Patent Publication Number: US-6338994-B1

Title: Semiconductor device and method of fabricating thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 08/755,602 filed on Nov. 25, 1996, now U.S. Pat. No. 6,097,051 the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device having capacitors using a ferroelectric film such as ferroelectric nonvolatile memory or a dynamic rondam access memory (DRAM). 
     Some ferroelectric materials have extremely large relative dielectric constants ranging from several hundreds to several thousands. Therefore, use of a thin film made of these ferroelectric materials for a capacitor dielectrics provides a capacitor of small area and large capacity suitable for large scale integration (LSI) devices. Also, the ferroelectric material has spontaneous polarization that can be inverted in direction by an applied electric field, thereby providing a nonvolatile memory. 
     As described in Japanese unexamined Patent Application No. 5-90606 and referring to FIG. 14, the conventional ferroelectric memory is fabricated by forming on an interlayer insulating film  144  with a bottom Pt electrode  145 , ferroelectric film  146 , and a top Pt electrode  147  in this order, thereby forming a ferroelectric capacitor. However, in the conventional ferroelectric memory, each of the layers is formed with an independent mask, which makes the memory cell area large because of critical dimension uniformity and alignment tolerance, thereby making it difficult to fabricate highly integrated memory devices. The conventional technique also involves a problem of thinning the interlayer insulating film  144  for the conventional technique repeats the patterning on it for forming the ferroelectric capacitors. 
     To solve the above-mentioned problems, a method was proposed as described in Japanese unexamined Patent Application No. 2-288368, in which a top electrode  158 , a ferroelectric film  157 , and a bottom electrode  156  are collectively dry-etched with the photoresist used as a mask as shown in FIG.  15 . This method uses polysilicon for the top and bottom electrodes  158  and  156 , which are dry-etched with C 2 Cl 2 F 4 , SF 6 , and Ar gases. 
     However, forming a ferroelectric film directly on polysilicon, a silicon oxide film of a low dielectric constant is formed at the interface. The silicon oxide film thus formed significantly deteriorates capacitor characteristics. To avoid this deterioration, it is necessary to use electrodes made of noble metals such as platinum and palladium or conductive oxides such as IrO 2 , RuO 2 , and ReO 3 . 
     Of the above-mentioned electrode materials, platinum is considered best suited for the application. Therefore, in the memory cell forming process described in Japanese unexamined Patent Application No. 5-299601 collectively dry-etches a top electrode  45 , a ferroelectric film  44 , a bottom electrode  43 , and a conductive diffusion barrier layer  169  with the photoresist used as the mask as shown in FIG.  16 . Use of such a structure can implement microscopic capacitors without losing their properties. 
     Actually, however, platinum cannot be converted to a highly volatile reaction product to be dry-etched. It was observed that, if platinum is dry-etched, a redeposited material forms a wallshaped residue (hereinafter referred to as a platinum-contained deposit) on the capacitor side wall due to the low volatility. In this structure, the above-mentioned platinum-contained deposits short-circuit the top electrode  45  and the bottom electrode  43 . 
     It is therefore an object of the present invention to provide a capacitor in which the top and bottom electrodes thereof will not be short-circuited when the top electrode, the ferroelectric film, and the bottom electrode are etched with single photolithography process step. 
     SUMMARY OF THE INVENTION 
     This object is achieved by setting the taper angle of the side wall of the ferroelectric film constituting the ferroelectric capacitor to less than 75 degrees to the main surface of the substrate on which the ferroelectric capacitor is formed. That is, the taper angle of the cross side wall of the ferroelectric capacitor to the plane on which the bottom electrode is formed is set to a value not reaching 75 degrees or more. 
     Referring to FIG. 13, there is shown a relationship between the taper angle of the cross side wall of the ferroelectric capacitor to the main surface of the substrate and short-circuit. It is assumed herein that a short-circuit has occurred when a leakage current density at an applied voltage of 3V became 10 −5  A/cm 2  or higher. In the above-mentioned prior art, the etching is performed at nearly 90 degrees, so that, after etching of the platinum top electrode  45 , the platinum of the top electrode  45  redeposits to form a platinum-contained sidewall deposit  101  as shown in FIG.  10 A. After completion of dry-etching of the ferroelectric film  44 , a sidewall deposit  102  composed of elements constituting the ferroelectric film  44  remains along the platinum-contained sidewall deposit  101  as shown in FIG.  10 B. Although this sidewall deposit  102  is composed of the components of the ferroelectric film, the composition and crystal structure thereof are out of order, resulting in insufficient insulation. Referring to FIG. 10C, during etching of the platinum bottom electrode  43 , this deposit  102  composed of the components of the ferroelectric film is mostly removed. However, the platinum-contained sidewall deposit  101  still remains. Further, the platinum-contained sidewall deposit  103  may also be formed from the platinum bottom electrode. Thus, in the prior-art technology, depositing of platinum on the sidewall short-circuits the bottom and top electrodes  43  and  45  of the capacitor. 
