Historical Review of Solid-State Drive

The concept for developing a solid-state storage drive to replace the magnetic drive comes from long time ago. It was in 1971 when Dr. Matsuoka joined Toshiba [48].

Since then, it has been over 30 years. The dream still has not become a solid truth till today. The main reason is the high bit cost. Even though the solid-state storage is more user-friendly than the rotating magnetic disk drive, the much cheaper bit cost and higher density of hard disk drive still dominated the storage device for the past decades in the computer industry.

The mass data storage device is one of the necessary I/O devices in the computer

systems. In the modern computer system architecture, CPU, memory and its peripherals compose the main frame of the computer systems. The mass data storage device is one of the necessary peripherals of the CPU. It stores the data required for system, such as the OS (Operating Systems) kernel, device drivers, and the AP (Application Programs) files. It also keeps the saved data by users. It provides a permanent, non-volatile memory for the data used in the computer system operation. If we make the analogy of a computer system to a human body, the storage device is like the memory function of our brain. Then, we can imagine how important of the data storage device is.

The data storage device development in the IT (Information Technology) industry is never stopping, since the first computer, Apple II, was introduced in the world. In the very initial stage, the magnetic tape drive was adopted for the data storage device for the computer. The tape drive cannot support random data access and the accessing speed is very slow. It could not keep the steps with the rapid progressing of CPU technology.

Then, the rotating-type disk drive structure was introduced to have the random data access capability by cross tracks seeking. There were many formats and types of FDD (Floppy Disk Drives) and HDD (Hard Disk Drives) used for the computer systems. As the IT industry entered into the multi-media era, the large capacity, removable, and low cost ODD (Optical Disk Drives) were adopted into the computer systems. They are CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RW, DVD-RAM, etc. In the most popular of current notebook computer, a 2.5’’ 160GB HDD and a DVD-RAM drive are the standard equipments of storage devices.

The non-volatile solid-state memory was invented as the micro-electronic process technology introduced. It uses the keeping and identifying the change of the non-volatile states in physical or electrical properties to store the data bit is ‘0’ or ‘1’.

The first type of the non-volatile solid-state memory is the EPROM (Erasable Programmable Read-Only-Memory). The EPROM can be electrically programmed, but it needs UV (Ultra-Violet) light exposure to do the erase [1]. The EPROM can be used

for the computer micro-code storage, but it cannot meet the requirement or electrically erasable of the mass data storage device. The EEPROM was invented to support the electrical erasure function. It might be the first invented non-volatile solid-state memory for mass data storage device, and the world first solid-state drive was made then, even though it is very low speed and very small capacity. Eventually, it was not a successful storage device due to the un-acceptable high cost. The block-wise simultaneous erasure mechanism was invented to improve the speed of the EEPROM, it called as flash memory named by Ariizumi in 1984 [48]. In 1987, Toshiba introduced the NAND flash memory structure with the smaller area of a single memory cell. It was the first light for the opportunity of the non-volatile solid-state memory can be used for the mass data storage device. The rapid progressing in process technology shrunk and the invention of MLC (Multi-Level Cell) technology pushes the bits density from 16Mbits in 1995 to 32Gbits in 2007. The density of NAND flash memory has got 2000X (11 folds) progress in the past 12 years. It nearly doubles the density per year [46].

The non-volatile solid-state memory is quite excellent as comparing with the inescapable mechanical movements of HDD and ODD. The advantages are: low power consumption, low heat dissipation, compact in size, shock free, anti-vibration, no acoustic noise, etc. The characteristics of the non-volatile solid-state memory are quite amazing, but the high bit cost limited its application field being very narrow in the past decades, like in the industrial equipments, automatic machines, or aero-space crafts.

However, through the 10 folded of the past 10 years progressing in NAND flash memory density, the bit cost of the NAND flash memory was at around US$5 for 1 Giga byte now. It looks a 32GB solid-state drive with the price less than US$200 is foreseeable now. It is expected to see the solid-state drive will start displacing HDD in notebook computers from next year, 2008.

The solid-state drive era is coming based on the minimum required capacity at an affordable cost. To achieve the good performance of the solid-state drive, an excellent

architecture of the controller chip for high-speed high-capacity solid-state drive is very important. Besides the cost and the capacity, the solid-state drive needs good quality, high performance, and high reliability as well. The rapid development of the NAND flash memory has led widespread applications for the NAND flash memory based storage devices. Such as the flash memory cards and UFD (USB Flash Disks), are widely used in the 3C (Computer, Communication and Consumer electronics) products.

The shipment of flash memory cards and UFDs in 2007 are anticipated to be 660 millions of units, and 125 millions of units, respectively (Source: Gartner, IDC). The flash memory cards are used for the storage media of DSC, cellular phone, MP3 player, etc. The UFD has replaced the floppy disks and become the indispensable removable storage media for the data transfer among PC and notebook computers. However, the application for solid-state drive is still not booming. The reason is caused not only by the not enough capacity but also the relatively high cost per Mega byte. But from this year, the situation might be changed. The emerging multi-media applications show a strong demand for higher density and lower cost, and drive the continuous process technology shrinking and compel the MLC (Multi-Level Cell) technology of flash memory. Samsung Semiconductor Co. demonstrated their 32-Gb NAND flash memory in ISSCC 2007. Meanwhile, the cost per GB (Giga Byte) of NAND flash memory was declined to US$5.0 in the first quarter of this year. The improvement of the non-volatile solid-state memory will keep going via the huge investment of the world’s top semiconductor companies (e.g., Intel, Samsung, Micron, Toshiba, Hynix, etc.).

Consequently, the nice-enough capacity and reasonable cost of NAND flash memory is convincing the solid-state drive era to be coming. On the other hand, the progressing on the density increasing and multi-level cell technology induced the unwanted degradation effects on NAND flash memory. For example, the higher bit error rate, degraded program and erase endurance, more severe disturbance and interference phenomena during reading, programming, and erasing, problems for long-time data retention, and the restricted data accessing speed in reading, programming, and erasing of a single flash memory device, etc. These deficiencies in the advanced high-density non-volatile

solid-state memory need a stronger controller to resolve the problems. The stronger controller with the effective circuit architecture and efficient managing algorithm is compellingly compulsory to ensure the reliability, performance, and lifetime of the high-speed and high-capacity solid-state drive.