Abstract:
To prevent two contacts that have different heights, share at least one interlayer insulating film and are disposed close to each other from being short-circuited to each other due to misalignment thereof, a semiconductor device according to the invention has a recess in an interlayer insulating film in which a first contact hiving a lower height, the recess being formed by the upper surface of the first contact, and a silicon nitride sidewall is formed in the recess to extend from the upper surface of the first contact and along the side surface of the recess.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method of manufacturing the same. In particular, it relates to a structure of a device, such as a memory cell, that includes a capacitor contact and a bit contact disposed close to each other in which a short circuit between the contacts is prevented, and a method of manufacturing the same. 
     2. Related Art 
     Memory cells used in semiconductor devices, such as DRAM, include a memory cell transistor and a capacitor. As a memory device with high integration density, a Capacitor Over Bit-line (COB) type DRAM has been proposed in which capacitors are disposed in a layer above bit lines. 
       FIG. 11  is a cross-sectional view of a part of such a COB-type DRAM. Bit contact  12  and capacitor contact  13  are connected to respective cell contacts  11  connected to cell transistors (not shown), and bit line  14  is disposed on bit contact  12 . Capacitor contact  13  extends into an upper insulating layer that covers bit line  14  and is connected to a capacitor that includes lower electrode  15 , capacitor dielectric film  16  and upper electrode  17 . 
     To prevent a short circuit between a bit line and a capacitor contact, there has been proposed a method of forming a nitride sidewall on the side of the bit line in a self-alignment manner (see Japanese Patent Laid-Open Nos. 2002-231906 and 2003-7854). Alternatively, in Japanese Patent Laid-Open No. 2005-39189, there is proposed a method of preventing a short circuit between a bit line and a capacitor contact by covering both the bit line and the bit contact with a direct nitride film. 
     On the other hand, based on a different idea, there has been proposed a structure in which a nitride film surrounding an upper part of a bit contact is formed, which is manufactured by partially forming a contact hole for a bit contact, forming a sidewall nitride film on the inner surface of the partially formed contact hole, using the sidewall nitride film as a mask to complete the contact hole having a reduced diameter that extends to the lower structure, filling the contact hole with a conductive material, and planarizing the conductive material by CMP or the like to form the bit contact (Japanese Patent Laid-Open No. 2000-299437). 
     With the advance of miniaturization of the semiconductor device, the problem of a short circuit between a bit line and a capacitor contact is becoming more serious, and the short-circuit margin between the capacitor contact and the bit contact is decreasing. However, from the viewpoint of ensuring adequate electrical connection, it is not wise to reduce the diameter of the contacts. In addition, considering that alignment is performed on the upper layer, the contacts preferably have larger diameters in upper parts than in lower parts. 
     Thus, if misalignment of a capacitor contact occurs, the top part of the bit contact, which has the largest diameter, and the capacitor contact are short-circuited. As shown in  FIG. 12(   a ), the top part of bit contact  22  and capacitor contact  23  are in contact with each other and thus short-circuited.  FIG. 12(   b ) is a top view, in which reference numeral  24  denotes a bit line, reference numeral  25  denotes the top surface of bit contact  22 , and reference numeral  26  denotes the top surface of capacitor contact  23 .  FIG. 12(   a ) is a cross-sectional view taken along the line A-A in  FIG. 12(   b ), in which reference numeral  21  denotes a cell contact. In  FIG. 12(   a ), illustration of the bit line, a capacitor and a cell transistor is omitted. 
     As disclosed in Japanese Patent Laid-Open No. 2005-39189, a short circuit can be prevented by covering both the bit line and the bit contact with a nitride film. However, to cover the bit contact with a nitride film, the interlayer insulating film covering the bit contact has to be previously removed. Furthermore, in the case of a contact that has a larger diameter in an upper part thereof as shown in  FIGS. 12(   a ) and  12 ( b ), the contact covered with a nitride film is increased in diameter in a lower part thereof, so that there is a possibility that the upper surface of the cell contact in the lower layer that is to be connected to the capacitor contact is partially covered in such a miniaturized structure, and the contact resistance of the capacitor contact increases. 
