METHOD FOR MANUFACTURING A CONDUCTIVE BRIDGING MEMORY DEVICE

A method for manufacturing a conductive bridging memory device includes the following steps. First, a bottom electrode is formed on a substrate. Next, a switching layer is formed on the bottom electrode. The switching layer is made of a semiconducting metal oxide and free of gallium. Then, a surface of the switching layer is subjected to an oxygen plasma surface treatment. Afterwards, a blocking layer including a conductive material is formed on the treated surface of the switching layer, and an upper electrode is formed on the blocking layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Invention Patent Application No. 110109341, filed on Mar. 16, 2021.

FIELD

The disclosure relates to a method for manufacturing a memory device, and more particularly to a resistive memory device.

BACKGROUND

As display panel industry matures, the market demands display panels that have higher resolutions and larger sizes. In turn, memory devices disposed in the display panels are required to process massive amount of data and therefore have to be continuously operated under high driving current. In addition, in order to enhance resolution of images and reduce the size of the devices, conventionally, a thin-film transistor is disposed in each of the pixels of the display panels, and a switching layer of a resistive memory device is connected in series with an active channel of the thin film transistor to form a one-transistor one-resistor (1T1R) structure.

Amorphous metal oxides, which are known to have high carrier migration rate, high light transmittance and low-temperature film-forming properties, are promising materials for making the switching layer of the resistive memory device. Among which, amorphous indium zinc oxides, such as indium gallium zinc oxide (a-IGZO), indium tungsten zinc oxide (a-IWZO) and indium tin zinc oxide (a-ITZO) draw attention because of their superior current driving capability and stable thin film forming property.

However, since the resistive memory device is required to carry increasing driving current and be operated under quick transition between high-resistance-state (HRS) and low-resistance-state (LRS), problems of fluctuating resistive switching parameters and unsatisfactory endurance may arise. In addition, the switching layer made of indium zinc oxide may have far more oxygen vacancies for carrier migration formed on a surface of the switching layer than within the switching layer, which might lead to current overshoot when a voltage is applied, thereby reducing the service life of the device. Therefore, it is important to improve stability of the current resistive memory device in order to prolong the service life of the device.

SUMMARY

Therefore, an object of the disclosure is to provide a method for manufacturing a conductive bridging memory device that can alleviate at least one of the drawbacks of the prior art.

According to this disclosure, the method includes the steps of:

(a) forming a bottom electrode on a substrate;

(b) forming a switching layer on the bottom electrode, the switching layer being made of a semiconducting metal oxide and free of gallium;

(c) subjecting a surface of the switching layer to an oxygen plasma surface treatment;

(d) forming a blocking layer on the treated surface of the switching layer, the blocking layer including a conductive material; and

(e) forming an upper electrode on the blocking layer.

DETAILED DESCRIPTION

Referring toFIG. 1, the disclosure provides an embodiment of a method for manufacturing a conductive bridging memory device3shown inFIG. 2.

The conductive bridging memory device3includes a bottom electrode31, a switching layer32, a blocking layer33, and an upper electrode34that are sequentially disposed on a substrate30in such order.

The bottom electrode31includes a first metallic material, which may be a relatively inert conductive material. Examples of the first metallic material may include, but are not limited to, platinum, palladium, and tungsten. The bottom electrode31may have a thickness ranging from 80 nm to 100 nm. In this embodiment, the bottom electrode31includes platinum, and has a thickness of approximately 80 nm.

The switching layer32is made of a semiconducting metal oxide that is free of gallium, such as indium tungsten zinc oxide. The semiconducting metal oxide may be amorphous. The switching layer32may have a thickness ranging from 8 nm to 12 nm. In this embodiment, the switching layer32includes amorphous indium tungsten zinc oxide, and has a thickness of approximately 8 nm.

The blocking layer33includes a conductive material. Examples of the conductive material for making the blocking layer33may include, but are not limited to, titanium tungsten, titanium, and titanium nitride. The blocking layer33may have a thickness ranging from 1 nm to 2.5 nm. In this embodiment, the blocking layer33includes titanium tungsten, and has a thickness of approximately 1 nm.

The upper electrode34includes a second metallic material having a higher activity than that of the first metallic material, which is conducive for the upper electrode34to release metal ions when a positive bias voltage is applied thereto. Examples of the second metallic material may include, but are not limited to, copper, silver, and gold. The upper electrode34may have a thickness ranging from 80 nm to 100 nm. In this embodiment, the upper electrode34includes copper, and has a thickness of approximately 80 nm.