     Referring to FIG. 13, it is clear that setting the angle of the cross sidewall of the platinum bottom electrode, the ferroelectric film and the top electrode to the main surface of the substrate to less than 75 degrees prevents the platinum deposits from being formed on the capacitor sidewall. 
     In FIG. 13, the angle of the cross sidewall of the platinum bottom electrode, the ferroelectric film, and the top electrode to the main surface of the substrate is shown; however, it is not always necessary to set the cross sectional sidewall of the entire capacitor to less than 75 degrees. For example, tilting the sidewall of only the ferroelectric film  44  relative to the main surface of the substrate by less than 75 degrees also provides an effect of preventing the platinum deposition from occurring. The effect can be made more conspicuous, however, by tilting together the sidewall of the platinum bottom electrode by less than 75 degrees. 
     It will be apparent that, instead of platinum, the top electrode  45  may be another rare metal such as iridium or ruthenium or a conductive oxide such as IrO 2 , RuO 2 , or ReO 3 . If platinum is not used for the top electrode  45 , the platinum-contained deposit is formed on the capacitor sidewall only when the platinum bottom electrode  43  is etched. As described above, tapering the capacitor side walls to the main surface of the substrate by less than 75 degrees prevents the short-circuit between the top electrode and the platinum bottom electrode. 
     The angle of the cross sidewall of the ferroelectric capacitor to the bottom surface of the bottom electrode is determined by the angle of the etching mask sidewall to the bottom surface of the bottom electrode. In the present invention, tungsten is used for the etching mask. When tungsten is etched by anisotropic dry etching, the angle of the tungsten sidewall to the bottom surface of the bottom electrode is determined by the angle of the photoresist side walls. FIG. 11 shows a relationship between the sidewall taper angle of photoresist sidewall and resist baking temperature. Shown are test results obtained from two types of photoresists A and B. The results indicate that the sidewall taper angle gets larger as the baking temperature rises for both the photoresists. The photoresist A is composed of a material having a flat distribution over molecular weights of 100 to 30,000, while the photoresist B is composed of a material having a peak over molecular weights 2,000 to 3,000. For the photoresists shown, a preferable result is obtained by setting the baking temperature to a range of 140° C. to 160° C. The method of controlling the sidewall taper angle by the resist baking temperature is also applicable to the case in which materials such as SiO 2  for which isotropic tapering is difficult is used for the etching mask. 
     When tungsten is etched by isotropic dry etching, the angle of the tungsten sidewall to the bottom surface of the bottom electrode can be controlled by the over-etching time of tungsten. FIG. 12 shows a relationship between the over-etching time of tungsten and the angle of the tungsten sidewall to the main surface of the substrate. As the tungsten over-etching time is increased, line width becomes narrower, while the sidewall approaches vertical angle. A preferable result will be obtained when the tungsten over-etching time is set to a range of 5% to 10%. 
     However, when etching the ferroelectric capacitor such that the sidewall taper angle thereof becomes less than 75 degrees relative to the main surface of the substrate, the ferroelectric sidewall is exposed to plasma, which may cause an etching damage, resulting in an increase in the leakage current on the sidewall. This problem is overcome by performing oxygen plasma processing after dry-etching of the bottom electrode and before etching the conductive diffusion barrier layer (hereinafter referred to simply as the diffusion barrier layer). 
     It should be noted that performing oxidization processing for etching damage recovery after etching TiN of the diffusion barrier layer oxidizes the TiN under the bottom platinum electrode to cause peel-off or the like trouble. The peel-off can be prevented from occurring by performing oxygen plasma processing before etching the TiN. 
     These above and further objects and features of the invention will be seen by reference to the description, taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, forming a part thereof, in which like reference characters denote like parts in the various views. 
