TW201528492A - Resistive random access memory and method of manufacturing the same - Google Patents

Abstract

The present disclosure provides a resistive random access memory, including: a substrate; a stack including a bottom electrode and a resistance-switchable layer; an interlayer dielectric layer disposed over the stack; wherein the interlayer dielectric layer has an opening aligned with the stack, a sidewall of the opening is aligned with sidewalls of the bottom electrode and the resistance-switchable layer; a top electrode disposed over the resistance-switchable layer; and a contact plug disposed in the opening and electrically connected to the top electrode. The present disclosure also provides a method for manufacturing the resistive random access memory.

Description

Resistive memory and manufacturing method thereof

The present invention relates to a resistive memory and a method of fabricating the same, and in particular to a self-aligned resistive memory and a method of fabricating the same.

In various non-volatile memories, flash memory that can be quickly written and erased is generally used. Each memory block in the flash memory can only be erased a certain number of times. When the erased number of a memory block exceeds a critical value, the memory block will not be correctly written, and an error may occur when the data is read by the memory block. And as components continue to shrink, flash memory is gradually facing the dilemma of excessive write voltage, excessive write time and too thin gates, resulting in shortened memory time.

In order to overcome the above shortcomings, all parties are constantly striving to develop new non-volatile memory to replace flash memory. Resistive random access memory (RRAM) is a series of novel memory developed by the industry. First, it uses the principle of variable resistor to make non-volatile memory, with short write erasing time, low operating voltage and current, long memory time, multi-state memory, simple structure, simplified writing and reading. The advantages of the method and the required area are a highly promising product that has received attention from all walks of life. Therefore, how to further reduce the area of components in resistive memory, optimize its process and reduce the cost of its process is the current development goal of the industry.

The present invention provides a resistive memory comprising: a substrate; a stack comprising: a lower electrode disposed on the substrate; and a resistive transition layer disposed on the lower electrode; an interlayer dielectric layer covering the stack, wherein the interlayer dielectric layer Having an opening alignment stack, the sidewall of the opening is aligned with the sidewall of the lower electrode and the resistance transition layer; the upper electrode is disposed on the resistance transition layer; and the contact plug is disposed in the opening and electrically connected to the upper electrode.

The invention further provides a method for manufacturing a resistive memory, comprising: providing a substrate; forming a stack on the substrate, the stacking comprises: a lower electrode disposed on the substrate; a resistive transition layer disposed on the lower electrode; and an upper electrode And a sacrificial layer disposed on the upper electrode; forming an interlayer dielectric layer covering the stack; removing an interlayer dielectric layer over the stack to expose the sacrificial layer; removing the sacrificial layer to form an alignment The openings are stacked, and the sidewalls of the openings are aligned with the sidewalls of the lower electrode, the resistance transition layer and the upper electrode; and the contact plug is filled into the opening and electrically connected to the upper electrode.

The invention further provides a method for manufacturing a resistive memory, comprising: providing a substrate; forming a stack on the substrate, the stacking comprises: a lower electrode disposed on the substrate; a resistive transition layer disposed on the lower electrode; and a sacrificial layer, Provided on the resistive transition layer; forming an interlayer dielectric layer covering the stack; removing the interlayer dielectric layer above the stack to expose the sacrificial layer; removing the sacrificial layer to form an opening of the alignment stack, and the sidewall of the opening The lower electrode and the sidewall of the resistance transition layer are aligned; the upper electrode is formed on the resistance transition layer, and the upper electrode compliance covers the sidewall and the bottom of the opening; and the contact plug is filled into the opening and electrically connected to the upper electrode.

The above and other objects, features, and advantages of the present invention are made. It will be more apparent that the preferred embodiments are described below, and are described in detail below with reference to the accompanying drawings.