When the conductive bridging memory device is in use, during the SET process, a positive bias voltage (VSET) is applied to the upper electrode34, the upper electrode undergoes oxidation to release metal ions (i.e., copper ions in this embodiment) that drift toward the bottom electrode31due to the electric field and reach the blocking layer33. The blocking layer33can serve as a buffer region to prevent a large amount of copper ions from entering the switching layer32within a short period, so as to avoid damage to the conductive bridging memory device3from a sudden voltage drop between the upper electrode34and the switching layer32. Then, the copper ions reaching the switching layer32migrate to the bottom electrode31along the conductive pathways formed by the oxygen vacancies of the switching layer32, and are reduced to copper metal deposited at the bottom electrode31, so as to form conductive filaments (Cu filaments) between the upper electrode34and the bottom electrode31. As a result, the conductive bridging memory device3is at low-resistance-state (LRS).

During the RESET process, when a negative bias voltage (VRESET) is applied to the upper electrode34, a Joule Heating effect is induced to rupture the conductive filaments near the bottom electrode31, and the conductive bridging memory device3is switched back to high-resistance-state (HRS). By adjusting the applied voltage, transition of the conductive bridging memory device3between the LHS and the RHS can be well controlled.

According to this disclosure, the embodiment of the method for manufacturing the above mentioned conductive bridging memory device3includes the following steps21to step25.

In step21, the bottom electrode31is formed on the substrate30by, e.g., sputtering deposition.

The substrate30may be made of a desired material according to practical needs. In certain embodiments, the substrate30may be a hard substrate such as a quartz substrate or a sapphire substrate. In other embodiments, the substrate30may be flexible glass, or a soft substrate made of polymeric materials. In this embodiment, the substrate30is a silicon substrate formed with a titanium metal layer (not shown in figures) to enhance bonding between the substrate30and the bottom electrode31. The titanium metal layer may have a thickness ranging from 5 nm to 6 nm.

In step22, the switching layer32is formed on the bottom electrode31opposite to the substrate by, e.g., sputtering deposition.

In this embodiment, the switching layer32is made of amorphous indium tungsten zinc oxide. Since indium zinc oxide is highly light transmissible, it is advantageous for the conductive bridging memory device3to be applied in a display panel or in an integrated circuit. In addition, indium zinc oxide has an excellent carrier migration rate which favors formation and rupture of conductive filaments in the switching layer32when a bias voltage is applied to the conductive bridging memory device3. Furthermore, relatively high bond dissociation energy between tungsten and oxygen allows the switching layer32to form fewer oxygen vacancies, such that the conductive filaments can be formed in the switching layer32in a controlled manner to improve the stability of the conductive bridging memory device3. If conduction filaments are formed too dense, current passing through the switching layer32would be too large, which may cause damage to the conductive bridging memory device3. In addition, since tungsten is more readily accessible when compared to other trace elements such as gallium which is not used in this embodiment, the switching layer32may be made in a manner that reduces manufacturing costs.

In step23, a surface of the switching layer32is subjected to an oxygen plasma surface treatment, which may be conducted under a pressure that is not greater than 5×10−1Torr.

Specifically, the surface of the switching layer32is exposed to an oxygen plasma which is generated by subjecting a reactant gas mixture that includes oxygen to an electric field in a vacuum environment. The reactant gas mixture may also include an inert carrier gas, such as argon. In the reactant gas mixture, oxygen may be present in an amount that ranges from 8% to 34% based on the total volume of the reactant gas mixture. When the amount of oxygen in the reactant gas mixture is too low, the oxygen plasma surface treatment is ineffective for reducing the amount of oxygen vacancies on the surface of the switching layer32. When the amount of oxygen present in the reactant gas mixture is above 34%, the oxygen plasma surface treatment may remove too many oxygen vacancies on the surface of the switching layer32to form sufficient conduction filaments between the upper electrode34and the bottom electrode31, which may result in ineffective switching between the high-resistance-state and the low-resistance-state of the conductive bridging memory device3. In this embodiment, the amount of oxygen in the reactant gas mixture is 33%. The oxygen plasma surface treatment is conducted by placing the product obtained in step22in a chamber (not shown in figures) that is vacuumed to have a background pressure ranging from 2.5×10−2Torr to 7×10−2Torr. Then, argon gas is introduced into the chamber at a flow rate ranging from 200 SCCM to 210 SCCM, and oxygen gas is introduced into the chamber at a flow rate ranging from 20 SCCM to 100 SCCM. The resultant reactant gas mixture is subjected to an electric field that is provided by applying a power level that ranges from 120 W to 180 W under a working pressure that is controlled within a range of 2×10−1Torr to 5×10−1Torr. The oxygen plasma is thus formed to treat the surface of the switching layer32for a time period ranging from 60 seconds to 150 seconds. The temperature within the chamber is controlled at a temperature ranging from 80° C. to 120° C.

Since oxygen vacancies on the surface of the switching layer32are found to be much more abundant than oxygen vacancies within the switching layer32due to surface state effect, the oxygen plasma surface treatment is capable of reducing oxygen vacancies on the surface of the switching layer32by incorporating oxygen atoms into the oxygen deficient sites. As such, difference between the amount of oxygen vacancies in the interior and that on the surface of the switching layer32is reduced, resulting in improved uniformity of the resistive switching parameters.

