Patent Application
20110001179
The present application claims priority from Japanese Patent Application No. 2009-158497 filed on Jul. 3, 2009, the content of which is hereby incorporated by reference to this application.
The present invention relates to a semiconductor device including a non-volatile memory and a manufacturing method of the same. More particularly, it relates to a technology effectively applied to a non-volatile memory having a MONOS structure and a manufacturing method of the same.
Nowadays, the large scale integration (LSI) in which semiconductor elements are integrated is used in control of various systems and is becoming an infrastructure to support society. The operation of LSI today is based on the execution of arithmetic processing in accordance with programs. Therefore, the capability of storing programs is an essential requirement in many cases, and as a semiconductor element therefor, a non-volatile memory which is one of semiconductor memories incorporated in the LSI is becoming extremely important. In order to use the LSI in various applications, programs have to be restructured for these applications. Therefore, the non-volatile memory which is rewritable and retains stored information even when the power source of the LSI is turned off can be said to be essential.
A so-called floating gate type memory and a memory using insulating films as a charge accumulating layer are known as typical non-volatile memories. Particularly, the latter memory, in which the insulating films are stacked and the charge is accumulated in their interfaces and the traps and others of the films, does not have to form a new conductive layer like the floating gate type memory, and therefore, it is known that the memory can be formed with good compatibility with the complementary metal oxide semiconductor (CMOS)-LSI process. The stacked layer of silicon nitride films and silicon oxide films is widely used as the insulating films to be the charge accumulating layer because it can achieve both charge retention characteristic and rewriting characteristic. Such a non-volatile memory having the stacked insulating films is commonly called a metal-oxide-nitride-oxide-semiconductor (MONOS) type memory.
A typical example of the MONOS type memory is a two-transistor cell in which a memory transistor and a select transistor are connected in series. The memory transistor uses the Fowler-Nordheim (FN) tunnel current and direct tunnel current generated by applying a bias between a channel and a gate electrode, and injection/discharge of the electric charge is carried out by the entire channel surface.
However, the above-described MONOS type memory is required to have a sufficient charge retention characteristic while performing the injection/discharge of the electric charge, which causes the various problems. For example, in practice, when the thickness of the stacked insulating films is increased to sufficiently ensure the charge retention characteristic, write/erasure becomes difficult, and the time of write/erasure therefore exceeds a practical range.
For its solution, a method of rewriting stored information by injecting two types of electric charge having different signs (electrons and holes) by using hot carriers instead of discharging electric charge has been proposed in U.S. Pat. No. 6,215,148 (Patent Document 1) and others. This enables efficient injection of electric charge by injecting the hot carriers even in the case of a thick insulating film. According to this method, since electrons and holes can be locally injected alternately, different charge injection states can be generated at end portions in the channel direction of a planar type MOS transistor, that is, at end portions of a source and a drain and can be read as charge information.
A MONOS type memory employing the above-described hot carrier injection method basically employs the device structure of a MOS transistor, and the gate insulating film thereof, which is commonly a silicon oxide film, is replaced by three layers of insulating films, that is, a silicon oxide film, a silicon nitride film and a silicon oxide film. Also, conceivable examples of a forming method of a memory array include a method of forming sources and drains below a thick isolation oxide film and a method of forming linear sources and drains in the extending directions of gate electrodes so as to use them as wiring. In any memory arrays, when attention is focused on one memory cell, basic memory cell operations are similar in many cases and are described as follows.
The planar arrangement and the cross-sectional structure of the above-described MONOS type memory are shown in
A writing operation, an erasing operation and a reading operation of the above-described MONOS type memory will be described with reference to
In the writing operation, as shown in
In the erasing operation, as shown in
In the reading operation, as shown in
In the case of the above-described MONOS type memory, the threshold voltage is strongly affected by the charge injected into the silicon nitride film 92 (charge accumulating portion) that is in the vicinity of the end portion of the diffusion layer to be the source side in reading, and the threshold voltage depends not so much on the charge state of the silicon nitride film 92 (charge accumulating portion) that is in the vicinity of the end portion of the diffusion layer to be the drain side. Therefore, by using the above-described diffusion layer 20 and diffusion layer 30 in a switched manner, one memory cell can be used as 2 bits.
U.S. Pat. No. 5,969,383 (Patent Document 2) and U.S. Pat. No. 6,477,084 (Patent Document 3) disclose a memory cell called a split gate structure as another example of the MONOS type memory. In this memory cell, two MOS transistors (select transistor 40 and memory transistor 41) basically based on n channel type MOS transistors are connected with each other in the state in which the memory transistor 41 is vertically stacked by the side of the select transistor 40.
Herein, the operation method of the memory cell will be first described, and the manufacturing method thereof and others will be described in detail when embodiments and the problems to be solved by the present invention are described. Moreover, a circuit configuration of a memory array using the memory cell is shown in
Typical writing/erasing operations of the above-described memory cell are shown in
First, the writing operation is shown in
A characteristic of the hot electron injection in this method is that, since the electric field is concentrated in the vicinity of the boundary between the select gate 52 and the memory gate 55, the injection is intensively carried out to the end portion of the memory gate 55 on the select gate 52 side. Further, although a charge retention layer is made up of a conductive film in a floating gate type memory, since electrons are accumulated in the insulating film in the insulating film type memory, the electrons are retained in an extremely narrow region.
Next, the erasing operation is shown in
In the charge retention, charge is retained as the charge of the carriers injected into the gate insulating film 95. The barrier film (silicon oxide film) in the gate insulating film 95 has a high potential barrier, and the charge is retained well.
Also, in the reading, a positive electric potential is applied to the diffusion layer 20 of the select gate 52 side, and a positive electric potential is applied to the select gate 52, thereby turning the channel below the select gate 52 to the ON state. Herein, an appropriate memory gate electric potential capable of determining the threshold voltage difference of the memory gate 55 imparted by the write/erasure state (in other words, an intermediate electric potential between the threshold voltage of the write state and the threshold voltage of the erasure state) is applied, thereby reading the retained charge information as a current.