100‧‧‧Substrate

110‧‧‧ lower electrode

120‧‧‧resistive transition layer

130‧‧‧Upper electrode

160‧‧‧Stacking

170‧‧‧Interlayer dielectric layer

180‧‧‧Contact opening

DE‧‧‧ dry etching step

W1‧‧‧Width

W2‧‧‧Width

200‧‧‧Base

210‧‧‧ lower electrode layer

210'‧‧‧ patterned lower electrode

210’a‧‧‧ side wall

220‧‧‧resistive material layer

220'‧‧‧ patterned resistive layer

220’a‧‧‧ side wall

230‧‧‧Upper electrode layer

230'‧‧‧ patterned upper electrode

230’a‧‧‧ side wall

240‧‧‧Sacrificial material layer

240'‧‧‧ patterned sacrificial layer

250‧‧‧patterned mask

260‧‧‧Stacking

260’‧‧‧Stacking

270‧‧‧Interlayer dielectric layer

280‧‧‧ openings

280a‧‧‧ side wall

290‧‧‧Diffusion barrier

300‧‧‧Contact plug

310‧‧‧Stacking

310’‧‧‧Stacking

400‧‧‧Resistive memory

410‧‧‧Resistive memory

θ‧‧‧An angle between the inner wall of the stack and the surface of the base

1-7 are cross-sectional views of various stages of the resistive memory of the embodiment of the present invention in its manufacturing method; and FIGS. 8-13 are diagrams showing the stages of the resistive memory of another embodiment of the present invention in its manufacturing method. Sectional view.

The resistive memory of the present invention will be described in detail below. It will be appreciated that the following description provides many different embodiments or examples for implementing the invention. The specific elements and arrangements described below are intended to provide a brief description of the invention. Of course, these are by way of example only and not as a limitation of the invention. Moreover, repeated numbers or labels may be used in different embodiments. These repetitions are merely for the purpose of simplicity and clarity of the invention and are not to be construed as a limitation of the various embodiments and/or structures discussed. Furthermore, when a first material layer is on or above a second material layer, the first material layer is in direct contact with the second material layer. Alternatively, it is also possible to have one or more layers of other materials interposed, in which case there may be no direct contact between the first layer of material and the second layer of material.

It is to be understood that the elements specifically described or illustrated may be in various forms well known to those skilled in the art. In addition, when a layer is "on" another layer or substrate, it may mean "directly" on another layer or substrate, or a layer on another layer or substrate, or between other layers or substrates. Other layers.

The method for manufacturing a resistive memory provided by the present invention defines a contact opening by means of a self-alignment method to prevent the dry etching step from injuring the components of the resistive memory.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view showing a resistive memory of an embodiment of the present invention in a dry etching step for defining a contact opening. As shown in the figure, a stack 160 is disposed on the substrate 100. The stack 160 includes a lower electrode 110, a resistance transition layer 120, and an upper electrode 130. The substrate 100 and the stack 160 are covered by an interlayer dielectric layer 170. In FIG. 1, the interlayer dielectric layer 170 is subjected to a dry etching step DE to etch the contact opening 180 in the interlayer dielectric layer 170. However, the above-described steps of forming the opening 180 using the dry etching step DE may cause damage to the component. For example, the charge accumulated on the component may cause damage to the resistive transition layer 120, and change the electrical properties of the resistive transition layer 120 to reduce product yield. And the width W1 of the stack 160 must be greater than the width W2 of the bottom of the contact opening 180 to preserve process tolerances to prevent misalignment from occurring and over-etching of components causing damage and short circuits. Therefore, another embodiment of the present invention defines a contact opening using a self-aligned manner to solve the above problems.

2 to 7 are cross-sectional views for explaining another embodiment of the method of manufacturing the resistive memory of the present invention. Referring to FIG. 2, a substrate 200 is first provided, and a lower electrode layer 210, a resistive material layer 220, an upper electrode layer 230, and a sacrificial material layer 240 are sequentially formed on the substrate 200. The substrate 200 can be a germanium substrate, a germanium substrate, other semiconductor compound substrates, an insulating layer overlying germanium (SOI), or any other suitable substrate.

The materials of the lower electrode layer 210 and the upper electrode layer 230 may be the same or different, and may be, for example, TaN, TiN, TiAlN, TiW, Ag, Cu, AlCu, Pt, W, Ru, Al, Ni, combinations of the above or any other suitable electrode material. The lower electrode layer 210 and the upper electrode layer 230 may be formed by a sputtering method, a plating method, a resistance heating vapor deposition method, an electron beam evaporation method, or any other suitable deposition method.