     FIG. 1 is a cross section illustrating a semiconductor device according to the present invention; 
     FIG. 2 is a cross section illustrating a process of fabricating a memory cell based on the present invention; 
     FIG. 3 is a cross section illustrating a fabrication process next to that of FIG. 2; 
     FIG. 4 is a cross section illustrating a fabrication process next to that of FIG. 3; 
     FIG. 5 is a cross section illustrating a fabrication process next to that of FIG. 4; 
     FIG. 6 is a cross section illustrating a fabrication process next to that of FIG. 5; 
     FIG. 7 is a cross section illustrating a fabrication process next to that of FIG. 6; 
     FIG. 8 is a cross section illustrating a fabrication process next to that of FIG. 7; 
     FIG. 9 shows characteristics curves indicating a relationship between the voltage and leakage current density of the capacitor according to the invention after the fabrication processes of FIGS. 6 through 8; 
     FIG. 10A is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the top electrode; 
     FIG. 10B is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the ferroelectric film; 
     FIG. 10C is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the bottom electrode; 
     FIG. 11 shows characteristics curves indicating a relationship between baking temperature and photoresist sidewall taper angle; 
     FIG. 12 shows characteristics curves indicating a relationship between over-etching time and tungsten mask taper angle; 
     FIG. 13 shows capacitor short-circuit test results; 
     FIG. 14 is a cross section illustrating a prior-art memory cell; 
     FIG. 15 is a cross section illustrating another prior-art memory cell; and 
     FIG. 16 is a cross section illustrating still another prior-art memory cell. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Now, referring to FIGS. 2 through 8, there are shown cross sections illustrating a method of fabricating a memory of FIG. 1 in the order of main processes, the method being practiced as one preferred embodiment of the present invention. 
     First, as shown in FIG. 2, a switching transistor is formed by the conventional MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) forming process. In the figure, reference numeral  21  indicates a p-type semiconductor substrate, reference numeral  22  indicates an isolation dielectric, reference numeral  23  indicates a gate oxide film, reference numeral  24  indicates a word line that provides a gate electrode, reference numerals  25  and  26  indicate phosphorous-doped n-type regions, and reference numeral  27  indicates an interlayer insulating film. A deposit of SiO 2    28  of about 600 nm in thickness is formed by the known CVD (Chemical Vapor Deposition) all over the transistor. Then, the formed deposit is reflowed at 850° C. to be etched back by about 300 nm, thereby smoothing steps caused by the word line. 
     Next, an opening is formed in the SiO 2    28  so that a bit line can comes into contact with the n-type region  25 . The opening is made by known photolithography and dry etching techniques. Then, the bit line  31  is formed. This bit line  31  is composed of a film stack made of metal silicide and polysilicon. This film stack is etched by the known photolithography and dry etching to a desired bit-line pattern. 
     An insulating film  32  of silicon oxide film type such as BPSG (Boron-doped Phosphor-Silicate Glass) is deposited to planarize. It should be noted that this insulating film  32  needs to be thick enough for to planarize the substrate surface. In the present embodiment, the insulating film  32  was formed to a thickness of about 600 nm and planarized by etching back. 
     Referring to FIG. 3, a contact hole  33  of memory section is opened to provide access for the storage capacity section to comes into contact with the substrate. On the insulating film  32  and inside the contact hole  33 , polysilicon  41  was deposited CVD dry etching to a thickness of about 350 nm. Then, by dry etching, the polysilicon  41  was etched back by the film thickness to fill the contact hole  33  as shown in FIG.  4 . 
     By sputtering, TiN is formed to a thickness of about 50 nm as a diffusion barrier layer  42  and then a bottom electrode  43  is formed. In the present embodiment, a Pt film about 200 nm thick was deposited as the bottom electrode  43 . The TiN of the diffusion barrier layer  42  is provided to prevent the platinum of the bottom electrode  43  and the oxygen from diffusing into the polysilicon  41 . Then, a ferroelectric film  44  is formed. In the present embodiment, a lead zirconate titanate (Pb(Zr 0.5 Ti 0.5 )O 3 ) film was formed to a thickness of about 150 nm by reactive evaporation and then crystallized by heat treatment in oxygen atmosphere at 650° C. for 30 seconds for obtaining the ferroelectric film  44 . It will be apparent that the ferroelectric film  44  may also be formed by high-frequency magnetron sputtering, Sol-Gel method, MOD (Metal Organic Decomposition), or CVD. Then, by sputtering, a Pt film about 50 nm thick was formed as a top electrode  45  and a tungsten  46  was deposited to a thickness of about 350 nm for the mask as shown in FIG.  4 . 
     Referring to FIG. 5, the tungsten  46  is patterned by dry-etching with SF 6 , a photoresist  51  being used as the mask. After the photoresist  51  has been removed, the top electrode  45  is patterned by the sputter etching with the tungsten  46  used as the mask as shown in FIG.  6 . In doing so, the dry etching conditions were adjusted such that isotropic etching is provided, and the etching was performed such that the cross section of the tungsten  46  becomes a trapezoid, the angle of each of the sides thereof relative to the substrate being less than 75 degrees. In the present embodiment, microwave dry etching was used with the conditions that an SF 6  gas flow of 10 SCCM, a pressure of 2 mTorr, and a microwave power of 400 W. 