     Thus, there is a demand for a structure of a semiconductor device having a miniaturized cell structure or the like in which a short circuit between two contacts that have different heights and are disposed close to each other, such as a bit contact and a capacitor contact, due to a misalignment thereof is prevented without increasing the contact resistance of the capacitor contact. 
     SUMMARY 
     The present invention seeks to solve one or more of the above problems, or to improve upon those problems at least in part. 
     In one embodiment, there is provided a semiconductor memory device that includes at least a first contact and a second contact having a larger height than the first contact that share at least one interlayer insulating film and are disposed close to each other, wherein the upper surface of the first contact is lowered from the upper surface of an interlayer insulating film, in which the first contact is formed, to form a recess, and a silicon nitride sidewall extends in the recess from the upper surface of the first contact and along the side surface of the recess. 
     In another exemplary embodiment, there is provided a method of manufacturing a semiconductor device that includes at least a first contact and a second contact having a larger height than the first contact that share at least one interlayer insulating film and are disposed close to each other, comprising: 
     etching back the upper surface of the first contact to form a recess in an interlayer insulating film in which the first contact is formed; and 
     forming a silicon nitride film in the recess and etching back the silicon nitride film to form a sidewall that extends from the upper surface of the first contact and along the side surface of the recess. 
     According to the these embodiments, even if the capacitor contact is formed close to an upper edge of the bit contact, the silicon nitride film blocks etching, so that a short circuit between the capacitor contact and the bit contact is prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which 
         FIG. 1  is a schematic cross-sectional view of an apparatus that prevents a short circuit between a capacitor contact and a bit contact according to an exemplary embodiment; 
         FIGS. 2 to 9  are schematic cross-sectional views showing different steps in a method of manufacturing a semiconductor device according to an exemplary embodiment; 
         FIG. 10  is a schematic cross-sectional view showing a step in a method of manufacturing a semiconductor device according to another exemplary embodiment; 
         FIG. 11  is a schematic cross-sectional view of a semiconductor memory device having a typical COB structure; 
         FIG. 12(   a ) is a schematic cross-sectional view for illustrating a situation in which a capacitor contact and a bit contact are short-circuited; and 
         FIG. 12(   b ) is a schematic plan view showing the situation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purpose. 
     A semiconductor device according to an exemplary embodiment is a semiconductor memory device, such as DRAM, shown in  FIG. 1  and in a cell transistor part, bit contact  4  and capacitor contact  6  are adjacent to each other, a recess above bit contact  4  is formed in interlayer insulating film  3  in which bit contact  4  is formed, and silicon nitride sidewall  9  covering the side surface of the recess extends from the upper surface of bit contact  4 . This drawing shows an example in which bit contact  4  comprises barrier film  8  and bit metal film  7 . Reference numeral  1  denotes a cell contact to a cell transistor formed in a lower layer (not shown), reference numerals  2 ,  3  and  5  denote an interlayer insulating film. In this exemplary embodiment, bit contact  4  serves as the first contact, and capacitor contact serves as the second contact. Bit contact  4  and capacitor contact  6  share interlayer insulating film  3 . Illustration of a bit line is omitted. 
     As shown in this drawing, since silicon nitride sidewall  9  is formed on the upper part;of the edge surface of bit contact  4 , even in a case where an edge of bit contact  4  would otherwise be etched in formation of the capacitor contact, silicon nitride sidewall  9  functions as an etching stopper, so that bit contact  4  is prevented from being exposed. As a result, a short circuit between bit contact  4  and capacitor contact  6  is prevented. 
     Next, exemplary embodiments will be described. 