In “2005 IEEE, International Electron Device Meeting, Technical Digest” pp. 547-550 (Non-Patent Document 1), the technique of injecting holes without using hot carriers is described. In a conventional stacked gate insulating film structure in which a silicon oxide film is used as a barrier layer, the tunnel injection of holes from a channel is impossible because the silicon oxide film formed between a silicon nitride film and a silicon substrate is thick. Therefore, in Non-Patent Document 1, instead of the silicon oxide film, a stacked film of an extremely thin silicon oxide film, a silicon nitride film and a silicon oxide film is formed so that holes can easily tunnel from the silicon substrate side when an electric field is applied thereto.
Japanese Patent Application Laid-Open Publication No. 2004-303918 (Patent Document 4) discloses the technique in which the structure in which a silicon oxide film, a silicon nitride film and a silicon oxynitride (SiON) film are stacked in this order from the silicon substrate side is employed as the structure of a gate insulating film, and holes are injected from the gate electrode side by applying a positive voltage to a gate electrode.
Generally, it is conceivable to improve the state of the interface between a polycrystalline silicon film making up the gate electrode 50 and the silicon oxynitride film 93 by sandwiching a thin silicon oxide film of about 1 nm at the interface between the gate electrode 50 and the silicon oxynitride film 93 (not shown). In this case, the thin silicon oxide film of about 1 nm does not serve as a main factor to determine the transmission rate in the hole injection because holes can directly tunnel therethrough from the gate electrode 50. The main factor that determines the injection efficiency of holes is the film thickness of the silicon oxynitride film 93 and the band offset with respect to the holes.
Japanese Patent Application Laid-Open Publication No. 2008-211162 (Patent Document 5) discloses the technique in which the stacked structure of a non-doped polysilicon layer (though it is difficult to form a completely non-doped polysilicon layer, a deposited polysilicon layer from which impurities are intentionally eliminated is called a non-doped polysilicon layer, and the concentration and others thereof are noted in accordance with needs in the present specification) and a p+ type polysilicon layer containing a high-concentration p type impurity disposed in this order from the side close to a memory insulating film is employed as the gate structure, thereby enhancing the injection efficiency of holes in the hole injection from the gate. This utilizes the phenomenon that the band of the non-doped silicon layer bends when a positive voltage is applied to the gate. When the band of the non-doped silicon layer bends, a quantum level is formed at the interface between the memory insulating film and the gate, the height of the barrier of the insulating film viewed from holes is lowered by the amount of the energy of the quantum level, and highly efficient hole injection is carried out. According to this method, highly efficient hole injection from the gate can be carried out even without reducing the thickness of the barrier insulating film between the gate electrode and the charge accumulating layer and lowering the barrier.
As described above, the carrier injection using hot carriers has the characteristic that injection can be efficiently carried out even if the insulating film between the silicon substrate 1 and the silicon nitride film which is the charge accumulating layer is thick. This is because the energy of the hot carriers has the energy that is approximately comparable to the magnitude of the barrier potential of the insulating film measured from above the silicon band. However, the injection of the carriers in the high-energy state from the silicon substrate 1 side via the insulating film causes many defects at the interface between the channel of the silicon substrate 1 and the gate insulating film 95, and when carriers are trapped thereat in the reading, faults in the reading such as deterioration and variation of the threshold voltage are caused.
Particularly, since the band offset of the silicon oxide film, which is the gate insulating film 90 immediately above the channel, with respect to silicon is higher to holes than to electrons, hot hole injection requires the holes with higher energy than those of hot electron injection. Therefore, when the hot hole injection is employed, the damage imparted to the interface between the channel and the insulating film is more serious than that of the hot electron injection.
Also, it is known that, since injection of carriers is locally carried out in hot carrier injection methods such as source side injection (SSI) and band-to-band tunnel hot hole injection (BTBTHH injection), the carriers injected into the charge accumulating layer (silicon nitride film) diffuse with time and cause variation in the threshold voltage and deterioration of the charge retention characteristic.
These problems are caused because the channel interface is deteriorated when holes are injected from the silicon substrate by using hot carriers. Also, from another viewpoint, the problems are caused because the holes are locally injected from a high-electric-field position. Therefore, when holes are to be injected from the silicon substrate, the measures for enabling the nonlocal injection of holes with low energy while maintaining a film thickness that ensures a sufficient charge retention characteristic are necessary.
Furthermore, the number of carriers generated when BTBTHH injection is carried out is extremely large, and most of them flow to the silicon substrate, and therefore, a large current has to be supplied in the hole injection. Therefore, the area of a charge pump in a peripheral circuit region other than the memory cell part is increased, which makes it difficult to reduce the module size.
The above-described Non-Patent Document 1 is known as a method for solving the above-described problem. However, since the silicon substrate side has to be used as a channel in the reading, the silicon oxide film has to be formed to be thick in order to maintain a channel characteristic and suppress variation in the threshold voltage caused by disturbances. Moreover, since charge is moved through the barrier film in the injection of holes, holes pass through the channel interface. As a result, deterioration in the interface characteristic is also inevitable. Furthermore, it is conceived that carriers are trapped by the silicon nitride film during the writing/erasing operation in the stacked film of the extremely thin silicon oxide film, the silicon nitride film and the silicon oxide film, and variation in the threshold voltage due to detrapping of the carriers is inevitable.
A nonlocal hole injection from a gate electrode is conceivable as the nonlocal hole injection method that does not deteriorate the interface. However, since the valence band offset of silicon oxide and silicon is as high as 4.7 eV, if the insulating film below the gate electrode is a thick silicon oxide film, the hole injection from the gate electrode is difficult.
