Haruji Ishihara. FLASH MEMORY STRUCTURES AND FEATURES Flash memory is classified into two types by

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1 High-Density Flash Hitachi Memory Review and Vol. Flash 47 (1998), Memory No. Card High-Density Flash Memory and Flash Memory Card Haruji Ishihara ABSTRACT: Flash memory is becoming a key component for mobile computing and counication systems because of its nonvolatility, field prograability, high density, and low cost. It is used as working memory in applications such as cellular phones. It is also used as file memory in applications such as flash cards. The flash card is a medium for transferring data between two mobile PCs, or between a mobile PC and peripheral equipment such as a digital still camera. Various types of data are stored in flash cards; not only text data but also still picture, voice, and full-motion video. These data require high-density storage media. Based on these needs, Hitachi has developed high-density flash memory with Hitachi s unique AND cell structure and applied it to CompactFlash * as small formfactor flash card. The AND cell 64-Mbit flash memory and 45-Mbyte CompactFlas are available now. INTRODUCTION FLASH memory had been expected to replace dynamic random access memory (DRAM) and hard disk drive (HDD) when it was introduced. The application of flash memory is gradually expanding even though the flash memory capacity is still less than that of DRAMs and far from that of HDDs. Recently Hitachi introduced 45-Mbyte CompactFlah and 150-Mbyte PC-ATA cards that incorporate Hitachi 64-Mbit flash memory. These high-density flash cards will replace low-density HDDs in industrial applications. FLASH MEMORY STRUCTURES AND FEATURES Flash memory is classified into two types by application: working memory and file memory, as shown in Fig. 1. Data in working memory is directly executed by a processor CPU, including code stored in a cellular phone. Therefore working memory must have random access capability. Data in file memory cannot be directly executed by a CPU. The data in file memory must be transferred from file memory to main memory for execution. This means that file memory does not require random-access capability, and serial access may be sufficient. Typical types of working memory are NOR- and DINOR-cell flash memory that feature random-access capability. Typical types of file memory are NANDand AND-cell flash memory that feature high density and low cost. Application System Optimum flash Working memory Direct execution by CPU as ordinary memory PC BIOS Cellular phone Digital still camera GPS NOR DINOR GPS: global positioning system BIOS: basic I/O system HPC: handheld PC Flash memory Execution by CPU after transfering data to RAM as HDD File memory Flash card for digital camera and HPC silicon disk NAND AND Fig. 1 Flash Memory Applications. Flash memory is classified into two types by application: working memory and file memory. * CompactFlash is a trademark of SanDisk Corporation and is licensed royalty-free to the CFA (CompactFlash Association) which in turn will license it royalty-free to CFA members.

2 Hitachi Review Vol. 47 (1998), No W0 W1 Wm NOR type AND type NAND type Minimum erase unit Minimum erase unit D1 D2 Dn D1 D2 Dn Minimum erase unit D1 D2 Dn W0 Select W1 W0 W1 S Wm Select S Wm Select S Metal layer Contact hole Floating gate Word line (Control gate) Embedded diffusion layer Floating gate Word line (Control gate) Fig. 2 Flash Memory Cell Structure. AND-type flash memory cells eliminate contact hole so that die size can be reduced. The cell structure of flash memory is shown in Fig. 2. The drain and source of all memory cells in NOR and DINOR flash memory are connected to data lines to provide random-access capability. The space for connection between data lines and the drains and sources of each memory cell increase the die size of flash memory. However, the drains and sources of all memory cells are not connected to data lines in AND and NAND flash memory. A drain and source of an entire block of memory cells are connected to a data line. This means that the area for connection between data lines and drains and sources of memory cells is negligibly small, and smaller die size can be realized than for NOR and DINOR flash memory. However