JPH02260461A - Thin-film memory element - Google Patents

Abstract

PURPOSE:To stabilize a readout operation by a method wherein a first gate insulating film is formed of a dielectric film and of an insulating film which has a charge storage function, a second gate insulating film is formed of an insulating film which does not have the charge storage function and a first gate electrode and a second gate electrode are used respectively for write/erasure use and for readout use. CONSTITUTION:A first gate insulating film 12 having a charge storage function is formed as a two-layer film where a dielectric film 12a is formed on the side of a first gate electrode G1 and an insulating film 12b having the charge storage function is formed on the side of a semiconductor layer 13. A second gate electrode D2 facing the semiconductor layer 13 via a second gate insulating film 15 which does not have the charge storage function is formed in a thin-film transistor which is composed of the first gate electrode G1, the first gate insulating film 12, a semiconductor layer 14, a source electrode and a drain electrode S, D. The first gate electrode G1 is used for a write operation and an erasure operation and the second gate electrode D2 is used for a readout operation. Thereby, even when the readout direction is repeated, it can be executed stably because a threshold voltage is not changed. Dielectric breakdown strength also becomes good.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrically writable/readable/erasable thin film memory.

[Conventional technology]

Recently, thin film memory devices using thin film transistors have been developed as electrically writable/readable/erasable memory devices.

FIG. 7 shows the conventional thin film memory element described above, in which an inverted staggered thin film transistor is used. This thin film memory element has an inverted staggered thin film transistor having a memory effect formed on an insulating substrate 1 made of glass or the like.This inverted staggered thin film transistor has a gate electrode G formed on the substrate 1.
A gate insulating film 2 is formed on this gate electrode G over almost the entire surface of the substrate 1, and type 1 amorphous silicon (i) is formed on this gate insulating film 2 to face the gate electrode G. -a-81) and an n-type semiconductor layer 4 made of amorphous silicon (n”-a-8i) doped with n-type impurities on top of this i-type semiconductor device. Formed source electrode S
and a drain electrode. Note that the gate electrode G and the source and drain electrodes S and D are each connected to wiring not shown. The gate insulator M2 is an insulating film having a charge storage function in order to provide a memory effect to the thin film transistor.
This gate insulation IN! 2, for example, the composition ratio Sj/N of silicon atoms S1 and nitrogen atoms N is determined by the stoichiometric ratio (S1/N
-0,75) greater than (Sl/N-0,85~1
．． 1) It is made of silicon nitride (SIN).

This thin film memory element has a gate voltage V (1 - drain current (current flowing between source and drain) 1), has hysteresis characteristics, and has a memory effect that can be electrically written/read/erased.

FIG. 9 shows the V of the thin film memory element. -1, characteristics as 8th
This shows the results measured by the measurement circuit shown in the figure, and the vG-iD characteristics of the thin film memory element are the 9th.
It has hysteresis as shown in the figure.

Then, a drain current i of 1 nA is applied to the thin film memory element.
Voltage ΔVth when o flows (hereinafter referred to as threshold voltage)
When we measure the threshold voltage (ΔV th
-n) becomes -15V, and the thin film memory element has a gate voltage ■. Drain current even when set to OV! Figure 10 shows the characteristics of a Debre silicon type transistor, such as characteristic curve a in Figure 9, where D flows, and Figure 10 (
When a voltage of +30V is applied to the gate electrode G as in b), the threshold voltage (ΔVtl+-9) becomes +12V,
A thin film memory element has a gate voltage of V. The characteristic curve of FIG. 9 shows an enhancement type transistor characteristic in which the drain current ID does not flow unless OV is made higher than OV.

1. Therefore, in order to use the above thin film memory element, it is only necessary to control the voltage applied to its gate electrode G.
When a voltage of -30V is applied to the gate electrode G and +IOV to the drain electrode and the source electrode S is grounded as shown in Fig. Figure 10(b)
+30v1 to the gate electrode G and +30v1 to the drain electrode as shown in
When a voltage of 10 V is applied and the source electrode S is grounded, the thin film memory element exhibits enhancement number type transistor characteristics and enters a write state.