For its solution, in Patent Document 4, the structure in which a silicon oxide film, a silicon nitride film and a silicon oxynitride (SiON) film are stacked in this order from the silicon substrate side is employed as the structure of the gate insulating film.
However, in the case of the above-described document, hole injection from the gate electrode employs the method in which the holes which are not hot carriers are injected in the state in which a positive voltage is applied to the gate electrode, and therefore, the reduction in erasure efficiency due to electron injection from the silicon substrate in the hole injection from the gate electrode side is a problem. Particularly, when the film thickness of the insulating film between the gate electrode and the charge accumulating layer is thick or when the barrier of the insulating film to holes is high, the electron injection amount from the silicon substrate surpasses the injection amount of holes, and the erasure becomes difficult. Moreover, as a matter of course, in the hole injection not using hot carriers, the injection speed is slow compared with that using hot carriers, and the rewriting speed becomes slow.
In order to solve these problems and realize hole injection from the gate electrode at high efficiency, the thickness of the insulating film between the gate electrode and the charge accumulating layer has to be reduced, or the valence band offset corresponding to the barrier for holes in the gate electrode has to be lowered. However, both the reduction in the band offset and the thickness reduction of the insulating film between the gate electrode and the charge accumulating layer deteriorate the retention characteristic of electrons and holes trapped in the silicon nitride film.
As is understood from the above-described Patent Documents, in a nonlocal hole injection method, the hole injection from the gate electrode is effective to suppress the deterioration of the channel interface and reduce the current due to the erasure. Moreover, it can also be understood that it is preferable to employ the stacked structure of a non-doped polysilicon layer and an impurity-doped p+ type polysilicon layer for the gate in order to increase the efficiency of hole injection from the gate without impairing the retention characteristic of the memory. This stacked gate structure has a merit that it can be formed without major changes in processes such as introduction of new materials to the gate because it can be formed only by providing two layers, that is, the non-doped polysilicon layer and the p+ type polysilicon layer and performing the activation annealing by laser annealing in the device formation. The degree of the effect thereof is extremely large, and highly-efficient hole injection that is faster than that of a single-layer p+ type gate by one or more orders of magnitude can be realized.
However, when employing the stacked structure of the non-doped polysilicon layer and the impurity-doped p+ type polysilicon as the gate structure, attention has to be paid for a thermal process in the device formation.
Hereinafter, problems will be specifically described along with an example of the MONOS formation process.
First, as shown in
The four-layer film is formed by, for example, forming the silicon oxide film 91 having a film thickness of 4 nm by thermally oxidizing the surface of the silicon substrate 1 and then sequentially depositing the silicon nitride film 92 having a film thickness of 6 nm and the silicon oxynitride film 93 having a film thickness of 5 nm on the silicon oxide film 91 by using the chemical vapor deposition (CVD) method. The deposition of the silicon nitride film 92 by the CVD method uses SiH2Cl2+NH3 as a source gas. Subsequently, an oxidizing agent (N2O) is added to the source gas, and the flow rate of NH3 is regulated, thereby forming the silicon oxynitride film 93. The silicon oxynitride film 93 has a characteristic that the band gap thereof is larger than that of the silicon nitride film 92. Herein, the composition ratio of oxygen and nitrogen in the silicon oxynitride film 93 is 1:1. Then, the thin silicon oxide film 94 having a film thickness of about 1 nm is formed on the silicon oxynitride film 93. The extremely-thin silicon oxide film 94 like this can be formed by the In-Situ Steam Generation (ISSG) oxidation to the surface of the silicon oxynitride film 93. The film thickness of the silicon oxynitride film 93 is determined in consideration of the reduction in film thickness due to the formation of the silicon oxide film 94. The four-layer insulating film making up the gate insulating film can be obtained for example through the process above.
Then, a non-doped amorphous silicon layer is deposited to about 6 nm. Thereafter, the amorphous silicon layer is crystallized by carrying out thermal treatment at 900° C. in an N2 atmosphere for 30 minutes, thereby forming a polysilicon layer 54. Next, an amorphous silicon layer 48 which is doped with boron in the film formation is deposited to 100 nm. The concentration of boron is 7×1020/cm3. Herein, the reason why the amorphous silicon layer deposited on the gate insulating film 95 is crystallized into the polysilicon layer 54 by N2 annealing is that, compared with amorphous silicon, polysilicon is more capable of suppressing impurity diffusion from the boron-doped amorphous silicon layer 48 deposited thereon.
Next, as shown in
Herein, the implantation energy of arsenic is 5 keV, and the dose amount thereof is 2×1015 atom/cm2. Problems in the step of FIG. 14 will be described below.
The gate insulating film 95 includes a charge accumulating film and barrier films and is one of the most important parts of a memory that stores information by accumulated charge. Therefore, wet etching that causes small damage is desired to be used as the etching of the gate insulating film 95.
This is for the reasons that etching of the gate insulating film 95 by dry etching may damage end portions of the gate insulating film 95 and defects may occur in the silicon oxide film 91, the silicon oxynitride film 93 and the silicon oxide film 94 which are the barrier films, so that the defects may become a cause of leakage in the charge retention.
When the treatment is carried out by wet etching, first, the thin silicon oxide film 94 closest to the gate electrode is removed by hydrofluoric acid cleaning. Then, the silicon oxynitride film 93 is removed by using hot phosphoric acid or hydrofluoric acid. The silicon oxynitride film can be removed by hydrofluoric acid when the oxygen content thereof is high and can be removed by hot phosphoric acid when the oxygen content thereof is low. Alternatively, the film can be removed also by double etching treatment in which etching by hot phosphoric acid is carried out after etching by hydrofluoric acid. Then, the silicon nitride film 92, which is the charge accumulating film, is removed by hot phosphoric acid. In the etching of the silicon oxynitride film 93 and the silicon nitride film 92 by hot phosphoric acid, there is a problem that the silicon of the gate electrode is also reduced because hot phosphoric acid has the property of dissolving polysilicon and amorphous silicon. The reduction in the gate electrode due to the wet etching leads to variation in finishing among samples when many memories are formed, and as a result, the variation in the memory characteristics occurs.