only serial access is available because of this cell structure. HITACHI AND FLASH MEMORY CELLS Hitachi developed the 64-Mbit flash memory HN29W6411 based on the AND cell structure for file memory applications. One of the most important factors for file memory is cost. Two new technologies are used to reduce cost. One is the AND cell structure mentioned above. The other is mostly-good memory technology. Generally speaking, memory ICs are not always perfect and they have defect bit(s). Memory Block 1 Block 2 Block 3 Block 2046 Block 2047 Block 2048 Data area 512 byte Control area 16 byte Sector 1 Sector 2 Sector 3 Sector 4 Sector 5 Sector 6 Sector 7 Sector 8 Fig. 3 Flash Memory Cell Array. Write and erase unit of 64-Mbit AND flash memory is sector, which has 512-byte data area and 16-byte control area. ICs that have defect bits cannot be used and discarding them increases costs. The mostly-good memory technology is used for solving this problem. The HN29W6411 memory array is shown in Fig. 3. The minimum-size unit of a memory array is a sector that has a 512-byte data area and a 16-byte control area. A flag is raised in the control-

3 High-Density Flash Memory and Flash Memory Card 150 TABLE Mbit Flash Memory Features and Specifications The 64-Mbit AND flash memory has sector erase function, and provides fast and constant speed. (B /MO) 80 Item Memory cell structure Power supply voltage Random access time Serial access time Write unit Specification Pseudo contactless 2 switch gates/128 cells 5-V and 3.3-V single 5 µs 50 ns 512 byte 80 Penetration to consumer electronics Moving picture 60 Audio HPC/PDA Dgital camera Write time Erase unit Erase time Write transfer rate Block erase max. min. Sector erase 1 ms typ./512 byte 512 byte 1 ms 443 kbyte/s (4-kbyte erase) 57 kbyte/s (4-kbyte erase) 256 kbyte/s (512-byte erase) Demand forecast by application Fig. 4 Flash Card Demand Forecast. Flash card market is growing with demand for use in digital cameras, HPCs and PDAs (personal digital assistance). Audio and full-motion video applications are good candidates for further growth. byte area for every sector that is a perfect sector. The defective sectors that do not have a raised flag in the 16-byte control area are replaced by a good sector. Based on this technology, mostly-good memory that has defect bits can be used. The HN29W6411 specifies mostly-good memory as a memory that has 98% or more good sectors out of 16,384 sectors. The write operation of HN29W6411 is done in sector units. The erase operation of HN29W6411 can be done by either sector unit, or by a block unit that has 8 sectors. The sector erase function makes the flash memory controller simple because the erase unit is the same unit as the write unit. The rewrite speed with sector erase is faster than with block erase when the number of rewrite sectors is 4 or less. It is a constant 256 kbyte/s. The rewrite speed with block erase is faster than sector erase when the number of rewrite sectors is 5 or more. Thus rewrite speed depends on the number of rewrite sectors. The peak speed is 443 kbyte/s. The serial read speed is very fast. The burst access time is 50 ns even though the first byte access time is 5 µs. Thus the average read speed is 17 Mbyte/s. The HN29W6411 can operate at either 3.3 V or 5 V. This dual-voltage operation capability provides a flexibility of system design and compatibility between 3.3-V systems and 5-V systems. Table 1 shows the 64-Mbit AND flash specifications and features. TOTAL FILE STORAGE SOLUTION Hitachi provides a total file storage solution that includes not only flash memory but also the flash memory controller and flash card. The total solution provides three benefits: (1) The latest flash memory can be used in a timely fashion because the controller for the flash memory is available at the same time that the flash memory is introduced. (2) The optimum controller is provided because the flash memory supplier knows the characteristics of the flash memory better than others. (3) The knowhow that has been obtained through flash card development and business can be utilized for developing better flash memory for file storage. FLASH CARD MARKET The demand forecast for flash cards is shown in Fig. 4. The present main applications are digital still cameras and handheld PCs. The flash card for a digital still camera stores a digital image of a photo. The data size of a digital photo image depends on resolution of the charge-coupled device (CCD) image sensor in the camera, color depth, and the compression ratio of the

