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Lithography S

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UV Inspection of EUV and EPL Reticles Donald W. Pettibone, KLA-Tencor Corporation Alan R. Stivers, Components Research, Intel Corporation P. J. S. Mangat, Motorola *DigitalDNA™ Laboratories Michael Lercel, NGL MCoC, Photronics/IBM Anthony Novembre, Bell Laboratories, Lucent Technologies

A UV inspection tool has been used to image and inspect Next Generation Lithography (NGL) reticles. Inspection images and simulations have been used to provide feedback to mask makers so that inspectability of NGL masks can be optimized.

SCALPEL masks have high optical contrast and look much the same in reflection as conventional chrome-on-glass masks do in transmission. EPL stencil masks can be imaged well in reflection, but defects below the top surface (in the cutouts) may not be detectable optically. EUV masks made to date tend to have relatively low contrast, with line edge profiles that are complex due to interference effects. Simulation results show that improved EUV inspection images can be obtained with a low reflectivity absorbing layer and the proper choice of buffer layer thickness.

Introduction

A partnership, partially sponsored by NIST-ATP Cooperative Agreement #70NANB8H44024, has been formed to retire the technical risks associated with optical inspection of EUV and SCALPEL reticles. The members of this partnership are KLA-Tencor, Lucent Technologies, the EUV-LLC, Photronics, and Dupont Photomasks. The EUV-LLC is comprised of AMD, Infineon, Intel, Micron, and Motorola. In addition, Motorola has provided SCALPEL masks to the program.

This program has three phases, each about one year in duration. In the first year, KLA-Tencor built a research tool and gathered information to support modeling efforts. In this, the second year, we are imaging and inspecting NGL reticles. The main goal this year is to establish the feasibility of optical inspection of NGL reticles at the 70 and 100 nm nodes. In the third year, KLA-Tencor plans to design a production prototype inspection system for NGL reticles.

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The emphasis to date in this program has been on providing feedback to mask makers so that mask design can be optimized for inspection. A number of NGL reticles have been imaged with a specially modified UV inspection system. Based on images and simulation results, recommendations have been made for changes in mask design that can improve the inspectability of NGL masks. Images and preliminary inspection results on some NGL masks will be presented in this paper.

Simulations have been carried out which indicate that EUV masks can be optimized for inspectability. In particular, the absorber reflectivity at the inspection wavelength should be minimized, and the buffer layer thickness can be chosen to improve contrast.

Research tool description

The research tool used in these studies is based on a KLA-Tencor high-NA UV inspection system. The operating wavelength is 364 nm, with a minimum pixel size of 150 nm. The system has been modified to accept all NGL reticle types. Due to the fact that NGL reticles do not have pellicles, special care has to be taken to avoid contaminants. A reticle SMIF pod developed by Asyst Technologies can be used to keep the reticles clean when not undergoing inspection. A transfer station has been built to transfer reticles from the SMIF pod to special

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adapters that are mounted in the inspection tool. New computing capabilities have been added to the research tool, resulting in improved defect sensitivity, which is needed to meet the defect sensitivity requirements of NGL. NGL images and inspection results

SCALPEL SCALPEL (Scattering with Angular Limitation Projection Electron-beam Lithography) is an Electron Projection Lithography (EPL) technology developed by Lucent Technologies1. The mask is constructed from a 200 mm diameter silicon wafer on which a thin silicon rich nitride (SiN x) layer, typically 100 nm, and a metal scattering layer, typically 30 nm of tungsten (W) and chromium (Cr), are deposited. The silicon wafer is patterned and etched to produce areas of freestanding membranes. The remaining silicon substrate forms a grillage structure and provides support and mechanical strength (Figure 1). The mask pattern is etched into the metal (scatterer) layer and the mask is imaged in transmission by scanning a 1 mm x 1 mm 100 kV electron beam along the membrane stripes, with adjacent stripes being stitched together at the wafer. The mask image is projected with 4X-reduction electron optics onto a resist-coated wafer. At UV wavelengths the membrane is transparent, so defects on the backside of the membrane are visible provided they are not behind the metal pattern.