The material of the resistance change material layer 220 may be Al, Hf, Cr, Cu, Ti, Co, Zn, Mo, Nb, Fe, Ni, W, Pb, Ta, La, Zr oxide, PrCaMnO ₃ (PCMO) , SrTiO ₃ (STO), SrZrO ₃ , combinations of the above or any other suitable resistance transition material. The method of forming the resistive material layer 220 can be formed by atomic layer deposition, sputtering, resistance heating evaporation, electron beam evaporation, or any other suitable deposition method. For example, in one embodiment, the electrically resistive material layer 220 is formed using radio frequency magnetron sputtering. The thickness of the electrically resistive material layer 220 is from about 1 nm to about 100 nm, such as from about 1 nm to about 50 nm.

The sacrificial material layer 240 is disposed on the upper electrode layer 230. This sacrificial material layer 240 will be patterned in a subsequent process to define a location where the contact plug is to be formed, and subsequent processes will selectively remove the patterned sacrificial material layer to form a contact opening. In order to be able to selectively remove the sacrificial material layer 240 in a subsequent process, the material must be different from the material of the upper electrode layer 230 and the material of the subsequently formed interlayer dielectric layer. The material of the sacrificial material layer 240 may be tantalum nitride, hafnium oxynitride, poly-Si or any other suitable material. The sacrificial material layer 240 can be formed using chemical vapor deposition. The chemical vapor deposition method can be, for example, a low-level chemical vapor deposition method, a low-temperature chemical vapor deposition method, a rapid temperature chemical vapor deposition method, a plasma-assisted chemical vapor deposition method, or an atomic layer chemical vapor deposition atomic layer. Deposition or other commonly used methods.

Next, referring to FIG. 2, a patterned mask 250 is formed on the sacrificial material layer 240. The position of the patterned mask 250 corresponds to a subsequent shape The location of the memory component. The patterned mask 250 can be a patterned photoresist or a patterned hard mask. The patterned hard mask can be tantalum nitride, tantalum oxide, amorphous carbon, polycrystalline germanium, combinations of the foregoing, or any other suitable masking material. The hard mask layer may be first deposited by chemical vapor deposition, and then the blanket hard mask layer is defined by a lithography and dry etching step to form a patterned hard mask layer. This dry etching step includes reactive ion etching, plasma etching, or other suitable dry etching.

Next, referring to FIG. 3, the underlying sacrificial material layer 240, the upper electrode layer 230, the resistive material layer 220, and the lower electrode layer 210 are sequentially etched by using the patterned mask 250 as a mask. This dry etching step includes reactive ion etching, plasma etching, or other suitable dry etching. The lower electrode 210, the resistance transition layer 220, the upper electrode 230, and the sacrificial layer 240 are etched to form a stack 260, including a patterned lower electrode 210', a resistance transition layer 220', an upper electrode 230', and a sacrificial layer. 240'. The inner wall of the stack 260 has an angle θ with the surface of the substrate 200, and the angle of the included angle θ can be adjusted by adjusting the parameters of the dry etching step. The angle of the included angle θ can be from about 80 degrees to about 90 degrees. After the dry etching step is completed, the wet masking, plasma ashing, or a combination thereof may be performed to remove the patterned mask 250.

Next, referring to FIG. 4, an interlayer dielectric layer 270 is blanket formed on the substrate 200 and the stack 260. As shown in FIG. 4, the interlayer dielectric layer 270 completely covers the stack 260. The interlayer dielectric layer 270 may be composed of a tantalum oxide or a low dielectric constant dielectric material. The low dielectric constant dielectric material may be phosphor bismuth glass, borophosphoquinone glass, fluorocarbon glass, cerium oxycarbide, spin-on glass, spin-on polymer, cerium carbide material, the aforementioned compound, and the aforementioned composite Material or a combination of the foregoing. In a preferred embodiment, the interlayer dielectric layer 270 has a flat upper surface. The interlayer dielectric layer 270 can be formed using a chemical vapor deposition method. This chemical vapor deposition method can be, for example It is a low pressure chemical vapor deposition method, a low temperature chemical vapor deposition method, a rapid temperature chemical vapor deposition method, a plasma assisted chemical vapor deposition method, an atomic layer chemical vapor deposition atomic layer deposition method or other commonly used methods.

Next, referring to FIG. 5, the interlayer dielectric layer 270 over the stack 260 is removed to expose the sacrificial layer 240'. For example, the interlayer dielectric layer 270 over the stack 260 can be removed by etch back or chemical mechanical polishing.