In step24, the blocking layer33is formed on the treated surface of the switching layer32by, e.g., sputtering deposition.

In step25, the upper electrode is formed on the blocking layer33by, e.g., sputtering deposition.

It is noted that since sputtering deposition can be conducted at a temperature ranging from room temperature to 120° C., the choice of material for the substrate30may be more flexible. In addition, sputtering deposition is capable of forming films that have a relatively large area, and an accurately controlled film thickness. Therefore, in this embodiment, the bottom electrode31, the switching layer32, the blocking layer33, and the upper electrode34are formed by sputtering deposition.

To verify the effect of the oxygen plasma surface treatment on the switching layer32, the (untreated) surface of the switching layer32(i.e., prior to the oxygen plasma surface treatment) and the treated surface of the switching layer32(i.e., after the oxygen plasma surface treatment) were subjected to X-ray photoelectron spectroscopy (XPS) to measure elemental composition and chemical state thereof, thereby determining the amount of oxygen vacancies on the surface of the switching layer32. The oxygen plasma surface treatment was conducted under 3×10−1Torr and at 100° C. for 60 seconds, in which the oxygen plasma was generated from a reactant gas mixture including 33% oxygen (formed by introducing argon at 200 SCCM and oxygen at 100 SCCM into the chamber) under an electric field which was provided by applying a power level of 150 W.

As shown inFIGS. 3 and 4, O1sof the untreated surface of the switching layer32has a peak value of 4400 a.u., while O1sof the treated surface of the switching layer32is increased to reach 4500 a.u. without position shift, indicating that the amount of oxygen ions present on the treated surface of the switching layer32increases. In addition, the area under the curve labeled “lattice-O” increases from 65.2% to 74.9% after the oxygen plasma surface treatment, indicating that more metal-oxygen bondings are present on the treated surface of the switching layer32. The area under the curve labeled “oxygen vacancy” decreases from 24.9% to 18.7% after the oxygen plasma surface treatment, indicating that the amount of oxygen vacancies present on the treated surface of the switching layer32decreases. Based on these results, it may be inferred that the oxygen plasma surface treatment is effective in reducing the amount of oxygen vacancies present on the surface of the switching layer32.

The conductive bridging memory device3made by the method of this disclosure which includes the treated switching layer32was subjected to 100 cycles of DC positive bias voltage and negative bias voltage, i.e., the transition between high-resistance-state (HRS) and the low-resistance-state (LRS), so as to determine resistive switching performance of the conductive bridging memory device3. In comparison, a conventional resistive memory device which includes a switching layer that is not subjected to the oxygen plasma surface treatment was subjected to the same analysis. A respective I-V graph for each of the conductive bridging memory device3and the conventional resistive memory device was obtained.

Compared to the I-V graph of the conventional resistive memory device shown inFIG. 5, the conductive bridging memory device3of this disclosure exhibits less fluctuation, that is, a more uniform resistive switching performance with fewer changes of resistive switching parameters (seeFIG. 6).

In addition, as shown inFIG. 7, compared with the conventional resistive memory device, the conductive bridging memory device3of this disclosure exhibits a relatively smaller set voltage with less deviation, indicating that the oxygen plasma surface treatment can improve the stability of the conductive bridging memory device3, which is conducive for the operation of writing information under lower voltage.

Referring toFIG. 8, an embodiment of the conductive bridging memory device3manufactured by the method according to the disclosure is connected to a thin film transistor4to form a one-transistor one-resistor (1T1R) structure which is adapted to be applied in a display device. The thin film transistor4includes a transistor substrate41, a dielectric layer411formed on the transistor substrate41, a gate electrode42formed on the transistor substrate41opposite to the dielectric layer411, and a channel layer43, a source electrode44and a drain electrode45which are respectively disposed on the dielectric layer411.

In this case, the source electrode44and the drain electrode45are disposed on the channel layer43, and the upper electrode34of the conductive bridging memory device3serves as the drain electrode45of the thin film transistor4, such that the conductive bridging memory device3is electrically connected in series to the thin film transistor4. As such, the resultant 1T1R structure is simple and is favorable to be applied in the display device.

In sum, by virtue of subjecting the surface of the switching layer32to the oxygen plasma surface treatment, the amount of oxygen vacancies on the surface of the switching layer32is reduced, and the difference in the of amount of oxygen vacancies between the surface and interior of the switching layer32is reduced, such that overshooting can be greatly avoided when the conductive bridging memory device3is subjected to an external bias voltage, thereby improving the durability of the conductive bridging memory device3. In addition, the conductive bridging memory device3produced by the method of this disclosure can have a more uniform resistance switching performance and less fluctuation of resistive switching parameters.