The SCALPEL masks that were tested had trapezoidal struts that were wet etched. This permits inspection in either reflection or transmission mode. However, it is highly desirable to have vertical wall struts so as to maximize the usable space on a mask, which will be based on a 200 mm wafer when SCALPEL goes into production. Vertical struts pose an inspection problem for transmission imaging or for imaging from the backside (strut side) of the mask, the front side of the mask having the metal scatterer on it. This is because the vertical walls vignette the high-NA components of the light that are needed to obtain good resolution and high defect sensitivity. This results in spatial variations

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of both resolution and image intensity, with up to 75 percent loss of light at a corner of two intersecting struts. Therefore, reflection imaging from the front side of the mask is the preferred inspection mode for vertical strut SCALPEL masks.

We report here inspection results for three SCALPEL masks. The first, SCALPEL1, designed and fabricated

by Lucent Technologies, is made up of a stack of 27 nm of Won six nm of Cr on a 100 nm SiNx membrane, with the substrate being a four-inch silicon wafer. The struts are formed by an anisotropic, wet potassium hydroxide etching of <100> Silicon wafers. The membranes are 1.1 mm by 12.1 mm on a side. The base pattern of this programmed defect mask is a wiring pattern comprised of 3.0-micron lines and spaces. There are three types of defects: a bridge between lines, a break of a line, and a pindot between lines, at seven different nominal sizes ranging from 200 nm to 800 nm in 100 nm increments.

Figure 2 shows reflection and transmission images of the smallest pindot. The contrast of the defect observed in reflection mode is about twice that observed in transmission.

The wiring pattern exhibits good contrast to noise in both reflection and transmission. The line edge profiles are monotonic, with no edge ringing in the reflection image.

This is due to the fact that the SCALPEL mask stack is thin compared to a wavelength, about one-tenth of a wavelength at UV. Inspection results are shown in Table 1.

All but the smallest defects were found consistently in the six transmission inspections that were run.

Transmission

Reflection

F i g u r e 2. UV images o f Lucent SCALPEL mask with 200 nm nominal pind ot defect.

Type/Size 200 nm 300 nm 400 nm 500 nm 600 nm 700 nm 800 nm Bridge

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Break

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Spot

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Tabl e 1. SCA LPEL1 in spec tio n results ( 6 inspections, transmission).

F i g u re 1. SCALPEL mask layout and cr oss sect ion.

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Reflected

Reflected

Transmitted

Transmitted

40 nm

80 nm

120 nm

160 nm

40 nm

Nominal Sizes

80 nm

120 nm

160 nm

Nominal Sizes

F i g u re 3. SCA LPEL2 mask (Pho tronics /MCoC), defects A1, A3 , A5,

F i g u re 4 . SCALPEL3 mas k (Mo torola), d ef ec ts A1, A3, A5, A7 ,

A7, 400 nm L/S.

20 0 nm L/S.

The second mask, SCALPEL2, was made by Photronics/ MCoC. It has a programmed defect pattern designed by KLA-Tencor on a four-inch silicon wafer with trapezoidal struts. The mask stack is 27 nm of W on five nm of Cr on a 150 nm membrane of SiNx. There are 14 defect types at ten different defect sizes. Figure 3 shows a series of pinhole defects in a wiring pattern of 400 nm lines and spaces, which would print on the wafer at 100 nm lines and spaces. The nominal defect size in these defects ranges from 80 nm to 320 nm in 80 nm increments. The defect visibility is better in the reflection images than in transmission.

obtain mask contrast. We have imaged one stencil mask made by Photronics/MCoC (Figure 5). The reflection image of the top surface of the stencil mask shows good contrast and resolution. The transmission image shows poorer resolution and very low signal levels, on the order of a few percent. Therefore, it is likely that stencil mask inspection may not be possible using an optical system because defects that are not near the top surface of the

The third mask, SCALPEL3, made by Motorola, has the same programmed defect pattern as SCALPEL2, though it also includes the patterns scaled to smaller sizes. It is based on an eight-inch silicon wafer with trapezoidal struts. The mask stack is 30 nm of tantalum silicon nitride on 10 nm of Cr on a 100 nm SiNx membrane. The base pattern shown in Figure 4 has 200 nm lines and spaces, which would print as 50 nm lines and spaces on the wafer. The nominal defect size in these defects ranges from 40 nm to 160 nm in 40 nm increments. Again, better contrast is seen in the reflected images.