Next, referring to Fig. 6, the sacrificial layer 240' is selectively removed by a wet etching step to form a self-aligned opening 280. Since the material of the sacrificial layer 240' may be tantalum nitride, hafnium oxynitride, polycrystalline germanium, and the material of the interlayer dielectric layer 270 may be tantalum oxide or a low dielectric constant material, the wet etching step may be performed without etching the interlayer dielectric. The sacrificial layer 240' is selectively removed in the case of layer 270. For example, a phosphoric acid solution can be used to selectively remove tantalum nitride or hafnium oxynitride. The etch selectivity ratio of the interlayer dielectric layer 270/sacrificial layer 240' may be from about 1/20 to about 1/500, such as from about 1/30 to about 1/400.

After this wet etching step, the lower electrode 210', the resistance transition layer 220', and the upper electrode 230' are left together as a stack 260'. The opening 280 is aligned with the stack 260', and the sidewall 280a of the opening 280 is aligned with the lower electrode 210', the resistive transition layer 220', and the sidewalls 210'a, 220'a, and 230'a of the upper electrode 230'.

It will be appreciated that the step of forming the opening 180 using the dry etching step DE in Figure 1 may cause damage to the component. For example, the charge accumulated on the component may cause damage to the resistive transition layer 120, and change the electrical properties of the resistive transition layer 120 to reduce product yield. The embodiment of the second to seventh embodiments of the present invention defines the contact opening 280 by means of a self-alignment method, which can avoid damage to the component in the step of forming the opening 280 by dry etching, and improve the process yield.

Furthermore, the process steps shown in FIG. 1 require the formation of a stack 160 with a patterned mask, and the contact opening 180 is defined by a further patterned mask in the dry etching step DE. However, since the embodiment of Figures 2-7 does not require the use of a patterned mask in the step of defining the contact opening 280 in a self-aligned manner, only one patterned mask is used to define the stack 260. Thus, the embodiment of Figures 2-7 can save a patterned mask and reduce production costs compared to the embodiment of Figure 1.

In addition, in the embodiment shown in FIG. 1, the width W1 of the stack 160 must be greater than the width W2 of the bottom of the contact opening 180 to preserve process tolerance to prevent misalignment and over-etching causing damage and short-circuiting of components. However, the sidewalls 280a of the self-aligned opening 280 of the embodiment of the present invention can be combined with the sidewalls 210'a, 220'a and 230 of the lower electrode 210', the resistive transition layer 220' and the upper electrode 230'. 'a is aligned, so the width of the stack 260 need not be larger than the width of the contact opening 280. In other words, the width of the stack 260 is substantially equal to the width of the contact opening 280, so the process of the present invention can further reduce the components in the resistive memory. area.

Next, referring to FIG. 7, after the definition of the opening 280 is completed, the deposition diffusion barrier layer 290 conforms to cover the sidewalls and the bottom of the opening 280. The diffusion barrier layer 290 can assist in the adhesion of subsequent metals and prevent their diffusion. For example, suitable diffusion barrier layer materials include: tantalum (Ta), tantalum nitride (TaN), tungsten nitride (WN), or conventionally known. A metal or alloy such as titanium nitride (TiN) commonly used in the process.

Next, a metal layer as a contact plug is deposited on the diffusion barrier layer 290 by chemical vapor deposition, physical vapor deposition, or electroplating, and is filled with the opening 280. Preferably, an ionized metal plasma is used to deposit a seed layer (not shown) having a thickness of about 400 to 2500 Å on the substrate, and then depositing the conductive layer by electroplating. The material of the metal layer includes Cu, Al or W. Finish After deposition of the diffusion barrier layer 290 and the metal layer, planarization is performed by a chemical mechanical polishing method, and the metal layer other than the opening and the diffusion barrier layer 290 are removed to form the contact plug 300 and the resistance as shown in FIG. 7 is obtained. Memory 400. The contact plug 300 is electrically connected to the upper electrode 230'.