For the solution of the above-described problems, when the hot phosphoric acid is used for the removal of the silicon oxynitride film 93 and the silicon nitride film 92 after the silicon oxide film 94 is removed by hydrofluoric acid, the sidewalls of the gate electrode have to be oxidized to about 3 to 5 nm as shown in
Herein, since the thermal oxidation process requires a high temperature, there is a problem that the impurity (dopant) of the amorphous silicon layer 48 diffuses to the non-doped polysilicon layer 54 and the gate of the stacked structure cannot be formed.
When arsenic is implanted into the silicon substrate 1 for forming the n− type diffusion layers 20 and 30 after the removal of the silicon nitride film 92 and before the etching of the silicon oxide film 91, the silicon oxide film 91 can be used as a through film of the ion implantation. Moreover, after the arsenic ion implantation, the silicon oxide film 91 is removed by hydrofluoric acid, and re-oxidation of about 1 nm in terms of the silicon oxide thickness is carried out. By this means, the part of the silicon oxide film 91, the silicon nitride film 92 and the silicon oxynitride film 93 that is exposed below the polysilicon gate can be thinly oxidized, the damage caused during the arsenic ion implantation can be recovered, and the reliability of the memory can be improved. If this step is introduced, however, since this step also requires a high temperature, there is a problem that the impurity (dopant) of the amorphous silicon layer 48 diffuses to the non-doped polysilicon layer 54 and the gate of the stacked structure cannot be formed.
Next, as shown in
On the other hand, when activation by laser annealing is carried out after the impurities are implanted with increased implantation energy, the impurity concentration of the diffusion layers of the surface of the silicon substrate is lowered, which also causes the leakage currents to the silicon substrate and the channel leakage currents.
Therefore, when activation is carried out by using laser annealing, in order to obtain good memory transistor characteristics, the impurity implantation into the diffusion layers has to be carried out by multiple-level implantation so as to form a deep implantation profile having a high impurity concentration.
However, the impurity implantation into the silicon substrate with the increased implantation energy and the impurity implantation carried out multiple times increase the amount of the impurities which penetrate through the gate electrode in the implantation and are implanted also into the gate insulating film 95 therebelow, and the gate insulating film 95 is damaged. This causes defects in the barrier films which are sandwiching the charge accumulating film, and as a result, the retention characteristic of the memory may be impaired.
Moreover, the impurity implantation carried out multiple times with a high concentration may locally form an n type region in the p type gate, and problems may be caused in operation. Therefore, it is desirable to carry out the low-concentration implantation by which the largest value of the volume density of the impurity implantation profile becomes lower than the volume density of boron of the p type gate.
Moreover, since impurity diffusion is small in the activation by laser annealing, the impurity profiles at the pn junction parts between the silicon substrate and the channel and the diffusion layers become steep, and the leakage currents caused by gate induced drain leakage (GIDL) in the MONOS operation are increased. Further, the steep junction profiles of the diffusion layers cause the above-described erroneous write/erroneous erasure (disturbance) to non-selected cells in the write/erasure at the array operation.
Because of the above-described reasons, it is desirable that the implantation energy and the number of times of implantation for the formation of the diffusion layers are minimized and the diffusion layers having high-concentration broad profiles are formed by using thermal activation by conventional RTA. However, in the stacked structure of the non-doped silicon and impurity-doped silicon, the impurity therein diffuses to the non-doped silicon layer in the thermal process by RTA, and it is difficult to maintain the stacked gate structure.
Note that the Patent Document 5 describes a process (dummy gate process) in which a dummy memory gate is formed in advance, an impurity of diffusion layers is implanted with using the dummy memory gate as a mask, the activation annealing by RTA is carried out, and then the dummy memory gate is etched to form a true memory gate (stacked gate). When the dummy gate process is used, although the number of steps is increased, the above-described problem of the impurity implantation and the problem of the diffusion of the impurity in the stacked gate can be avoided because the thermal load of the diffusion layer activation by RTA is applied before the stacked gate is deposited.
With regard to the n type impurity implantation to the gate in the formation of the diffusion layers, when an oxide film is deposited on the boron-doped amorphous silicon layer 48 by the CVD method capable of carrying out the deposition at low temperature before the gate processing as described above, the implanted arsenic ions can be stopped in the oxide film and can be prevented from being implanted into the silicon of the gate although the gate height is increased.
Next, as shown in
It is known that there is the effect that the retention characteristic of a MONOS memory is improved by adding annealing treatment in a high-temperature H2 atmosphere (about 700° C.) before the salicide process. However, this also cannot be applied when the impurity diffusion into the non-doped polysilicon layer 54 in the stacked gate is taken into consideration.
Next, after a thick interlayer insulating film is deposited on the silicon substrate 1, wiring is formed on the interlayer insulating film by a known wiring process. A MONOS type memory having a NROM structure (not shown) is completed through the above-described steps.
As described above, as a problem of the MONOS memory having the gate electrode made up of the stacked structure of the non-doped polysilicon layer and the impurity-doped polysilicon layer, the processes accompanied by high-temperature thermal treatment such as thermal oxidation, activation annealing by RTA and high-temperature H2 annealing cannot be used after the formation of the memory gate, and as a result, there is a problem that it is difficult to improve the reliability of the memory.
If the dopant diffuses to the non-doped polysilicon layer through the high-temperature process and the gate becomes, for example, a p type single-layer gate, the high-speed erasure as described in Patent Document 5 and the tunnel hole injection from the gate in which erasure saturation Vth is also low cannot be realized.