For reading, as shown in FIG. 10(C), a voltage of +10V is applied to the drain electrode, the source electrode S is grounded, and the selection voltage is OV and the non-selection voltage is -20V to the gate electrode G.
This can be done by applying a pulse voltage of .

[Problem to be solved by the invention]

However, the above-mentioned conventional thin film memory element cannot write/write.
Since a voltage is applied to the same gate electrode G during erasing and reading, the threshold voltages ΔV th-n and Δv th-p change as shown in FIG. 11 as reading is repeated. However, when the number of readings exceeds several thousand times, stable reading becomes impossible. Moreover, in the conventional thin film memory element described above,
Only the gate insulating film 2 having a charge storage mechanism is interposed between the gate electrode G and the i-type semiconductor layer 3, and the gate insulating film 2 having a charge storage mechanism has low voltage resistance.
Another problem is that a short circuit occurs between the gate electrode G and the source and drain electrodes S and D, resulting in dielectric breakdown of the memory element.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to make it possible to perform semi-permanently stable reading without changing the threshold voltage even if reading 1 is repeated. Another object of the present invention is to provide a thin film memory element having excellent dielectric breakdown resistance.

[Means for Solving the Problems] In order to achieve the above object, the thin film memory element of the present invention includes a semiconductor layer, a source electrode and a drain electrode connected to the semiconductor layer, and a first electrode on one surface of the semiconductor layer. a second gate electrode opposite to each other with a gate insulating film interposed therebetween; and a second gate electrode opposite to the other surface of the semiconductor layer with a second gate insulating film interposed therebetween, and the first gate insulating film is a two-layer film in which a dielectric film is formed on the first gate electrode side and an insulating film with a charge storage function is formed on the semiconductor layer side, and the second gate insulating film is an insulating film without a charge storage function. In addition, the first gate electrode is used as a write/erase electrode, and the second gate electrode is used as a read electrode.

[Effect]

That is, the thin film memory element of the present invention is basically a thin film transistor having a memory effect, which is composed of a first gate electrode, a first gate insulating film having a charge storage function, a semiconductor layer, and source and drain electrodes. A second gate electrode is provided facing the semiconductor layer through a second gate insulating film having no charge storage function, and writing and erasing are performed using the original first gate electrode of the thin film transistor, and reading is performed using the first gate electrode of the thin film transistor. This is performed using a second gate electrode that faces the semiconductor layer through a gate insulating film that does not have a charge storage function, and in this way, reading can be performed using the second gate electrode. For example, since the threshold voltage does not change even if reading is repeated, stable reading can be performed semi-permanently. Further, in the thin film memory element of the present invention, the first gate insulating film having a charge storage function is
Since it is a two-layer film in which a dielectric film is formed on the first gate electrode side and an insulating film with a charge storage function is formed on the semiconductor layer side, the voltage resistance of the first gate insulating film is determined by the dielectric film. Therefore, this thin film memory element also has excellent dielectric breakdown resistance.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

