Programming and Erasing Flash Memory

Devices Using the Keithley S530 Pulse

Generator Option

Introduction and Background

Normally, in parametric test, the instrument used most is the

Source Measurement Unit (SMU). The SMU allows supplying

a DC voltage or current to the device under test (DUT) and

simultaneously measuring the resultant voltage or current.

However, there are some cases where it’s necessary to apply a

A pulse generator allows outputting a voltage in a time-

controlled, time-accurate manner, including control over the

amount of voltage (pulse height), the duration of the pulse

(pulse width), as well as the voltage ramp rate (rise and fall

time). This type of instrument also provides the ability to control

the number of pulses that are output and even to synchronize

multiple pulses.

The Keithley S530 Parametric Test System offers a pulse

generator option that offers two to six channels of pulse outputs,

each of which is capable of outputting a maximum of ±40 VDC

characterize flash memory cells.

Flash Memory Basics

Flash memory is currently the dominant form of solid-state, non-

volatile memory technology. It is used in a wide range of devices

and applications—everything from the common USB “thumb

drive” to smartphones, MP3 players, and digital cameras.

Flash memory is part of a class of MOS devices that use

floating gates. There are two types of flash cells: NOR and NAND.

In NOR technology, the storage cells can be programmed and

erased individually. Unfortunately, the storage densities for this

(

Figure 1

) usually have a control gate, drain, source, and bulk.

The memory cell is set (programmed) and reset (erased) by

applying or removing charge from the floating gate. Charge can

be applied or removed from the floating gate of any type of flash

memory cell via Fowler-Nordheim (FN) current tunneling or via

Hot Carrier Injection (HCI). In a normal CMOS transistor, both

of these mechanisms cause device degradation and are usually to

be avoided, but they are beneficial for flash memory. Moreover,

although FN tunneling and HCI are useful for programming and

When charge is applied to or removed from the floating gate,

the threshold voltage (V

T

(

Figure 2

). This threshold voltage change is what allows the flash

would cause the cell to be over-programmed or over-erased.

Once a cell is placed into one of these states, it cannot be set

to the opposite state, usually because the gate oxide has been

damaged in some way. The stimulus voltage must be applied

in a time-controlled manner, which is why a pulse generator

is required.

T

pulses are required to place the cell in each of its possible states,

which results in from four to eight possible V

Figure 4

To program the cell using FN tunneling, a positive pulse is

applied to the gate, while the drain, source, and bulk voltages are

set to 0V (or grounded). This causes charge to be pushed into the

floating gate. To erase the cell via FN tunneling, a negative pulse

is applied to the gate (with the drain, source, and bulk terminals

set to 0V or connected to ground).

Usually, the threshold voltage is measured afterward to

ensure that the cell has indeed been programmed or erased. If

one programs and erases the cells thousands of time, one can

monitor its lifetime. (For the sake of simplicity, this note focuses

only on single-bit flash memory cells.)

A parametric test system is oriented primarily toward performing

accurate DC measurements. Therefore, the switch matrix and

relays typically used are designed and optimized to ensure good

DC performance, such as low leakage current, minimal offset

In contrast, pulses are essentially AC signals. A square pulse