Hitachi
Review
47 (1998),
4
High-Density Flash
Memory
andVol.
Flash
MemoryNo.
Card
148
High-Density Flash Memory and Flash Memory Card
Haruji Ishihara
ABSTRACT: Flash memory is becoming a key component for mobile
computing and communication systems because of its nonvolatility, field
programmability, 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
PC BIOS
Cellular phone
Digital still camera
GPS
NOR
DINOR
Direct execution by CPU
as ordinary memory
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
* CompactFlash™ is a trademark of SanDisk Corporation and is licensed
royalty-free to the CFA (CompactFlash™ Association) which in turn will
NAND
AND
Fig. 1—Flash Memory Applications.
Flash memory is classified into two
types by application: working memory
and file memory.
license it royalty-free to CFA members.
Hitachi Review Vol. 47 (1998), No. 4
NOR type
W0
AND type
Minimum erase unit
D1 D2 ... Dn
W0
149
NAND type
Minimum erase unit
D1 D2 ... Dn
Minimum erase unit
D1 D2 ... Dn
Select
W0
W1
W1
W1
Wm
Wm
S
Wm
Select
S
Select
S
Metal layer
Contact hole
Floating gate
Floating gate
Word line
(Control gate)
Embedded
diffusion layer
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
Data area
512 byte
Block 1
Block 2
Block 3
Control
area
16 byte
Sector 1
Sector 2
Sector 3
Sector 4
Sector 5
Sector 6
Sector 7
Sector 8
Block 2046
Block 2047
Block 2048
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-
150
High-Density Flash Memory and Flash Memory Card
TABLE 1. 64-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
80
Penetration to consumer electronics
Item
Specification
Memory cell structure
Pseudo contactless
2 switch gates/128 cells
Power supply voltage
5-V and 3.3-V single
Random access time
5 µs
Serial access time
50 ns
Write unit
512 byte
Write time
1 ms typ./512 byte
Erase unit
512 byte
Erase time
1 ms
Moving picture
55
60
Audio
40
36
HPC/PDA
25
20
Dgital camera
12
2
3.5
6.3
’96
’97
’98
0
Write
transfer
rate
Block
erase
max.
443 kbyte/s (4-kbyte erase)
min.
57 kbyte/s (4-kbyte erase)
Sector erase
256 kbyte/s (512-byte erase)
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
’99
’00
’01
’02
’03
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.
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
Hitachi Review Vol. 47 (1998), No. 4
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
2M
Bundle
card
Option
card
15 Mbyte
30/45 Mbyte
Compatible model
0.8 M
–
1M
• Kodak
• Konica
2M
8 Mbyte
15/30 Mbyte
• Casio
• Canon
–
0.35 M
• NEC
1.4 M
Mbyte
60
’96
’97
’98
Compatible model
30–60 Mbyte
Flash card
• Casio
• HP
15–45 Mbyte
45
15–30 Mbyte
• IBM
• NEC
30
15
Main memory
0
’97
’98
DC210
DC50
QM100
QM3501
QV700
PS350
PS600
Picona
2/4 Mbyte 8/15 Mbyte
0.25 M
0
151
• Sharp
• PSION
CASIOPEIA
200LX
300LX
320LX
PalmTop100
MobilePro400
MobileGear (MC-CS12)
Power Zaurus
Series-5
’99
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
36.4
mm
33.0
mm
42.8
mm
Outline
3.3-mm thickness
Interface
Compatibility
High
density
Smart media
45.0
mm
38.0
mm
3.5-mm thickness
37.0
mm
0.8-mm thickness
PC-ATA standard
True-IDE standard
Flash memory
depended interface
NAND flash memory
interface
Ensured controller which
supports industrial standard
Depends on type of
flash memory and
density
May depend on type of
flash memory and
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 2M pixels in 1998.
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
High-Density Flash Memory and Flash Memory Card
3-chip controller
Controller technology
Relative cost/Mbyte
2-chip controller
152
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.
1-chip controller
1.0
New controller
64-Mbit flash
64-Mbit A-mask
0.1
84-Mbit flash
256-Mbit flash
1995
1996
1997
1998
1999
2000
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 2Mpixel 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
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
Fig. 8—Double Density Packaging
Technology.
Hitachi TCP technology facilitates
doubling CompactFlash card density
up to 45 Mbyte.
Flash memory
2-layer TCP technology
TQFP package
controller
2-layer TCP technology
flash memory
TQFP: thin quad flat package
TCP: tape carrier package
Hitachi Review Vol. 47 (1998), No. 4
153
4th generation
...
600 MB
...
150 MB
...
150 MB
75 MB
...
75 MB
...
1st
generation
75 MB
...
ATA
15 MB
15 MB
8 MB
15 MB
8 MB
4th generation
...
1996
Fig. 9—Hitachi Flash Card.
chip in 1997. The 3rd-generation card was introduced
with a 1-chip controller by using micro CBIC
technology in November 1997.
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
1998.
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 45Mbyte CompactFlash and a 150-Mbyte PC-ATA card
as shown in Fig. 9. The further expansion of flash card
8 MB
1997
...
8 MB
45 MB
30 MB
20 MB
15 MB
10 MB
8 MB
1998
1999
30 MB
...
...
1st
30 MB
generation
15 MB 15 MB
64 Mbit
84 Mbit
256 Mbit
180 MB
15 MB
...
CF
3rd
2nd generation
generation 45 MB
Componet
2000
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.
capacity is shown in Fig. 10. It is expected that a 180Mbyte CompactFlash and a 600-Mbyte PC-ATA card
will be available with 256-Mbit flash memory in the
2nd half of 1998.
CONCLUSIONS
High-density flash memory and flash cards will be
popularly used not only for mobile computing and
communication 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 1972. Belongs to the Memory
Product Marketing Dept. at the Semiconductor &
Integrated Circuits Div. Currently working on product
marketing for flash memory and flash card.
E-mail: [email protected]