In summary, the resistive memory 400 formed by the manufacturing steps of Figures 2-7 has a substrate 200 and a stack 260' disposed on the substrate 200. The stack 260' includes a lower electrode 210', a resistive transition layer 220', and an upper electrode 230'. The interlayer dielectric layer 270 covers the substrate 200 and the stack 260', and has an opening 280 aligned with the stack 260'. The sidewall 280a and the lower electrode 210' of the opening 280, the resistive transition layer 220' and the sidewall 210' of the upper electrode 230' a, 220'a and 230'a aligned. The diffusion barrier layer 290 is compliant with the sidewalls and the bottom of the opening 280. The contact plug 300 is disposed in the opening 280 and electrically connected to the upper electrode 230'.

8-13 illustrate another embodiment of the present invention. The difference from the foregoing embodiment is mainly that the upper electrode is not formed in the stack first, and the upper electrode is formed after the self-aligned opening is formed. It should be noted that elements or layers that are the same or similar to those in the foregoing will be denoted by the same or similar reference numerals, and the materials, manufacturing methods and functions thereof are the same or similar to those described above, and therefore will not be described later. Narration.

Referring first to FIG. 8, a substrate 200 is provided, and a lower electrode layer 210, a resistive material layer 220, a sacrificial material layer 240, and a patterned mask 250 are sequentially formed on the substrate 200.

Next, referring to FIG. 9, the sacrificial material layer 240, the resistive material layer 220, and the lower electrode layer 210 are sequentially etched by using the patterned mask 250 as a mask. The lower electrode 210, the resistive transition layer 220, and the sacrificial layer 240 are etched to form a stack 310, including a patterned lower electrode 210', a resistive transition layer 220', and a sacrificial layer Livestock 240'. The inner wall of the stack 310 has an angle θ with the surface of the substrate 200, and the angle of the included angle θ can be adjusted by adjusting the parameters of the dry etching step. The angle of the included angle θ can be from about 80 degrees to about 90 degrees. After the dry etching step is completed, the wet masking, plasma ashing, or a combination thereof may be performed to remove the patterned mask 250.

Next, referring to FIG. 10, an interlayer dielectric layer 270 is blanket formed on the substrate 200 and the stack 310. As shown in FIG. 10, the interlayer dielectric layer 270 completely covers the stack 310. Next, as in Fig. 11, the interlayer dielectric layer 270 over the stack 310 is removed to expose the sacrificial layer 240'.

Next, referring to Fig. 12, the sacrificial layer 240' is selectively removed by a wet etching step to form a self-aligned opening 280. The wet etching step can selectively remove the sacrificial layer 240' with almost no etching of the interlayer dielectric layer 270. The etching selectivity ratio of the interlayer dielectric layer 270/sacrificial layer 240' can be about 1/20 to about 1. /500, for example, may be from about 1/30 to about 1/400.

Continuing to Fig. 12, after performing this wet etching step, the lower electrode 210' and the resistive transition layer 220' are left together as a stack 310'. The opening 280 is aligned with the stack 310', and the sidewall 280a of the opening 280 is aligned with the lower electrode 210' and the sidewalls 210'a and 220'a of the resistive transition layer 220'.

Next, referring to Fig. 13, after the definition of opening 280 is completed, the upper electrode material is deposited to form an upper electrode layer on resistive transition layer 220' and compliantly cover the sidewalls and bottom of opening 280. Next, another deposition step is performed to form a diffusion barrier material layer compliant to cover the upper electrode layer.

Next, a metal layer as a contact plug is deposited on the diffusion barrier material layer and filled to fill the opening 280. After the deposition of the upper electrode layer, the diffusion barrier material layer and the metal layer is completed, planarization is performed by chemical mechanical polishing, and the opening is opened. The upper electrode layer, the diffusion barrier material layer and the metal layer are removed to form the upper electrode 230', the diffusion barrier layer 290, and the contact plug 300, and the resistive memory 410 as shown in Fig. 13 is obtained. The contact plug 300 is electrically connected to the upper electrode 230'.

The resistive memory 410 formed by the manufacturing steps of Figs. 8-13 has a substrate 200 and a stack 310' disposed on the substrate 200. This stack 310' includes a lower electrode 210' and a resistive transition layer 220' in this order. The interlayer dielectric layer 270 covers the substrate 200 and the stack 310', and has an opening 280 aligned with the stack 310'. The sidewall 280a and the lower electrode 210' of the opening 280 and the sidewalls 210'a and 220'a of the resistive transition layer 220' Align. The upper electrode 230' is compliant with the sidewalls and the bottom of the opening 280, and the diffusion barrier 290 compliantly covers the upper electrode 230'. The contact plug 300 is disposed in the opening 280 and electrically connected to the upper electrode 230'.