     First Exemplary Embodiment 
     According to a first exemplary embodiment, after a bit contact hole is formed, a TiN/Ti barrier film and a W film are buried in the bit contact hole, and then the TiN/Ti barrier film and the W film are polished by CMP, thereby forming a bit contact. Then, the bit contact is etched back by dry etching to form a recess structure in which the upper surface of the bit contact is recessed. In this process, the TiN/Ti barrier film surrounding the W plug is selectively etched so that the level of upper surface of the TiN/Ti barrier film is lower than that of the W plug. Then, a silicon nitride film is formed over the entire surface and then etched back so that a silicon nitride sidewall extending from the upper surface of the bit contact and covering the side surface of the recess structure is formed. 
     In the following, a manufacturing method according to the first exemplary embodiment will be described with reference to  FIGS. 2 to 9 . 
     First, to form a bit contact, interlayer insulating film (silicon oxide film)  3  is etched to form contact hole  4 a that exposes to the upper surface of a base cell contact (not shown) ( FIG. 2 ). 
     Then, a TiN film and a Ti film serving as barrier film  8  are formed. The Ti film is deposited by chemical vapor deposition (CVD) using TiCl 4 , and the TiN film is deposited by CVD using TiCl 4  and NH 3 . Then, tungsten (W) film  7  is formed. W film  7  is deposited by CVD using WF 6  and H 2  ( FIG. 3 ). 
     Then, CMP is performed to planarize the surface to form a plug ( FIG. 4 ). At this point in time, the upper surface of the plug is flush with the upper surface of interlayer insulating film  3 . Thus, then, the level of the upper surface of W film  7  and TiN/Ti barrier film  8  is lowered. W film  7  and TiN/Ti barrier film  8  are etched back by dry etching to form a recess. W film  7  is etched using a fluorine-based gas, and TiN/Ti barrier film  8  is etched using a chlorine-based gas. Since W film  7  is not substantially etched by the chlorine-based gas, and TiN/Ti barrier film  8  is not substantially etched by the fluorine-based gas, W film  7  and TiN/Ti barrier film  8  can be independently and selectively etched with high controllability. For example, etching of W film  7  involves an electron cyclotron resonance (ECR) etcher, CF 4 /O 2 /N 2  as a process gas, a pressure of 1.33 Pa (10 mTorr), a microwave power of 800 W and an RF power of 30 W. To lower the upper surface of the W plug by 40 nm, supposing that the etching rate for W is 100 nm/min, the etching can be performed for about 25 seconds. However, the fluorine-based gas can also etch interlayer insulating film  3 , which is a silicon oxide film, and therefore, the etching conditions are preferably adjusted to increase the selectivity to the oxide film by decreasing the RF power, for example ( FIG. 5 ). 
     On the other hand, for example, etching of the TiN/Ti barrier film involves the ECR etcher, BCl 3 /Cl 2  as a process gas, a pressure of 1.33 Pa (10 mTorr), a microwave power of 800 W and an RF power of 30 W. To lower the upper surface of the TiN/Ti barrier film by 80 nm, supposing that the etching rate for TiN/Ti is 100 nm/min, the etching can be performed for about 50 seconds. The chlorine-based gas can hardly etch interlayer insulating film  3 , and therefore, a high selectivity to the oxide film can be achieved. In addition, the W film is also unlikely to be etched ( FIG. 6 ). 
     Then, silicon nitride film  9  is formed by CVD ( FIG. 7 ). The thickness of silicon nitride film  9  is the lateral length of the sidewall to be formed later. For example, silicon nitride film  9  is formed to a thickness of 40 nm, for example, using SiH 2 Cl 2  as a process gas, under a pressure of 133 Pa (1 Torr) and at a temperature of 680° C. 
     Then, the silicon nitride film is etched back to form a sidewall. The etch back of the silicon nitride film is dry etching and involves a parallel plate type etcher, CF 4 /CHF 3 /Ar/O 2  as a process gas, a pressure of 40 mTorr and an RF power of 250 W, for example. Although the etching of the W film and the TiN/Ti barrier film described above is half etching, so that end point detection is impossible, the end point of the etching of the silicon nitride film can be detected using plasma emission. Typically, the end point detection of the etching is performed by detecting light emission of SiF (at 440 nm). In this way, a sidewall extending along the sidewall of the bit contact and the upper edge of the contact can be formed. 
