Magazine autumn00 mask making in130nm

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Mask Making in the 130 nm Technology Node: an Approach to Defect Free Manufacturing Naoya Hayashi, Shiho Sasaki, Toshifumi Yokoyama, Dai Nippon Printing Co., Ltd.

Specifications for advanced photomasks are becoming more stringent as the industry shifts to smaller lithography nodes.

Among various requirements for photomasks, the need for precise control of critical dimensions (CDs) and reduction of defects are the hottest issues for photomask manufacturers.

In this article a unique photomask manufacturing method for precise CD control is described and an approach to defect-free manufacturing (DFM) is discussed. To cancel CD errors, measuring resist CD after development is adopted. A mean to target (MTT) ratio of less than ±15 nm is achieved using this method.

Introduction

As semiconductor lithography development accelerates, the requirements for photomasks become more and more stringent. This is especially true for mean to target (MTT), uniformity of the critical dimension (CD) and tolerated defect size. According to the latest ITRS roadmap shown in Table 1, the specifications for photomasks of 130 nmlithography node are: CD-MTT ±10 nm, CD uniformity (3σ) <13 nm and defect size <104 nm. These specifications are difficult to achieve with current manufacturing processes. In this study issues in the manufacturing of 130 nm-node photomasks were investigated, and a new processing strategy to achieve the tight specifications were successfully developed.

In our previous paper1, 2 we reported that the combination of a high acceleration voltage e-beam writer, a chemically amplified resist (CAR) and dry etching was able to yield much smaller features. Sufficient CD uniformity is also shown as a fruit of the process with a CAR, but it was recognized that the technique to control CD-MTT has

to be improved to evade the influences of instabilities in the process. To compensate for the errors in the CAR process the CD had to be controlled during the actual patterning process by feed forwarding.

In addition, to meet the tight specification for defect size, we have to introduce defect free manufacturing (DFM) technology to photomask manufacturing. Currently the most dominant cause of defects is particles of human origin. The use of an automated cluster process tool, where all the process tools are connected by a robot handler with each other, may reduce defects.3 ’99 ‘00 ‘01

‘02

180

130

MMIS

560

OPC

280

CD MTT (+/- nm)

ITRS ’99 Technology Node

CD Unif. (3σ, nm) DRAM

‘03 ‘04

‘05

‘06 ‘07 ‘08

100

70

360

260

180

180

130

90

14

10

8

6

24

13/ * 26

10/ * 20

14*

7/ * 14

10*

21

15

MPU

16

10/ * 20

Image Placement (nm)

39

27

Defect Size (nm)

144

104

80

55

152

155/ 200

155/ 200

Mask Size (mm) (square/diameter)

152

Source: The International Technology Roadmap for Semiconductors * Alternating PSM only

Table 1. Mask technology roadmap.

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This article proposes an approach to achieving precise CD-MTT control and also provides an example of how a DFM concept in photomask manufacturing could be realized.

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CD control

CD-MTT control is one of the key technologies to satisfy the specifications of the photomasks. There are several errors having impact on CD-MTT. Differences in sensitivity between blank lots are one cause of errors. It is possible to correct exposure doses for each blank lot by use of the data of a sensitivity check. Furthermore, the influences of post exposure delay (PED) and post coating delay (PCD) on CD-MTT also cause errors.

Attention should be given to the fact that stabilizing those errors alone is not enough. Therefore, we have to introduce an error-compensation concept in our process to reduce the influence of those errors.

The method is based on the concept of correcting the errors by feed-forwarding information on the resist CD to the descum step before the dry-etch process. The process flow is illustrated in Figure 1. The resist CD is measured after development and the descum compensation step is done. The CD error is canceled by changing the descum

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Descum Time/sec Figure 2. CD control using descum compensation.

time. An appropriate descum time is calculated on the basis of the difference between the resist CD and final CD. Figure 2 shows an experimental result for CD control using the descum compensation, where CD shift (defined as the difference between the resist CD and final or target CD) is plotted as a function of the descum time.

The CD shift is found to increase linearly with the descum time. To adopt the descum compensation, the descum condition was refined to achieve a high sideetch rate and less reduction of the resist thickness. In this way, if the resist CD can be measured precisely, it is possible to control the final CD to a high degree.

Figure 3 is a result for the CD controllability obtained in production using the compensation. The final CD errors are plotted for a series of production plates. The solid line shows actual CD data, whereas the dotted line shows CD data predicted if this method were not adopted. The CD controllability is found to be about

Descum Compensation 0.05 Simulation

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CD MTT Error (nm)

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Figure 1. Process flow of the compensation.

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Figure 3. CD controlability for MTT using the compensation.

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CD Deviation (nm): SiScan

CD Deviation (nm): SEM

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Approach to a DFM process

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Designed CD (µm)

Figure 4. CD controllability for MTT using the compensation and optimized dr y-etching condition.