Also, even when processes such as high-temperature thermal oxidation and annealing are eliminated and activation of the diffusion layers and the gate is carried out by laser annealing after the gate of non-doped polysilicon and impurity-doped polysilicon is deposited, the diffusion of the impurity into the non-doped polysilicon cannot be completely suppressed.
An object of the present invention is to provide the technique that enable highly-efficient tunnel hole injection from a gate in a memory cell which has undergone thermal treatment in high-temperature thermal treatment processes and common processes after formation of a memory gate so as to improve the performance and reliability of the memory.
The above and other objects and novel characteristics of the present invention will be apparent from the description of the present specification and the accompanying drawings.
The typical ones of the inventions disclosed in the present application will be briefly described as follows.
A semiconductor device according to one invention of the present application is a semiconductor device including a memory cell formed on a main surface of a semiconductor substrate of a first conductivity type, wherein the memory cell comprises: a gate electrode formed on the semiconductor substrate via a gate insulating film; and a source region and a drain region formed in the semiconductor substrate in a vicinity of the gate electrode and respectively made up of semiconductor regions of a second conductivity type, the gate insulating film is made up of at least an electric potential barrier film and a charge retaining film stacked on the electric potential barrier film, and the gate electrode includes a polysilicon layer having an impurity concentration of 5×1019/cm3 or less and a metal material electrode layer formed on the polysilicon layer.
The effects obtained by typical embodiments of the invention disclosed in the present application will be briefly described below.
In a non-volatile semiconductor memory in which carriers are injected from a gate, efficiency enhancement of the carrier injection and improvement in the reliability of the memory can be both achieved.
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted.
The drawings are schematic, and the relation between thicknesses and planar dimensions, the ratios of the thicknesses of layers and the like should be determined in consideration of the following descriptions.
Also, the following embodiments exemplify devices and methods for realizing the technical ideas of the present invention, and the technical ideas of the present invention do not specify the materials, shapes, structures, arrangements, operating voltages and the like of the constituent parts to those in the embodiments.
Hereinafter, the process of forming an NROM-type MONOS cell of the present embodiment will be described with reference to
First, an isolation region (not shown) is formed in a main surface of a p type silicon substrate 1 by using a known shallow trench isolation process. Then, ion implantation of boron and activation annealing treatment are carried out, thereby forming a p type well region (not shown) in the surface of the silicon substrate 1. Next, as shown in
Next, a non-doped amorphous silicon layer is deposited. Although the effects of the film thickness of the non-doped silicon layer will be described later, the layer is deposited to 10 nm in the present embodiment. Then, thermal treatment at 900° C. is carried out in an N2 atmosphere for 30 minutes, thereby crystallizing the amorphous silicon layer to form a polysilicon layer 54, and a metal material electrode layer 59 made of TiN is deposited to 100 nm.
Next, as shown in
The etching of the gate insulating film 95 is carried out by using HF and hot phosphoric acid. First, after the silicon oxide film which is the uppermost layer of the gate insulating film is etched by HF, the silicon oxynitride film 93 is etched by HF and hot phosphoric acid, the silicon nitride film 92 is etched by hot phosphoric acid, and the silicon oxide film 91 is etched by HF. In this process, there is a problem that the polysilicon layer 54 is also etched when the hot phosphoric acid is used. Therefore, before the treatment of hot phosphoric acid, the sidewalls of the polysilicon layer 54 have to be oxidized to about 3 to 5 nm. In this structure, the non-doped silicon layer is not in direct contact with p type and n type impurities, and therefore, even through this thermal oxidation step, impurities do not diffuse to the non-doped silicon layer. Moreover, although the TiN surface is also thinly oxidized in this process, the oxidized part can simply be removed by etching in the later processing of forming a contact onto the gate.
Note that, when the implantation of arsenic for forming the n− type diffusion layers 20 and 30 is carried out before the etching of the silicon oxide film 91, the silicon oxide film 91 can be used as a through film of the ion implantation. Moreover, after the arsenic ion implantation, the silicon oxide film 91 is removed by hydrofluoric acid, and re-oxidation of about 1 nm in terms of the silicon oxide thickness is carried out. By this means, the part of the silicon oxide film 91, the silicon nitride film 92 and the silicon oxynitride film 93 that is exposed below the polysilicon gate can be thinly oxidized, the damage caused during the arsenic ion implantation can be recovered, and the reliability of the memory can be improved.
Next, as shown in
Next, as shown in
Next, as shown in
As a conceivable example of the operating method of this memory, in the writing, for example, 10 V is applied to the gate, 8 V is applied to the source, 0 V is applied to the drain, 0 V is applied to the silicon substrate 1, and write is carried out by using channel hot electrons, and in the erasure, a positive voltage (for example, 12 V) is applied to the gate, and erasure is carried out by hole injection from the gate. Moreover, it is also conceivable that a negative voltage (for example, −12 V) is applied to the gate in the writing and a positive voltage (for example, 12 V) is applied to the gate in the erasure, so that the write/erasure are carried out by injecting electrons and holes from the gate. This gate structure makes the highly-efficient injection possible even in the case in which electrons are injected from the gate.
Herein, the reason why the stacked gate electrode of non-doped polysilicon and TiN (metal) of the present embodiment enables highly-efficient hole injection will be described. First,
In the case of
Another part of the holes can pass through the silicon oxynitride film 93a by the tunneling phenomenon from a position that is higher than the formed quantum level 54b in terms of energy (B in
The lower the impurity concentration, the more likely the avalanche breakdown occurs even with a low electric field. This is because the band bending of polysilicon in the voltage application becomes larger and its distance becomes longer when the impurity concentration becomes lower and thus carriers are more likely to be accelerated, so that electron-hole pairs are easily generated. Generally, it is known that, when the concentration is about 1018/cm3 or less, avalanche breakdown occurs even with a low electric field of 0.1 MV/cm or less (for example, SEMICONDUCTOR DEVICES Physics and Technology 2nd Edition written by S. M. Sze, and Physics of VLSI Devices written by Mitsumasa KOYANAGI and Seigo KISHINO). If the concentration is in the range of about 1018/cm3 or more, the range in which the band bending occurs is shortened in terms of distance and the amount of band bending is also reduced, and therefore, acceleration of carriers becomes insufficient and avalanche breakdown is less likely to occur.