1 and 2 are a sectional view and a plan view of the thin film memory element of this example. To explain the structure of this thin film memory element, in FIGS. 1 and 2, 11 is an insulating substrate made of glass or the like, G1 is a first gate electrode formed on this insulating substrate 11, and 12 is the first A first Goo 1 insulating film is formed over almost the entire surface of the substrate 11 on the gate electrode Gl. This first gate insulation 1!1
2, tantalum oxide (
It is a two-layer film in which a dielectric film 12a made of (TaOx) is formed, and an insulating film (hereinafter referred to as SjN film) 12b made of silicon nitride (SjN) is formed on top of the dielectric film 12a. It is formed to the thickness of a human. In addition, the upper layer SIN film 12b has a composition ratio Si/N of silicon atoms S1 and nitrogen atoms N that is set to a stoichiometric ratio (SS/N
-0,75), and this SIN film 12b is formed to an extremely thin film thickness (500 layers in this example) in order to provide it with a charge storage function. has been done. That is, even an SjN film having a film quality in which the value of S1/N is comparable to the stoichiometric ratio has a charge storage function if the film thickness is made thin. In addition, 13 is the first
An i-type semiconductor layer S and D formed on the gate insulating film 12 of 1-a-5l, facing the first gate electrode G1, are formed on the n-type semiconductor layer 13. a
The source electrode and the drain electrode are formed through an n-type semiconductor layer 14 made of -8L, and the 19th gate electrode Gl and the gate insulating film 12 in which the upper layer SiN film 12b has a charge storage function, The type semiconductor layer 13, the n-type semiconductor layer 14, and the source and drain electrodes S and D constitute an inverted staggered thin film transistor having a memory effect.

Further, on the i-type semiconductor layer 13 and the source and drain electrodes S and D connected thereto via the n-type semiconductor layer 14, a second insulating JI is provided over almost the entire surface of the substrate 11.
115 is formed, and on this second insulating film 15, a second gate electrode G2 facing the i-type semiconductor layer 13 is formed. The second gate insulating film 15 is made of silicon nitride (SI N) in which the composition ratio of silicon atoms (Si) and nitrogen atoms (N) is approximately the same as the stoichiometric ratio (Si/N-0.75). The second gate insulating film 15 is made to have a thickness of aooo Å or more, so that it is an insulating film without a charge storage function. Note that the first gate electrode G and the source.

The drain electrodes S, D and the second gate electrode G2 are each connected to wiring not shown.

Further, 16 is a protective insulating film.

and the first and second gate electrodes Gl.

Among G2, the first gate insulating film 1 having a charge storage function
The first gate electrode G1, which faces the i-type semiconductor layer 13 through the insulator 2, is used as a write/erase electrode, and is connected to the i-type semiconductor layer 13 through the second gate insulator 1, which has no charge storage function. The opposing second gate electrode G2 is used as a readout electrode.

That is, in this thin film memory element, the first gate electrode G
l, a first gate insulating film 12 having a charge storage function,
A thin film transistor with a memory effect consisting of an i-type semiconductor layer 13, an n-type semiconductor layer 14, and source and drain electrodes S and D is provided with a second gate insulator 1 without a charge storage function.
A second layer facing the i-type semiconductor layer 13 via i115.
By providing the gate electrode G2, writing and erasing can be performed via the first gate insulating film 12 having a charge storage function. 1. , and reading is performed using the second gate electrode G2 facing the i-type semiconductor layer 13 via the second gate insulator 115 having no charge storage function.

FIG. 3 shows the manufacturing process of the thin film memory element.

Therefore, this thin film memory element is manufactured as follows.

First, as shown in FIG. 3(a), a first gate electrode G1 is formed on an insulating substrate 1 to a thickness of 1000 mm. This first gate electrode Gl is formed on the substrate 11 using chromium (C').
) is deposited as a film by vacuum evaporation or sputtering, and this metal film is patterned by photolithography.

Next, as shown in FIG. 3(b), a first gate insulating film 1 is formed on the substrate 11 on which the first gate electrode Gl is formed.
2, dielectric Ml 2a and S18M12b, i
A type semiconductor layer 13, an n-type semiconductor layer 14, and a metal film 17 that will become source and drain electrodes S and D are sequentially laminated.