Transmission, high contrast

In summary, SCALPEL masks, in reflection, look much the same as conventional chrome on glass masks do in transmission. The programmed defects we have studied to date show better visibility in reflection than in transmission. Programmed SCALPEL defects in the 100 to 140 nm size range are visible.

EPL Stencil Masks One type of EPL uses a stencil mask and images transmission electrons through the mask to the wafer with a 4x reduction.2 However, unlike SCALPEL, which uses a scattering layer to obtain mask contrast, stencil cutouts are made through a 2.0 Âľm thick silicon membrane to 8

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Reflection

F i g u re 5. Stencil mask ( Ph otronics/MCoC), reflection and transmission images , 400 nm line widths .

mask may not be detectable at the needed sensitivity levels. More work needs to be done to verify this.

EUV A general description of EUV lithography is provided in Reference 3. EUV masks are made at 4X, and are composed of an absorbing metal layer on top of a buffer layer on top of an EUV reflecting mirror (Figure 6).

This mirror is composed of alternating layers of Si and molybdenum (Mo), and typically forty pairs of layers are used to obtain a reflectivity of approximately 65 percent at the EUV wavelength of 13.4 nm. The buffer layer is there to prevent damage to the multilayer during the absorber etch process and mask repair process.

The patterned EUV masks are first inspected for hard defects after the absorber etch. The defects are then repaired, and the buffer layer is subsequently etched. A second inspection is performed after the buffer layer is etched. The absorber and buffer layer height add up to an optical path difference (OPD) of between approximately

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Nominal Size(nm)

F i g u re 6. EUV mask cross section.

120 nm to 180 nm, which is of the order of one-half of the UV or DUV inspection wavelength. Therefore, in reflection inspection images, the OPD difference between light reflected off of the absorber and light reflected off of the ML is about one wavelength. This rapid phase variation typically results in a pronounced dark fringe in the inspection image at the absorber-ML edges, unless the reflectivity of one of the materials is much higher than the reflectivity of the other. This effect will be discussed in the section on optimization of EUV masks.

The first mask we inspected, EUV1, was designed and made by Intel, and has a programmed defect pattern with wiring and contact defects of varying types and sizes.

The mask stack has 105 nm of titanium (Ti) absorber, on 85 nm of silicon dioxide (SiO2) buffer layer, on a silicon wafer substrate. Silicon is a reasonably good match to the EUV multilayer in terms of UV reflectance. The SiO2 has been etched on this mask, so it is only present under the absorber layer. Figure 7 shows reflection images of an absorber protrusion defect in the 400 nm/800 nm line/space pattern. The defect is 0.5-microns wide and protrudes into the space a distance of 80 nm, 100 nm, 140 nm, and 200 nm. The Protrusion A defect sizes have been confirmed with SEM measurements. Note that all of the defects are visible in the images. The dark fringes mentioned earlier are evident in these images. Inspection results for the 400/800 line/space pattern are shown in Table 2. Defects towards the bottom of the table that were not detected were checked with a SEM. These defects were shown to be either undersized or to have not resolved on the reticle at

PA 08 (80 x 500 nm)

PA10 (100 x 500 nm)

PA14 (140 x 500 nm)

Type Intrusion A Intrusion B Protrusion A Protrusion B Space Wiring Bridge Corner Hole 1 Corner Hole 2 Corner Hole 3 Hole 2 Hole 3

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140

100

80

6 6 6 6 6 6 4 6 6 6 3

6 6 6 6 6 4 0 0 0 0 0

0 5 6 6 5 0 0 0 0 0 0

0 0 0 4 0 0 0 0 0 0 0

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Table 2. EUV1 inspection results, 6 inspections, 400 nm lines/800 nm spaces.

Another mask, EUV2, was made by Motorola. The mask stack is 30 nm of Cr on 100 nm of silicon oxynitride (SiON) on 10 nm of Cr, on a Si/Mo reflective multilayer.