Note that, in the case of pn junction, breakdown due to the interband tunneling occurs in the range in which the concentration is about 1018/cm3 or more. However, this breakdown needs a larger electric field, and in a MOS structure, when holes are accumulated at the interface between the gate and the insulating film to some extent and the band bending becomes small, the interband tunneling phenomenon does not occur almost at all.
Experimental results showing the effects of the non-doped silicon/metal gate structure of the present embodiment are shown in
It can be understood from this result that the gate (B) of the non-doped polysilicon/TiN (metal) structure overwhelmingly exceeds the gate (A) in the injection amount and injection speed of holes. This is because the holes generated by the avalanche breakdown are injected from the energy position higher than the quantum level in addition to the holes highly-efficiently injected with the assist by the quantum level as described above.
According to the above-described results, when the impurity concentration of the non-doped polysilicon layer is smaller than 1018/cm3, the more efficient hole injection from the gate than that of the case in which the conventional stacked structure is used can be realized.
Moreover, since the same is true of the case in which electrons are injected from the gate, the above-described mechanism can also be utilized in electron injection from the gate.
Note that, in the non-doped polysilicon/TiN (metal) stacked gate structure, the influence of the damage imparted to the insulating film during rewriting by the holes, which are generated by the avalanche breakdown in the gate, is small. This is because the energy of the holes generated by the avalanche breakdown is at most 1 eV or less and this value is half or less compared with the band offset of the Si/SiON film of the valence band side. Therefore, since the holes pass through the SiON film consistently by the tunneling phenomenon, the damage is small.
In the MONOS with the non-doped silicon/metal stacked gate structure, the time required for the hole injection erasure from the gate is short, and therefore, the time during which voltage stress is applied to the insulating film is short, and deterioration of the insulating film is suppressed well. Furthermore, when the band of the non-doped silicon layer of the gate bends in the charge retention, the electric field applied to the insulating film is weakened. By virtue of these effects, the data retention characteristic of the MONOS with the non-doped silicon/metal stacked gate structure is excellent.
Regarding the thickness of the non-doped silicon layer, the present inventors have confirmed the operation of the non-doped layer of up to about 80 to 100 nm by experiments and also have confirmed the effects of highly-efficient hole injection. However, when the non-doped silicon layer becomes too thick, the effect of gate depletion excessively works also when a gate voltage is applied in the reading, and defective reading is caused. On the other hand, when the thickness is smaller than 5 nm, the distance and size of the band bending of the non-doped layer become small, and the effects of the highly-efficient hole injection are reduced. When the reading characteristic and the like in the practical usage are taken into consideration, it is preferred to use the non-doped layer of about 5 nm or more and 100 nm or less.
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in a split gate type MONOS cell will be described.
The drawings of
First, as shown in
Next, as shown in
Next, a non-doped polysilicon layer 54 made of non-doped polysilicon is deposited. A non-doped amorphous silicon layer may be used as the material of the polysilicon layer 54, and in that case, thermal treatment may be carried out at 900° C. in an N2 atmosphere for 30 minutes after the deposition so as to crystallize the amorphous silicon layer into polysilicon. As the film thickness of the polysilicon layer 54 becomes larger, the erasure speed is more improved and Vth in the erasure saturation becomes lower. However, if the thickness is increased too much, the response when a voltage is applied to the memory gate becomes slow, and the reading characteristic is deteriorated. In the present embodiment, the thickness of the non-doped layer is, for example, 10 nm. Then, a metal material electrode layer 59 made of TiN is deposited to 100 nm on the silicon substrate 1.
Next, the metal material electrode layer 59 and the polysilicon layer 54 are etched back, thereby forming sidewalls as shown in
Next, as shown in
Next, as shown in
The etching of the gate insulating film 95 is carried out by using HF and hot phosphoric acid. First, after the silicon oxide film which is the uppermost layer of the gate insulating film is etched by HF, the silicon oxynitride film 93 is etched by HF and hot phosphoric acid, the silicon nitride film 92 is etched by hot phosphoric acid, and the silicon oxide film 91 is etched by HF. In this process, there is a problem that the polysilicon layer 54 is also etched when the hot phosphoric acid is used. Therefore, before the treatment of hot phosphoric acid, the sidewalls of the polysilicon layer 54 have to be oxidized to about 3 to 5 nm. In this structure, the polysilicon layer 54 is not in direct contact with p type and n type impurities, and therefore, even through this thermal oxidation step, impurities do not diffuse to the polysilicon layer 54. Moreover, although the TiN surface is also thinly oxidized in this process, the oxidized part can be removed by etching in the later processing of forming a contact onto the memory gate 55.
Herein, when the implantation of arsenic for forming the n− type diffusion layers 20 and 30 is carried out before the etching of the silicon oxide film 91, the silicon oxide film 91 can be used as a through film of the ion implantation. Moreover, after the arsenic ion implantation, the silicon oxide film 91 is removed by hydrofluoric acid, and re-oxidation of about 1 nm in terms of the silicon oxide thickness is carried out. By this means, the part of the silicon oxide film 91, the silicon nitride film 92 and the silicon oxynitride film 93 that is exposed below the polysilicon gate can be thinly oxidized, the damage caused during the arsenic ion implantation can be recovered, and the reliability of the memory can be improved.