In summary, the present invention defines the contact opening by means of a self-alignment method, which can avoid damage to the component caused by the dry etching step to form the contact opening, thereby improving the process yield. Moreover, the present invention can reduce the production cost by reducing the number of patterned masks compared to the process of forming the contact openings by the dry etching step. In addition, the invention does not need to retain the process tolerance, so the area of the components in the resistive memory can be further reduced, and the component is miniaturized.

Although the embodiments of the present invention and its advantages are disclosed above, it should be understood that those skilled in the art can make modifications, substitutions, and refinements without departing from the spirit and scope of the invention. In addition, the scope of the present invention is not limited to the processes, machines, manufacture, compositions, devices, methods, and steps in the specific embodiments described in the specification. Any one of ordinary skill in the art can. Understand current processes, machines, manufacturing, material compositions, devices and methods developed in the future or in the future The steps can be used in accordance with the present invention as long as they can perform substantially the same function or achieve substantially the same result in the embodiments described herein. In addition, the scope of each of the claims constitutes an individual embodiment, and the scope of the invention also includes the combination of the scope of the application and the embodiments.

200‧‧‧Base

210'‧‧‧ patterned lower electrode

210’a‧‧‧ side wall

220'‧‧‧ patterned resistive layer

220’a‧‧‧ side wall

230'‧‧‧ patterned upper electrode

230’a‧‧‧ side wall

260’‧‧‧Stacking

270‧‧‧Interlayer dielectric layer

280‧‧‧ openings

280a‧‧‧ side wall

290‧‧‧Diffusion barrier

300‧‧‧Contact plug

400‧‧‧Resistive memory

θ ‧‧‧ the angle between the inner wall of the stack and the surface of the base

Claims (7)

A resistive memory comprising: a substrate; a stack comprising: a lower electrode disposed on the substrate; and a resistive transition layer disposed on the lower electrode; an interlevel dielectric layer covering the stack, wherein The interlayer dielectric layer has an opening aligned with the stack, the sidewall of the opening being aligned with the lower electrode and the sidewall of the resistive transition layer; an upper electrode disposed on the resistive transition layer; and a contact plug, Provided in the opening and electrically connected to the upper electrode. The resistive memory of claim 1, wherein the upper electrode extends and conforms to cover a sidewall of the opening. The resistive memory according to claim 1, further comprising a diffusion barrier layer disposed on the sidewall and the bottom of the opening. A method of manufacturing a resistive memory, comprising: providing a substrate; forming a stack on the substrate, the stack comprising: a lower electrode disposed on the substrate; a resistive transition layer disposed on the lower electrode; An upper electrode is disposed on the resistance transition layer; and a sacrificial layer is disposed on the upper electrode; an interlayer dielectric layer is formed to cover the stack; and the interlayer dielectric layer above the stack is removed to expose the layer Sacrificial layer Removing the sacrificial layer to form an opening aligned with the stack, and sidewalls of the opening are aligned with the lower electrode, the resistive transition layer and sidewalls of the upper electrode; and forming a contact plug to fill the opening And electrically connecting the upper electrode. The method for manufacturing a resistive memory according to claim 4, further comprising forming a diffusion barrier layer in the opening before forming the contact plug, and the diffusion barrier layer conforms to cover the sidewall of the opening With the bottom. A method of manufacturing a resistive memory, comprising: providing a substrate; forming a stack on the substrate, the stack comprising: a lower electrode disposed on the substrate; a resistive transition layer disposed on the lower electrode; a sacrificial layer disposed on the resistive transition layer; forming an interlevel dielectric layer covering the stack; removing the interlayer dielectric layer over the stack to expose the sacrificial layer; removing the sacrificial layer to form An opening of the stack is aligned, and a sidewall of the opening is aligned with the lower electrode and a sidewall of the resistive transition layer; an upper electrode is formed on the resistive transition layer, and the upper electrode conforms to cover the sidewall of the opening And forming a contact plug filled in the opening and electrically connecting the upper electrode. The method for manufacturing a resistive memory according to claim 6, further comprising forming a diffusion barrier layer in the opening before forming the contact plug, and the diffusion barrier layer conforms to the upper electrode in compliance.