With the resolution enhancement techniques such as OPC (Optical Proximity Correction) and SRAF (Sub-Resolution
Assist Features), the size of layout data have grown significantly. It is quite common now to find layout files that are tens
of GBs in size. Unlike GDSII which can store data hierarchically, mask data formats such as MEBES are essentially flat
and more voluminous. Moreover, polygonal data present in layout data files is fractured, thereby increasing the data
volume before getting stored in MEBES data format. This results in huge MEBES files. As per the ITRS roadmap of
2005, for a 45nm half-pitch node that is expected to be in use by 2010, the mask data volume for a single layer is
expected to reach up to 825 GB. Storing and transferring such large mask data are issues for which the mask industry
needs solutions.
Historically, MEBES is the most prevalent EB format in the industry. Moreover, in many Mask Data Preparation (MDP)
flows, the MEBES format is being used as the de-facto standard for specifying the fractured EB data even though the
final target EB machine might be different. In this paper we present techniques for lossless reversible compression of
MEBES data, i.e., when the compressed file is decompressed, the generated uncompressed file matches the original
MEBES file bit- by-bit. By applying these compression techniques a compression ratio of 5X to 15X can be obtained.
In practice, compressing MEBES files is usually a one-time task, but decompression of compressed files is expected to
be done multiple times as every time a compressed MEBES file needs processing, it has to be decompressed. MEBES is
essentially an efficient data format and the geometries are stored compactly. As a result the compression/decompression
techniques described in this paper are quite computation intensive in order to achieve higher compression ratio. This in
turn leads to higher CPU time for compression/decompression compared to generic compressors such as gzip. However,
as the format-specific compressors produce higher compression ratios, the disk I/O time for compression and
decompression is expected to be less compared to the generic compressors such as gzip and gunzip. In spite of this, the
format-specific decompressor is usually 3-5X times slower than the generic decompressor such as gunzip. Since
decompression is expected to be done more frequently as compared to compression, speeding up decompression is
highly desirable. We present a compression technique for MEBES data which enables multiple process threads to
decompress the compressed MEBES data. With the multi-core multiprocessor machines becoming quite inexpensive and
common, the multi-threaded decompression is expected to perform close to the disk I/O time on such machines.
The paper details out the techniques, experimental results in terms of comparative compression ratios, compression and
decompression speed using single threads and multiple threads. The possibility of getting higher compression ratios as
well as higher or comparable decompression speed makes it more practical to use format specific reversible compression
schemes rather than using generic compressors. Even though, the paper focuses on compression and decompression of
MEBES, it can be easily extended to the compression of GDSII [1].
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