4 Hitachi Review Vol. 47 (1998), No Fig. 5 CCD Resolution vs. Flash Card Capacity. Megapixel cameras need higher capacity flash cards such as 8 M, 15 M, 30 M, 45 Mbyte. CCD pixcel number 2 M 1 M 0.25 M 0.35 M 1.4 M 0.8 M 2 M Bundle card 15 Mbyte 8 Mbyte 2/4 Mbyte Option card 30/45 Mbyte 15/30 Mbyte 8/15 Mbyte Compatible model Kodak Konica Casio Canon NEC DC210 DC50 QM100 QM3501 QV700 PS350 PS600 Picona Mbyte Mbyte Compatible model Flash card Mbyte Mbyte Main memory Casio HP IBM NEC Sharp PSION CASIOPEIA 200LX 300LX 320LX PalmTop100 MobilePro400 MobileGear (MC-CS12) Power Zaurus Series-5 Fig. 6 HPC and PDA Memory Trend. Flash card capacity for HPCs and PDAs is increasing based on the need for high-density main memory. TABLE 2. Small Form-Factor Flash Card CompactFlash provides better compatibility and high-density capability. Item Compact flash Miniature card Smart media Outline thickness 3.5- thickness 0.8- thickness Interface PC-ATA standard True-IDE standard Flash memory depended interface NAND flash memory interface Compatibility Ensured controller which supports industrial standard Depends on type of flash memory and density May depend on type of flash memory and density High density Memories and controllers can be mounted as long as space is permitted. Limited by flash component density Limited by flash component density digital image. The CCD sensor resolution was 250-k pixels in 1996, then it increased to between 350-k and 1.4-M pixels in 1997, and it is expected to reach to 2- M pixels in The capacity of flash cards bundled with cameras in 1996 was 2 M to 4 Mbyte and option card capacities were 8 M to 15 Mbyte, because 50 k 100 kbyte is needed for each photo from a 250-kpixel CCD sensor. In 1997 the bundled card capacity was increased to 8 Mbyte and the option card capacities became 15 M to 30 Mbyte because 200 k 400 kbyte is needed for a 1-Mpixel CCD sensor. Thus in 1998 the bundled card

5 High-Density Flash Memory and Flash Memory Card 152 Relative cost/mbyte chip controller Controller technology 2-chip controller 1-chip controller 64-Mbit flash 64-Mbit A-mask 84-Mbit flash New controller 256-Mbit flash Fig. 7 Cost Reduction by New Technologies. Flash card cost will be continually reduced by new controller technologies as well as flash technologies. capacity is expected increase to 15 Mbyte and the option card capacities increase to 30 M to 45 Mbyte in 1998 because 400 k -800 kbyte are needed for a 2- Mpixel CCD sensor. Fig. 5 shows a chart of flash card capacity as a function of CCD sensor resolution. Other leading applications are handheld PCs and PDAs. These portable PCs do not have HDDs or FDDs due to space limitations, power consumption limitations, and mechanical shock considerations. User data and application programs are stored in DRAM main memory with battery backup. It is not reassuring for the user that user data is retained by battery backup because reliability is problematic. Also, the computer speed slows down when the user data and application programs occupy more than certain amount of main memory. For these reasons the user wants to use flash cards as mass storage for user data and application programs. In general, the user needs flash cards that have twice the capacity of main memory. Fig. 6 shows the trends of main memory capacity and flash card capacity. In 1998, 15 M- to 45-Mbyte flash cards will be needed. The flash card market will be enlarged by audiorelated applications such as voice recorders, cellular phones, pagers, and solid-state audio players as shown in Fig. 4. Full-motion video applications are also good candidates for future flash card business. HITACHI FLASH CARD The Hitachi flash card family has two form factors: PC-ATA (AT attachment) card and CompactFlash. The PC-ATA card was standardized by PCMCIA (Personal Computer Memory Card International Association) and JEIDA (Japan Electronic Industry Development Association), and all suppliers follow the standard. However the small form-factor flash card market segment is a