In this case, the dielectric film 12a, which is the lower layer film of the first gate insulator 112, is sputtered using a sputtering device using tantalum (Ta) as a target material. ) was used as an atmospheric gas to perform reactive soot treatment, and tantalum oxide (Ta
Ox (eg, Ta203) is deposited to a thickness of 3000 nm. In addition, the composition ratio of the SIN deposited on the substrate 11 is determined by using the SiN film 12 b let, which is the upper layer film of the first gate insulating film 12, using the Blaze vCVD method, 5IH4°NH3, N2, and adjusting the flow rate of each gas. is Sl/
The thickness was controlled to be N'-0.75 and deposited to a thickness of 500 mm. Further, the i-type semiconductor layer 13 and the mouth-type semiconductor layer 14 are formed by a plasma CVD method following the formation of the 5INI 112b. Note that the i-type semiconductor layer 13 is
The n-type semiconductor layer 14 is deposited to a thickness of 250 nm using 5IH4°H2, PH3.

Further, gold (J17), which becomes the source and drain electrodes S and D, is deposited with a metal such as chromium (Cr) to a thickness of 1000 nm by vacuum evaporation or sputtering.

Next, the metal film 17 is patterned by photolithography to form a source electrode S and a drain electrode as shown in FIG. The layer 14 is patterned into the same shape as the source and drain electrodes. In addition, the above money @c
III(Cr@)17 buttering is CCJ5.
02 as an etching gas or by active ions or etching to form a mouth-shaped semiconductor layer (n''7a=S
The patterning of layer I) 14 was performed using active ion etching using CF4.02 as an etching gas.

This is done by etching.

Next, as shown in FIG. 3(d), the i-type semiconductor layer 14 is patterned into an element shape by photolithography. The patterning of this i-type semiconductor layer (i-a-Si layer) 14 is CC, l? ,.

This is carried out by active ion φ etching using He as an etching gas.

Next, in order to remove dirt on the surface of the i-type semiconductor layer 13, the entire substrate 11 is immersed in a 1% HF solution for about 15 seconds to lightly etch the surface of the i-type semiconductor layer 13, and then washed with pure water. Immediately after drying, a second gate insulating film 15 is formed on the i-type semiconductor layer 13 and the source and drain electrodes S and D, as shown in FIG. 3(e).

This second gate insulating film (St thin film) 15 is made using a plasma CVD method using SiH4, NH3, and N2.
The flow rate of each gas was controlled so that the composition ratio of SIN deposited on the substrate 11 was 81/N+0.75.
Deposit to a thickness of 0,000 people.

Next, chromium (C
A film of a metal such as R2) is deposited by vacuum evaporation or sputtering, and this metal film is patterned by photolithography to form the second gate electrode G2 as shown in FIG. 3(e).

After this, a protective insulating film 16 made of silicon nitride (SI N÷0.75) or the like is deposited for 2,000 years using the plasma CVD method.
The thin film memory device shown in FIGS. 1 and 2 is completed by forming the thin film memory device to a thickness of 100 mL.

To explain the operation of the thin film memory element, FIG. 4 is a circuit diagram showing the driving states of the thin film memory element during erasing, writing, and reading. First gate electrode for writing/erasing (hereinafter referred to as writing/erasing gate electrode) G11. knee 30V, drain electrode D1. : A voltage of +10 V is applied, and the source electrode S and the second gate electrode for reading (hereinafter referred to as the gate electrode for reading) G2 are grounded. In this case, the thin film memory element is of a depression type as shown in the characteristic curve a shown in FIG. 9, in which a drain current of 1° flows even when the gate voltage vG applied to the write/erase gate electrode Gl is set to Ov. It exhibits the transistor characteristics of , and enters the erased state.

Also, when writing, write/write as shown in Figure 4(b).
+30v1 to erase gate electrode G1 +1 to drain electrode
It is sufficient to apply a voltage of 0V and ground the source electrode S and the read gate electrode G2. The transistor enters a write state exhibiting enhancement type transistor characteristics as shown in the characteristic curve shown in FIG. 9, in which no drain current ID flows.