     Then, a bit line is formed (not shown), interlayer insulating film (silicon oxide film)  5  is formed, and capacitor contact hole  6   a  is formed in interlayer insulating film  5 . Even in a case where patterning of the capacitor contact involves a significant misalignment, an edge of the bit contact would otherwise be etched in etching to form the capacitor contact hole, the sidewall silicon nitride film  9  blocks the etching (etching of the oxide film to form the capacitor contact hole is performed under etching conditions including a high selectivity to the silicon nitride film), and therefore, a short circuit between the capacitor contact and the bit contact is prevented ( FIG. 9 ). 
     Since the silicon nitride sidewall is formed along the sidewall and upper edge of the bit contact in this way, even if the capacitor contact is disposed close to an upper edge of the bit contact due to a misalignment in formation of the capacitor contact, the silicon nitride film blocks etching, so that a short circuit between the capacitor contact and the bit contact is prevented. 
     Consider a case where the W plug is etched back by 40 nm, and the silicon nitride film to form the sidewall is deposited to a thickness of 40 nm. Supposing that the depth of the bit contact is 180 nm, the amount of over etching of the capacitor contact is 50 nm, and the etching rate ratio between the oxide film and the silicon nitride film is 10, the thickness of the removed silicon nitride film is 23 nm. Since the initial height of the silicon nitride film sidewall is 40 nm, the thickness of the remaining silicon nitride film is 17 nm, and thus, a short circuit is prevented. 
     Since the silicon nitride sidewall is formed by etching back the TiN/Ti barrier film on the sidewall of the bit contact, no short circuit to the capacitor contact occurs also on the side surface of the bit contact. For example, in a case where the TiN film has a thickness of 15 nm, and the Ti film has a thickness of 10 nm, the silicon nitride film formed on the side surface of the bit contact has a thickness of 25 nm, which is a sufficient short-circuit margin. 
     If the W plug is excessively etched back, the bit line formed later experiences a large difference in height at the bit contact and thus can be broken at that part. Thus, the W plug is preferably etched back by an amount of about 30 to 60 nm. Furthermore, if the TiN/Ti barrier film is etched back to the bottom of the bit contact, an abnormal shape can occur. Thus, taking the controllability into account, the TiN/Ti barrier film is preferably etched back by an amount of about 30 nm to a half of the depth of the bit contact. 
     Second Exemplary Embodiment 
     In the exemplary embodiment described above, the W plug is formed by CMP. In the following, a method of etching back the W plug by dry etching, rather than using CMP, will be described. 
     First, after barrier film  8  and W film  7  are buried in the bit contact hole as shown in  FIG. 3 , W film  7  is etched back under the same conditions as described above. In this process, based on the emission spectrum, the end point of the etching can be detected when TiN/Ti barrier film  8  is exposed on flat interlayer insulating film  3  at a part other than the part in which the bit contact is to be formed. Furthermore, over etching is performed to lower the upper surface of the W plug to a desired level ( FIG. 10 ). Since the TiN/Ti barrier film is not substantially etched under the etching conditions for the W film, the TiN/Ti barrier film is then etched (under the same etching conditions as described in the first exemplary embodiment). Since the oxide film and the W film are not substantially etched under the etching conditions for the TiN/Ti barrier film, the TiN/Ti can be etched with high shape controllability. As a result, a structure in which barrier film  8  is retracted as shown in  FIG. 6  is formed. Then, a sidewall can be formed in the same manner as described above. 
     Prevention of a short circuit between a capacitor contact and a bit contact has been described as an example. However, the present invention can be applied to all the semiconductor devices having a structure in which a contact has to be formed by preventing a short circuit to another contact in a lower layer. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.