TECHNICAL FIELD

The present disclosure relates to the field of memory, in particular, to a method for programming flash memory.

BACKGROUND

Non-volatile memories (NVMs) are applied widely in semiconductor devices. A characteristic of an NVM is that it can save stored data even after it is powered off. The earliest NVM is erasable programmable read-only memory (EPROM). EPROM uses hot carrier injection (HCI) for programming and uses ultraviolet (UV) light for erasure. However, such devices require quartz glass for UV light erasure, resulting in high costs.

To reduce manufacturing costs, electrically erasable programmable read-only memory (EEPROM) that adopts the Fowler-Nordheim (FN) Tunneling effect to perform electrical erasure was invented. In EEPROM, when electrons are injected and stored in a floating gate, it indicates that information “0” is stored; when electrons are erased from the floating gate, it indicates that information “1” is stored. EEPROM requires lower costs than EPROM, but EEPROM has a disadvantage that its programming and erasure need to be performed byte by byte, resulting in a slow speed.

To increase the read speed, the circuit design of EEPROM devices was further improved, so that programming and erasure can be performed on multiple memory cells at the same time. This kind of improved EEPROM is what now commonly known as the Flash EEPROM (or Flash for short).

At present, flash memory has been widely utilized in various kinds of storage mediums (for example, smart cards and storage cards) and is an important semiconductor component. The most classic flash memory is EEPROM with tunnel oxide (ETOX), which uses HCI for programming.

Because of the physical mechanism of HCI, generated hot electrons are injected based on the Lucky Electron model, whose programming efficiency is relatively low (about 50% in general). Therefore, a higher voltage is needed to improve the programming efficiency, which however leads to high power consumption of the corresponding flash memory device. In addition, because the Punch Through effect is inherent in the Hot Electron effect, when the gate length of the flash memory is less than 110 nanometers, the Punch Through effect is likely to occur, restricting further miniaturization of the device.

To overcome the above-mentioned shortcomings, NXP Semiconductors N.V. invented a 2 T flash memory which is programmed based on the FN Tunneling effect. Hsu et al. invented a P-type channel flash memory, which adopts band-to-band tunneling (BTBT) for programming. However, because these devices mainly take advantage of quantum tunneling, which is physical, they have a relatively low read current for programming, and these programming methods may inflict considerable damage on the tunnel oxide layers of the devices, thereby posing a challenge to the reliability of these flash memory devices.

Thus, to come up with a method for programming flash memory that has a high programming efficiency, low power consumption, and large read current for programming, one that does not hinder further miniaturization of the device, has becomes an urgent problem facing those skilled in the art.

SUMMARY

In view of the shortcomings in the prior art, the present disclosure provides a method for programming flash memory, in order to resolve problems, for example, that the programming methods in the prior art have low programming efficiency, high power consumption, and a small read current for programming, that the prior art methods prevent further miniaturization of the device, and that they inflict severe damage on the device's tunnel oxide layer.

Thus, the present disclosure provides a method for programming a flash memory, which includes:

S1) providing a flash memory structure with a floating gate, and floating a source of the flash memory structure;

S2) separately applying voltages to a drain and a substrate of the flash memory structure, to form an electric field, generating electron-hole pairs, generating primary electrons, with the voltage applied to the substrate less than the voltage applied to the drain;

S3) accelerating holes downward under the action of the electric field to collide with the substrate in the flash memory structure within a preset time, to generate secondary electrons; and

S4) separately applying voltages to a gate and the substrate of the flash memory structure, with the voltage applied to the substrate less than the voltage applied to the gate, and enabling the secondary electrons to generate tertiary electrons under the action of a vertical electric field to inject the tertiary electrons into the floating gate of the flash memory structure, to complete a programming operation.

Optionally, the flash memory structure comprises an ETOX structure, a 1.5 T structure, a 2 T structure, or an EEPROM structure.

Optionally, the difference between the voltage applied to the drain and the voltage applied to the substrate in operation S2) is not less than 4 V.

Optionally, the preset time in operation S3) is from 10 ns to 100 ns.

Optionally, the difference between the voltage applied to the gate and the voltage applied to the substrate in operation S4) is not less than 7 V.

Optionally, before operation S1), the method further comprises: pre-erasing the flash memory structure to eliminate residual charges in the floating gate.

Optionally, the method of the pre-erasing further comprises: separately applying voltages to the gate and the substrate of the flash memory structure, with the voltage applied to the substrate higher than the voltage applied to the gate.

Optionally, the difference between the voltage applied to the gate and the voltage applied to the substrate is not less than 10 V.

As discussed above, the method for programming a flash memory according to the present disclosure has the following beneficial effects:

In the method for programming flash memory according to the present disclosure, by improving and optimizing the operations of programming voltage, tertiary electron excitation is triggered to perform programming, so that the programming efficiency of the flash memory can be improved, power consumption can be reduced, the read current for programming can be increased, and the device's tunnel oxide layer will only be subject to minor damage. In addition, the Punch Through effect can be prevented, thereby facilitating further miniaturization of the flash memory.