The buffer layer (SiON) has been etched. In Figure 8 we present images of a 400 nm L/S test pattern. In this image a 4X blow-up of five lines is shown. Dark interference fringes around the Cr absorber lines are evident.

We used TEMPEST, a software program that solves Maxwellâ&#x20AC;&#x2122;s equations for the case of monochromatic radiation incident upon a scattering structure in concert with aerial imaging software from Panoramic Technology5, to calculate and simulate the UV inspection image of EUV2. This simulated image is also shown in Figure 8, and we can see that the dark fringes are predicted by the simulation.

In the next section, these simulation tools are used to understand and optimize the absorber line edge profiles, leading to improved inspectability of the EUV masks.

Optimization of EUV masks

Work in the area of optimizing EUV mask inspectability has been done by Tejnil and Stivers 6. This work has focused on finding materials with good contrast. In

PA20 (200 x 500 nm)

F i g u re 7. EUV1 mask (Intel ), pro t rusion defect images .

4X Blowup

all. For the defects that were well resolved on the reticle, we were able to repeatedly detect defects in the 100 nm to 140 nm size range.

TEMPESTsimulation F i g u re 8. EUV2 (Motorola) images and TEMPEST simulation, 400 nm L/S.

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F i g u re 9. TEMPEST simulati ons of Ultraviolet High Resolution (UV HR) lin e profi les of absorbers of var ying reflectivity and width.

addition to the mean contrast of the absorber material being an important factor, the rapid phase ramp of the reflected light due to the mask topography significantly impacts the visibility of the absorber lines during inspection. In Figure 9 we show simulated images from a set of absorber lines of varying width. The lines have a width of, from left to right, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, and 500 nm. These simulations were run with the absorbing material being either Cr or titanium nitride, and with an absorber height of 50 nm, with no buffer layer under the absorber. The Cr has a reflectivity of 0.66 (in thick sections) and the TiN has a reflectivity of 0.22, both at a wavelength of 364 nm.

The multilayer reflectivity has been taken to be 0.50.

We can see how the edge interference changes the appearance of the line images. In the broad lines the Cr is brighter than the multilayer, as expected from their relative contrast. As the line narrows, the dark fringes merge and we undergo a contrast reversal. This makes it difficult to interpret the images. The *ave greater visibility than the Cr lines. Further simulations have been carried out which indicate that an absorber reflectivity of ten percent or less would be very desirable in terms of improving the line visibility. Conventional masks that use Cr with an antireflection coating can achieve such a low reflectivity. It is worth noting that it is somewhat easier to find candidate absorber materials with low reflectivity at DUV wavelengths than at UV wavelengths.

When the buffer layer is present, its thickness can be optimized for maximum line image contrast. In Figure 10 we show simulations of a constant width absorber 10

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line on varying heights of buffer layer, ranging from 40 nm to 80 nm in 10 nm steps. We see that the visibility of the line is a strong function of the height of the buffer layer. This effect is easily explained by the reflected light interference that happens when a small scatterer is positioned above a mirror that is normally illuminated. Interference minima occur when the scatterer is positioned at half-wave multiples above the mirror, and maxima are a quarter-wave away. The buffer layer height is thus an important variable that needs to be controlled to optimize line visibility.

Summary

In this paper we have demonstrated the feasibility of optical inspection of EUV and SCALPEL masks. We have imaged masks made by several sources and carried out limited inspections of the masks that had programmed defects. SCALPEL and EUV mask defects in the range of 100 to 140 nm were consistently detected.

In order to meet the stringent ITRS roadmap requirements for defect sizes that are 80 nm at the 100 nm node and 55 nm at the 70 nm node, we will extend this work to DUV (257 nm wavelength) inspection in the coming year. We will also optimize the defect detection algorithms specifically for EUV and SCALPEL reticles to further improve sensitivity.

We have supported NGL mask development with images and inspections from the research tool. A valuable collaboration has been established that provides rapid feedback to the mask developers based on the mask images and inspection results.

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F i g u re 10. T E M P E S T simulation of var ying b uf fer layer t hickness.

Using simulation tools and inspection images, we have found that EUV mask inspectability may be optimized.