On the other hand, for reading, a voltage of +IOV is applied to the drain electrode as shown in FIG.
2, this can be done by applying a pulse voltage in which the selection voltage is Ov and the non-selection voltage is 120V. in this case,
Drain current with respect to change in gate voltage vG applied to read gate electrode G2 1. When a non-selection voltage (-20V) is applied to the readout gate electrode G2, the change in is a depletion curve as shown in the characteristic curve a' shown in FIG.
When a selection voltage (Ov) is applied, an enhancement type transistor characteristic as shown in characteristic curve b' shown in FIG. 5 is exhibited.

In this manner, the thin film memory element changes the transistor characteristics caused by the voltage applied to the write/write/erase gate electrode Gl, that is, the depletion type or enhancement type state, to the read gate electrode Gl.
This can also be reproduced by applying a voltage to 2.

Then, as shown in the reading state of the thin film memory element, that is, as shown in FIG. 4(c), the drain electrode is +IOV.
The threshold value is obtained by applying a voltage of Voltage Δv th-n+ Δv th-
9, it was found that this threshold voltage ΔV th-n
, ΔVth-p hardly changes as shown in FIG. 6, and the threshold voltage ΔV t due to the repetition of reading
Almost no changes were observed in hn and Δv th-9.

In this way, the thin film memory element has a first gate electrode (write/erase electrode) Gl, a first gate insulating film 12 having a charge storage function, an l-type semiconductor layer 13, and an n-type semiconductor layer. 14 and sauce.

A thin film transistor having a memory effect consisting of drain electrodes S and D is provided with a second gate electrode (for reading Writing and erasing are performed using the original first gate electrode CI of the thin film transistor, which faces the l-type semiconductor N113 via the gate insulating film 12 which has a charge storage function, and reading is performed using the charge storage function. Since the second gate electrode G2 facing the L-type semiconductor layer 13 via the gate insulating layer 115 which does not have a −p does not change, so according to this thin film memory element,
Semi-permanently stable reading can be performed.

Moreover, in this thin film memory element, the first gate insulating film 15 having a charge storage function is formed with a dielectric film 12a on the side of the first gate electrode Gl, and the SiN film with a charge storage function is formed on the side of the L-type semiconductor layer 13. Film (Sl/N: 0.75. Film thickness 500
Since it is a two-layer film with 12b formed on it, this first
S for providing a charge storage function to the gate insulating film 12 of
Even if the IN film 12b is thin, this first gate insulating film 12
The dielectric film 12a can ensure the voltage resistance of 100 nm, and therefore this thin film memory element also has excellent dielectric breakdown resistance.

In the above embodiment, the thickness of the dielectric film 12a below the first gate insulating film 12 is 3000, but the dielectric film 12a may have a thickness in the range of 1000 to 5000. For example, the voltage resistance of the gate insulating film 12 of the tube 1 can be sufficiently ensured. Further, this dielectric layer 12a is not limited to tantalum oxide, and may be formed of titanium oxide, barium titanate, titanium zinc zirconate, or the like. Further, in the above embodiment, the thickness of the insulating film 12b (SiN film of SS /N ÷ 0.75) having a charge storage function of the first gate insulating film 12 is 500. , 5
A film thickness in the range of 0 to 2000 people has a sufficient load accumulation mechanism. Further, the insulating film 12b having the charge storage function may be a SiN film with S1/N-0, 85 to 1.1, in which case the thickness of the insulating film 12b is 20
The thickness may be 00 Å or more. Further, the second gate insulator 1!115, which does not have a charge storage function, is not limited to a SiN film.