Next, as shown in
In the present embodiment, the memory gate 55 has the stacked gate structure of non-doped silicon and TiN. Also, since the implantation depth (Rp) of the impurities of the diffusion layers implanted into TiN and thermal diffusion of the impurities are extremely small as described above, even when activation annealing is carried out by RTA at 950° C. to 1000° C. for 10 seconds or more, there is no need to worry about the diffusion of the impurities in the gate, and the problems can be avoided.
Next, as shown in
Note that the effect that the retention characteristic of the MONOS memory can be improved by adding the annealing treatment in a high-temperature H2 atmosphere (about 700° C.) before the salicide process has been known. Further, since there is no need to worry about the diffusion of the impurities in the gate in the case in which the stacked gate electrode of non-doped silicon and TiN like the present embodiment is used, such high-temperature treatment can be carried out.
Next, as shown in
As an example of the operating method/operating voltages of the memory described above, for example, in the writing, 1.5 V is applied to the select gate 52, 9 V is applied to the memory gate 55, 5 V is applied to the diffusion layer 21 of the memory gate side, 0 V is applied to the diffusion layer 31 of the select gate side, and 0 V is applied to the silicon substrate 1 so as to inject electrons into the silicon nitride film 92 by using the source side injection (SSI) method. In the erasure, for example, 12 V is applied to the memory gate 55, and 0 V is applied to the select gate 52, the diffusion layer 21 and the silicon substrate 1 so as to carry out the erasure by injecting holes from the memory gate 55.
Moreover, it is conceivable that, in the writing, a negative voltage (for example, −12 V) is applied to the memory gate 55, and in the erasure, a positive voltage (for example, 12 V) is applied to the memory gate 55 so as to carry out the writing and the erasure by injecting electrons and holes from the memory gate 55.
In the present embodiment, in the impurity implantation of the diffusion layer, the impurities are implanted also into an exposed part of an upper portion of the non-doped polysilicon layer 54 (the part shown by a broken-line circle of
Moreover, after the n type diffusion layers 21 and 31 are formed in this structure, the exposed part of the polysilicon layer 54 shown by the broken-line circle of
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in an NROM-type MONOS cell will be described.
Particularly, in the present embodiment, the processes and structure of the case in which a so-called gate-last process is used will be described.
First, an isolation region (not shown) is formed in a main surface of a p type silicon substrate 1 by using a known shallow trench isolation process. Also, ion implantation of boron and activation annealing treatment are carried out, thereby forming a p type well region (not shown) in the surface of the silicon substrate 1. Next, as shown in
Then, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
According to the manufacturing method of the present embodiment, the non-doped polysilicon/TiN (metal) stacked gate is formed after the diffusion layers are formed and the activation annealing is carried out, and therefore, the impurities do not enter TiN of the memory gate 55 at all in the formation of the diffusion layers, and impurity diffusion to the non-doped polysilicon layer can be prevented almost completely. As a result, highly-efficient hole injection from the gate can be realized.
Note that, although the silicon oxide film 88 is formed as a dummy gate in the present embodiment, a gate similar to the gate insulating film 95, the polysilicon layer 54 and the metal material electrode layer 59 may be formed as a dummy gate instead of the silicon oxide film 88, and after removing it after the formation of the diffusion layers 21 and 31, a gate made up of the gate insulating film 95, the polysilicon layer 54 and the metal material electrode layer 59 may be formed again.
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in an NROM-type MONOS cell will be described.
Particularly, in the present embodiment, the processes and structure of the case in which a so-called gate-last process is used will be described.
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Thereafter, wiring which is conducted to the diffusion layers 20, 21, 30 and 31 and the gate is formed on the interlayer insulating film 51 by a known wiring process, and an NROM-type MONOS memory of the present embodiment is completed.
According to the manufacturing method of the present embodiment, since the non-doped polysilicon/TiN (metal) stacked gate is formed after the diffusion layers 20, 21, 30 and 31 are formed and the activation annealing is carried out, the impurities of the diffusion layers 20, 21, 30 and 31 do not enter TiN at all, and the impurity diffusion to the non-doped polysilicon layer 54 can be prevented almost completely. As a result, highly-efficient hole injection from the gate can be realized.
Moreover, since high-temperature thermal treatment such as activation annealing is not carried out after the metal material electrode layer 59 is deposited, a metal material having a low thermal resistance can also be used, and the range of material selection is expanded. Therefore, work function control of the gate becomes easier.
Moreover, the non-doped polysilicon layer 54 can have the shape of a sharp-edged corner like the parts surrounded by circles shown in
A point that should be taken into consideration in this manufacturing method is that, if the thickness of the polysilicon layer 54 is too large, the metal material electrode layer 59 cannot be embedded. Moreover, if the polysilicon layer 54 is thick, the region of the electric field effect that can be exerted by the metal material electrode layer 59 to the channel is reduced, which leads to the reduction in current in the reading. Particularly, in a scaled-down fine cell, the thickness of the polysilicon layer 54 has to be determined in consideration of the gate length thereof.
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in an NROM-type MONOS cell will be described.
Particularly, in the present embodiment, the processes and structure of the case in which a so-called gate-last process is used will be described.
First, as shown in
Next, as shown in
Next, as shown in
Then, wiring conducted to the diffusion layers 20, 21, 30 and 31 and the gate is formed on the interlayer insulating film 51 by a known wiring process.
In this manner, the stacked gate of non-doped silicon and metal (silicide) can be formed as the gate electrode. Note that the height of the non-doped polysilicon layer 54 is set in consideration of the amount removed in the CMP process and the amount of the reaction between Si and a metal such as Co in the salicide process.
According to the manufacturing method of the present embodiment, since the non-doped polysilicon/silicide (metal) stacked gate is formed after the diffusion layers 20, 21, 30 and 31 are formed and the activation annealing is carried out, the impurities of the diffusion layers 20, 21, 30 and 31 do not enter the polysilicon at all, and the impurity diffusion to the non-doped polysilicon layer 54 can be prevented almost completely. As a result, highly-efficient hole injection from the gate can be realized.