Specifically, it is desirable that the mask absorber reflectivity at the inspection wavelength be reduced to approximately 10 percent. In concert with this, the mask buffer layer thickness can be optimized so that the absorber visibility is enhanced.

for work on the EUV1 mask and for helpful technical discussions. Finally, we would like to thank Yalin Xiong, Jacobus Koster, and Matt DiLorenzo of KLA-Tencor Corporation for their technical support of the research tool used in this work.

EPL stencil masks may not be inspectable optically.

The problem is that very little light is transmitted deep into the membrane cutouts, so that if a defect were to be 1 to 2 microns below the surface of the mask, it would not be visible in an optical inspection image. This should be regarded as a tentative conclusion since we have only inspected one such mask.

1 . J. A. Liddle, et al, “The SCALPEL Lithography System”, Japan. J. Appl. Phys., 34, 12B, 6663 (1995).

2 . Hans C. Pfeiff e r, “PREVAIL - IBM’s E-Beam Technology for Next Generation Lithography”, SPIE Vol. 3997, 206 (2000).

3. John E. Bjorkholm, “EUV Lithography - The Successor to Optical Lithography?”, Intel Technology Journal, Q3’98.

4. A l f red K. Wong, “Rigorous Three-dimensional Ti m e - D omain Finite-Difference Electromagnetic Simulation”, Ph.D. d i s s e r-tation, Engineering- Electrical Engineering and Computer Sciences, University of California at Berkeley, 1994.

5 . Panoramic Te c h n o l o g y, www. p a n o r a m i c t e c h . c o m .

6 . Edita Tejnil and Alan R. Stivers, Components Researc h , Intel Corporation, private communication.

At DUV wavelengths the SCALPEL membrane is nearly opaque. This poses a problem for inspection of backside SCALPEL defects. As mentioned earlier, vertical wall struts hamper inspecting SCALPEL masks in transmission from the frontside or in reflection from the backside. If front side optical inspection at UV or longer wavelengths proves not to be sensitive enough to detect printing backside defects, it may be necessary to develop another inspection technique for backside inspection.

Acknowledgements

The authors would like to thank Bing Lu and K. Smith of the Motorola *DigitalDNA™ Laboratories, and PSRL, Motorola Labs, Tempe AZ for making NGL masks used in this work. We would also like to thank Edita Tejnil, Components Research, Intel Corporation,

References

A version of this ar ticle was originally published in SPIE Pro c e e d i n g s 4186, pp. 250-258 (2001) entitled “UV Inspection of EUV and SCALPEL Reticles” by Donald W. Pettibone, Noah Bareket, KLA-Tencor Corporation; Ted Liang, Alan R. Stivers, Components Research, Intel Corporation; Scott D. Hector, P. J. S. Mangat, Motorola * DigitalDNA Labs.; D. J. Resnick, PSRL M o t o rola Labs.; Micheal Lercel, Mark Lawliss, Chris Magg, NGL M C o C , P h o t ronics/IBMN; Antho ny N ovembre, Reginald Farro w, Bell Labs, Lucent Te c h n o l o g i e s .

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Yield Management Seminar A valuable venue for innovative ideas KLA-Tencor’s Yield Management Seminars (YMS) focus on value-added, integrated process module control solutions for defect reduction, process parametric control and yield management.

Key topics include navigating the transition to the sub-0.13 µm technology node, with special emphasis on copper/low-κ interconnect, sub-wavelength lithography, and the 300 mm wafer.

To register online for the upcoming YMS, please visit us at: http://www.kla-tencor.com/seminar Date: Wednesday, October 17, 2001 Time: 10:00 am – 6:00 pm Location: Four Seasons Hotel, Austin, Texas

Call for future papers Papers should focus on using KLA-Tencor tools and solutions to enhance yield through increased productivity and performance. If you are interested in presenting a paper at one of our upcoming yield management seminars, please submit a one-page abstract to: Cathy Silva by fax at (408) 875-4144 or email at cathy.silva@kla-tencor.com.

YMS at a Glance

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October 17

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December 6

Makuhari, Japan

February 6

Seoul, Korea

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