fused silica induced by ultrafsat laser radiation

In this paper we present the method for high-speed 3D structures patterning inside the volume of photomask substrate without affecting front mask surface and absorber layer using ultrafast laser and the results of investigation of DUV radiation attenuation by created structures.