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in a split gate type MONOS cell will be described. In the present embodiment, particularly, the processes in which the diffusion layer formation is carried out before the memory gate formation by using a dummy gate will be shown.
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Note that the effect that the retention characteristic of the MONOS memory can be improved by adding the annealing treatment in a high-temperature H2 atmosphere (about 700° C.) before the salicide process has been known. Further, since there is no need to worry about the diffusion of the impurities in the gate in the case in which the stacked gate electrode of non-doped silicon and TiN like the present embodiment is used, such high-temperature treatment can be carried out.
Then, after a thick interlayer insulating film 51 is deposited on the silicon substrate 1, wiring 60 is formed on the interlayer insulating film 51 by a known wiring process. A MONOS type memory having a split gate structure capable of injecting holes from the memory gate electrode side is completed through the above steps.
According to the manufacturing method of the present embodiment, since the non-doped polysilicon/TiN (metal) stacked gate is formed after the diffusion layers 20, 21, 30 and 31 are formed and the activation annealing is carried out, the impurities of the diffusion layers 20, 21, 30 and 31 do not enter TiN at all, and the impurity diffusion to the non-doped polysilicon layer 54 can be prevented almost completely. As a result, highly-efficient hole injection from the gate can be realized.
In the present embodiment, the structure and manufacturing method of a memory cell having a non-doped polysilicon/metal stacked gate structure capable of carrying out highly-efficient hole injection in a split gate type MONOS cell will be described. In the present embodiment, particularly, the processes in which the diffusion layer formation is carried out before the memory gate formation by using a dummy gate will be shown.
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Subsequently, as shown in
Then, as shown in
Note that the effect that the retention characteristic of the MONOS memory can be improved by adding the annealing treatment in a high-temperature H2 atmosphere (about 700° C.) before the salicide process has been known. Further, since there is no need to worry about the diffusion of the impurities in the gate in the case in which only the non-doped silicon like the present embodiment is used for the gate electrode, such high-temperature treatment can be carried out.
Then, although it is not shown in the drawings, after a thick interlayer insulating film is deposited on the silicon substrate 1, wiring is formed on the interlayer insulating film by a known wiring process. A MONOS type memory of the split gate structure capable of injecting holes from the gate electrode side is completed through the above steps.
According to the manufacturing method of the present embodiment, since the non-doped polysilicon/TiN (metal) stacked gate is formed after the diffusion layers 20, 21, 30 and 31 are formed and the activation annealing is carried out, the impurities of the diffusion layers 20, 21, 30 and 31 do not enter TiN at all, and the impurity diffusion to the non-doped polysilicon layer 54 can be prevented almost completely. As a result, highly-efficient hole injection from the gate can be realized.
In the first to seventh embodiments above, it has been described that the impurity concentration in the non-doped polysilicon of the gate is desirably 1018/cm3 or less, and ideally, the hole injection efficiency is more enhanced when the concentration comes close to 0/cm3.
When the impurity concentration of the polysilicon layer of the gate is further lowered, the gate is depleted, and the initial threshold voltage of the MONOS transistor is lowered. Therefore, compared with MONOS having a high initial threshold voltage, the threshold voltage of the memory is more lowered when the same amount of holes are accumulated in an erasure state (hole accumulated state). As a result, even at a memory gate voltage of 0 V, a large reading current can be obtained. The increase in the reading current enables high-speed operation of the module, for example, when the MONOS is used for a memory that is mounted together with a microcomputer.
On the other hand, there is also a case in which a high reading current is not required in the erasure state as described above or a circuit in which the threshold voltage of the memory has to be at a position higher than 0 V even in the erasure state.
In that case, MONOS having a high threshold voltage may be utilized by setting the impurity concentration of the non-doped layer of the non-doped silicon/metal stacked gate structure to about 1018 to 5×1019/cm3. As a matter of course, due to the increase in the impurity concentration of the gate, the amount of the holes generated by avalanche breakdown is reduced, and the band bending amount and the band bending distance in the voltage application are also reduced, and therefore, the hole injection efficiency is lowered. Nevertheless, compared with the p+ type and n+ type polysilicon gates (>1020/cm3), the effect of gate depletion is observed in the voltage application if the concentration is 1018 to 5×1019/cm3, and holes can be highly efficiently injected into the charge accumulating film with the assist of the quantum level formed by the band bending of the polysilicon in the gate.
As the manufacturing method thereof, in the processes shown in the above embodiments, the impurity concentration is simply adjusted in the deposition of the polysilicon of the gate.
Meanwhile, as shown in
A manufacturing method pursuant to the non-doped polysilicon/metal stacked gate structures shown above will be described. This process is a simple formation method with a small number of steps. The case of NROM type is taken as an example for the description. As shown in
Thereafter, similarly to the embodiments described above, after a thick interlayer insulating film is deposited, wiring is formed on the interlayer insulating film by a known wiring process. A MONOS type memory capable of injecting holes from the gate electrode side is completed through the above steps. The same is true of the formation of split gate type MONOS.
According to the present embodiment, the structure pursuant to the non-doped polysilicon/metal stacked structure can be formed by extremely simple processes. However, sufficient attention has to be paid for the impurity concentration in the diffusion layer formation and the following thermal processes.
In the embodiments above, the embodiments of the MONOS type memories using a silicon nitride film as a charge accumulating film have been shown. The present invention is effective also in a floating gate type memory using polysilicon as a charge accumulating film when the rewriting using carrier injection from a gate is carried out.
In the foregoing, the invention made by the inventors of the present invention has been concretely described based on the embodiments. However, it is needless to say that the present invention is not limited to the foregoing embodiments and various modifications and alterations can be made within the scope of the present invention.
The present invention can be applied to a semiconductor storage device having a non-volatile memory.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2009-158497 | Jul 2009 | JP | national |