     If the dry etching is performed with high anisotropy, the cross section of the tungsten  46  departs from the trapezoid and approaches a rectangular, leaving projecting deposits on the tungsten  46  and the photoresist  51  at the sidewalls thereof. As shown in FIG. 10A, after the photoresist  51  has been removed, platinum-contained projecting sidewall deposits  101  remain. As shown in FIG. 10B, after the ferroelectric film  44  has been dry-etched, sidewall deposits  102  composed of the components of the ferroelectric film remain on the periphery of the platinum-contained projecting sidewall deposits  101 . 
     When the ferroelectric film  44  has been etched by use of a mixed gas composed of CF 4  and Ar, the bottom electrode  43  is etched by sputter etching. It should be noted that 30% over-etching was performed in order to remove the platinum deposited on the sidewalls as shown in FIG.  7 . 
     Oxygen plasma is generated in the same chamber in which the above-mentioned etching was performed to recover the etching damages of the cross section of the ferroelectric film  44 . In the present embodiment, the oxygen plasma processing was performed at an oxygen flow of 25 SCCM, a pressure of 30 mTorr, an RF power of 150 W for three minutes as shown in FIG.  8 . Then, by the dry etching with SF 6 , the diffusion barrier layer  42  and the remaining tungsten  46  were removed simultaneously to complete the ferroelectric capacitor of FIG.  1 . Although not shown, the memory device is completed by performing wiring like an ordinary semiconductor memory chip. 
     According to the above-mentioned processes, the increase in leakage current on the sidewalls and the decrease in breakdown voltage can be prevented at the same time, thereby allowing fabrication of microscopic ferroelectric memory cells suitable for high integration. The leakage of the ferroelectric capacitor can also be decreased by performing the oxygen plasma processing after etching the diffusion barrier layer  42  by the dry etching using SF 6 . However, this causes oxidization of the diffusion barrier layer  42  left under the bottom electrode  43  from the sides of the layer, resulting in peeling off of the bottom electrode  43  from the bottom electrode/diffusion barrier layer interface. This problem can be avoided if the oxygen plasma processing is performed before dry-etching the diffusion barrier layer  42 . 
     FIG. 9 shows comparisons of the leakage current density and voltage characteristics of the ferroelectric capacitor obtained after the etching of the top electrode shown in FIG. 6, after the etching of the bottom electrode shown in FIG. 7, after the over etching, after the oxygen plasma processing of FIG. 8, and after the etching of the diffusion barrier layer. After the top electrode etching, the leakage current density is on a order of 10 −7  A/cm 2 . After the bottom electrode etching, the platinum-contained deposits on the ferroelectric film sidewalls short-circuit the top and bottom electrodes. Removing these platinum-contained deposits by 30% over-etching decreases the leakage current density to an order of 10 −5  A/cm 2 . However, this value is larger than that obtained after the top electrode etching by an order of magnitude or more because the ferroelectric film sidewalls are exposed to the plasma to cause oxygen defects. When the oxygen defects on the ferroelectric film sidewalls are remedied by oxygen plasma processing, the leakage current density decreases to the generally same level as that observed after the top electrode etching. Etching of the diffusion barrier layer does not indicate an increase in the leakage current density either. 
     In the present embodiment, the tungsten  46  that provides the mask is formed by isotropy dry etching into a trapezoid in cross section. It will be apparent that, as described above with reference to FIG. 11, the tungsten  46  maybe formed by anisotropic dry etching into a trapezoid in cross section after forming the photoresist  51  into a trapezoid in cross section by setting the baking temperature of the photoresist  51  to a range of 140° C. to 160° C. 
     In the present embodiment, lead ziroconate tintanate is used for ferroelectric film  44 . It will be apparent that the material for the ferroelectric film is not limited to lead zirconate titanate; also available are, by way of example, perovskite-type oxides such as lead titanate, strontium titanate, and barium titanate, solid solutions of these, and bismuth-type layer-structured ferroelectric oxides. 
     In the present embodiment, TiN is used for the diffusion barrier layer  42 . It will be apparent that the same effect can be obtained by use of Ti or Ta or by stacking a plurality of materials selected from TiN, Ti, and Ta. 
     As described and according to the invention, etching the top electrode, the ferroelectric film and the bottom electrode with single photolithography process step does not cause short-circuit between the top and bottom electrodes, thereby allowing the fabrication of the memory cell of a small cell area suitable for highly integrated memory devices. Use of the memory cell according to the present invention can implement not only a high integrated DRAM (Dynamic Random Access Memory) and a highly integrated ferroelectric nonvolatile memory, but also a high-performance LSI (Large Scale Integration) in which these memory cells and a logic LSI are integrated on one chip and a field-programmable logic LSI that allows modification of wiring by the ferroelectric nonvolatile memory. It will be apparent that the effect of the present invention is by any means restricted to the memory cell of the above-mentioned embodiment; rather, the effect of the present invention extends to all semiconductor devices including LSIs for communications applications that use the ferroelectric capacitor. 
     While the preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.