±15 nm; this is, however, not enough for manufacturing 130 nm-node photomasks.

One of the reasons for this insufficient CD controllability is thought to be the dependence of the pattern densities for various photomasks. Therefore, further investigation of the CD shift on pattern density in dry etching is required.

Additionally, the accuracy of measurement of the resist CDs is speculated to have affected CD controllability.

In this production, the resist CDs were measured near the linearity limit of the measurement tool. Therefore, the linearity error was included in the previous measurement. This error needs to be reduced for meeting the specification of 130 nm-node and beyond.

Defect control is growing in importance as smaller geometries, tighter specifications and adoption of optical proximity correction patterns are required. Particles residing on the plate during the manufacturing process are thought to be the dominant cause of defects. In particular, particles found on alternating phase shift masks during their process easily yield shifter defects, which are now classified as killer defects because they are barely removed by any current repair technology.

Therefore, the development of a particle-free manufacturing process is essential.

Figure 5 illustrates the cluster system installed in our process line, which consists of an oven for post exposure bake for CARs, a developer, a stripper, a dry etcher, CD measurement tools, and a loader station. Each module is combined by a robot handler. In this system, an operator does not need to touch masks once they are placed them on the loader station.

The performance of the cluster tool was evaluated along with that of human handling. Human handling was performed as follows: An operator carries a mask in a carrying case from the oven to the developer, to the measurement tool, then to the dry-etcher (one cycle). In this one cycle the operator opens the case at the loader of each tool and puts the mask into the cassette in the loader by hand. When the process of the tool finishes, the mask is put back into the case. During the cycle the operator uses only one case. Four operators F, N, S, and T, carried the test for four consecutive cycles each. Among them, operators F and N were new to the job. Operator S also used a commercial mask handler (or a pick) instead of handling by her hands in the same procedure.

In order to reduce the errors described above, an advanced measurement tool is required. One of the candidates is a CD-SEM which has much better linearity to measure the resist CDs. Figure 4 shows the measurement linearity data of the current SiSCAN system and a KLA-Tencor reticle CD SEM, the 8100XP-R. The CD SEM system has better linearity than the SiSCAN. The limitation of the SiSCAN is 0.8 µm and its linearity worsens below 1.4 µm. The KLA-Tencor 8100XP-R keeps linearity down to 0.2 µm. In conclusion, the measurement with the SEM is advantageous for fine patterns less than 1.0 µm. Figure 5. Schematic representation of the fully automated cluster process tool.

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Added Particle/ Plate

0.25 – 0.3

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Opaque Defect/Plate @ 1st Inspection

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With Standalone Systems

9

With Cluster Tool

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Product Plate Figure 7. The effect of defect control with cluster tool.

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and feed-forwarding its deviation from the target to the descum time.

Operators and Methods Figure 6. Performance of the cluster tool compared with human handling for reducing the particle count.

Figure 6 shows the results of this evaluation, where the performance of the cluster tool is compared with that of human handling. Here the count of particles (with classification in size) for each operator represents a sum of the counts of the particles detected on the four plates handled by the operator. The cluster tool yielded one or zero particles. In the cases of human handling (both by hands and pick), the particle counts are found to correlate with the amount of experience an operator had; that is, the inexperienced operators splashed many particles on the plates.

Figure 7 shows defect control with the cluster tool. Here, the count of opaque killer defects is shown for 125 commercial plates of 250-150 nm nodes. Even after adopting the cluster tool, opaque killer defects were still detected on some plates. Although the origins of all killer defects, could not always be identified, it was proven that the killer defects occurring in the EB delineation process are successfully reduced by adopting the cluster tool.

Conclusions

We have successfully developed a CD-MTT control method with compensation. This compensation is conducted by measuring the post-development resist CD

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Several CD metrology tools were compared to select the most suitable one for compensation step. A KLATencor CD SEM tool demonstrated excellent capability for measuring the post-development resist CD, suitable for 130 nm-node photomasks.

Furthermore, a fully automated cluster tool which consists of the process tools combined by a robot handler demonstrated a possibility of defect free manufacturing.

Acknowledgements

The authors would like to acknowledge Daisuke Totsukawa, Hiro-o Nakagawa, Shigekazu Fujimoto and Hiroshi Mohri of Dai Nippon Printing for technical support, helpful discussions, and contributions to the experiment. Special thanks to Masaru Endo of Dai Nippon Printing Fine Electronics for his technical support in developing the new controlling method.

Reference 1. M. Kurihara et al., “Performance of a chemically amplified positive resist for next generation photomask fabrication,” SPIE Vol.3412, 279(1988) 2. T. Abe et al., “Comparison of etching methods for sub-quarter micron rule mask fabrication,” SPIE Vol.3412, 163(1998) 3. A. Oelmann et al., “Results from the first fully automated PBS-maskprocess and pelliclization,” SPIE Vol.2087, 57(1993)