different story. There are three major small form-factor flash cards on the market. They are CompactFlash, Miniature Card, and SmartMedia as shown in Table 2. Hitachi selected CompactFlash for its small form factor flash card for the following reasons. (1) Compatibility with PC related systems. (2) Compatibility with PC-ATA card. (3) Independence of flash memory variations. CompactFlash maintains compatibility with an internal intelligent controller. This controller buffers the flash memory variations and maintains the CompactFlash specification. But a disadvantage is the added cost of the controller. On the other hand, the Miniature Card and SmartMedia do not have an internal intelligent controller. Therefore, it is difficult to maintain compatibility among systems and the interface depends on the flash variation. But lower cost is expected than for CompactFlash. Hitachi thinks that compatibility is the most important factor for general users even with added cost. Cost reduction is one of the keys to expanding flash card applications and markets. Hitachi is endeavoring to continuously reduce costs of controller technology and flash memory technology, as shown in Fig.7. When Hitachi introduced the first-generation flash card in 1996, the controller consisted of three chips: a microcontroller, gate array, and 4-Mbit DRAM. Then the number of controller chips was reduced to two in the 2nd-generation card by eliminating the DRAM Flash memory 2-layer TCP technology Fig. 8 Double Density Packaging Technology. Hitachi TCP technology facilitates doubling CompactFlash card density up to 45 Mbyte. TQFP package controller 2-layer TCP technology flash memory TQFP: thin quad flat package TCP: tape carrier package

6 ATA 1st generation 75 MB 3rd 2nd generation generation 45 MB CF 1st 30 MB 30 MB generation Hitachi Review Vol. 47 (1998), No MB 75 MB 4th generation 600 MB 150 MB 75 MB 4th generation 180 MB 45 MB 30 MB 20 MB 10 MB Componet 64 Mbit 84 Mbit 256 Mbit Fig. 9 Hitachi Flash Card. Fig. 10 Hitachi Flash Card Road Map. It is expected that 180-Mbyte CompactFlash and 600-Mbyte PC-ATA card will become available with 256-Mbit flash memory. chip in The 3rd-generation card was introduced with a 1-chip controller by using micro CBIC technology in November Flash memory chips now dominate the cost of flash cards, especially for high capacity cards. The 2ndgeneration 64-Mbit flash memory is under development to reduce the die size and enhance performance. Then 84-M and 256-Mbit flash memory chips will be introduced for further cost reduction in the 2nd half of Based on digital still camera and handheld PC trends, Hitachi is focusing on high-capacity flash cards. Two key technologies are used to implement high-capacity flash cards. One is high-density flash memory chips as mentioned above. The other is packaging technology. Fig. 8 shows the cross section of CompactFlash. There are 4 positions for mounting controller and flash memory chips. The 1-chip controller occupies one position and flash memory chips occupy 3 positions. A 24-Mbyte card is the maximum capacity when conventional packaging technology is used with 64-Mbit flash memory. However tape carrier packages facilitate stacking two 64-Mbit flash memory chips at one position so that six 64-Mbit flash memory chips can be mounted. Based on this technology, Hitachi introduced a 45- Mbyte CompactFlash and a 150-Mbyte PC-ATA card as shown in Fig. 9. The further expansion of flash card capacity is shown in Fig. 10. It is expected that a 180- Mbyte CompactFlash and a 600-Mbyte PC-ATA card will be available with 256-Mbit flash memory in the 2nd half of CONCLUSIONS High-density flash memory and flash cards will be popularly used not only for mobile computing and counication systems but also for industrial applications such as file storage because of advantages such as high portability, low power consumption, high reliability and reasonable cost. Hitachi will continue to focus on high-density flash technology for leadingedge file storage applications. ABOUT THE AUTHOR Haruji Ishihara Joined Hitachi, Ltd. in Belongs to the Memory Product Marketing Dept. at the Semiconductor & Integrated Circuits Div. Currently working on product marketing for flash memory and flash card. ishiharh@cm.musashi.hitachi.co.jp