Further, in the above embodiment, the lower gate electrode Gl (on the substrate 11 side) is used as the first gate electrode for writing/erasing, and the upper gate electrode G2 is used as the second gate electrode for reading. Conversely, the upper gate electrode G2 is written/
The first gate electrode for erasing is used as the lower gate electrode Gl.
may be used as the second gate electrode for reading, in which case, the gate insulating film 15 between the upper gate electrode G2 and the l-type semiconductor layer 13 is replaced with a dielectric material PI412a on the upper gate electrode G2 side. The gate insulating film 12 between the lower gate electrode Gl and the l-type semiconductor layer 13 has a charge storage function. An insulating film with no function may be used. In this case, the thin film transistor with the memory effect is a Cobraner type consisting of the upper gate electrode G2, the gate insulating film 15, the l-type semiconductor layer 13, the n-type semiconductor layer 14, and the source and drain electrodes S and D. The thin film memory element has a structure in which a read gate electrode Gl is provided below the thin film transistor and faces the L-type semiconductor layer 13 via a gate insulating film 12 having no charge storage function. In addition, the thin film transistor having the above-mentioned memory effect is of an inverted stagger type, and has a second readout transistor that faces the l-type semiconductor layer through a gate insulating film that does not have a charge storage function.
By providing a gate electrode of 1, it is possible to obtain a thin film memory element having the same effects as those of the embodiments described above.

〔Effect of the invention〕

The thin film memory element of the present invention includes a semiconductor layer, a source and a drain electrode connected to the semiconductor layer, a first gate electrode that faces one surface of the semiconductor layer with a first gate insulating film interposed therebetween, and a semiconductor layer that is connected to the semiconductor layer. a second gate electrode facing each other with a second gate insulating film on the other side of the layer, and the first gate insulating film forms a dielectric film on the first gate electrode side, and the semiconductor The second gate insulating film is an insulating film without a charge storage function, and the first gate electrode is a write/erase electrode. Since the second gate electrode is used as a readout electrode, the threshold voltage does not change even if readout is repeated, and stable readout can be performed semi-permanently. is also excellent.

[Brief explanation of drawings]

1 to 6 show an embodiment of the present invention, FIG. 1 and FIG. 2 are a cross-sectional view and a plan view of a thin film memory element, and FIG. 3 is a manufacturing process diagram of a thin film memory element. FIG. 4 is a circuit diagram showing the driving states of the thin film memory element during erasing, writing, and reading, and FIG. 5 is a V, -ID characteristic diagram when a gate voltage is applied to the gate electrode, which becomes the read electrode.
FIG. 6 is a diagram showing changes in threshold voltage with respect to the number of readings. Fig. 7 is a cross-sectional view of a conventional thin film memory element, Fig. 8 is a circuit diagram for measuring V and -ID characteristics of the thin film memory element, Fig. 9 is a diagram of V and -ID characteristics of the thin film memory element, and Fig. 10 is a conventional FIG. 11 is a circuit diagram showing the driving states of the conventional thin film memory element during erasing, writing, and reading, and FIG. 11 is a diagram showing the change in threshold voltage with respect to the number of readings of the conventional thin film memory element. 11... Insulating substrate, Gl... First gate electrode (writing/erasing electrode), 12... First gate insulating film,
12a... Dielectric film, 12b... Insulating film with charge storage function, 13... 1 type semiconductor layer, 14... N type semiconductor layer, S... Source electrode, D... Drain electrode,
15... Second gate insulating film (insulating film without charge storage function), G2... Second gate electrode (reading electrode)
, 16... protective insulating film. Applicant: Casio Computer Co., Ltd. Figure 1 Figure 2 Figure 3 Liao Figure jIs Figure 9 Jing 1 Gorge (a) (b) (c) Figure 10 Number of nods (b) Figure 11

Claims (1)

[Claims] In an electrically writable/readable/erasable thin film memory element, a semiconductor layer, source and drain electrodes connected to the semiconductor layer, and a first gate insulating film that faces one surface of the semiconductor layer with a first gate insulating film interposed therebetween. and a second gate electrode facing the other surface of the semiconductor layer with a second gate insulating film interposed therebetween, and the first gate insulating film has a dielectric layer on the first gate electrode side. The second gate insulating film is an insulating film without a charge storage function, and the second gate insulating film is an insulating film having no charge storage function. A thin film memory element, wherein the second gate electrode is a write/erase electrode, and the second gate electrode is a read electrode.