Abstract:
The object of the present invention is to provide a method of manufacturing an improved semiconductor device in which overlay-accuracy can be enhanced even when a halftone mask is used. An oxide film is formed on an antireflection film. Resist films are selectively irradiated with light using a halftone phase shift mask. Subsequently, it is developed to form resist patterns for a connecting hole and an overlay mark. According to the, present invention, the provision of an antireflection film under an oxide film prevents formation of a ghost pattern in an overlay mark portion.

Description:
This application is a divisional of application Ser. No. 08/988,210 filed Dec. 10, 1997 now U.S. Pat. No. 6,005,295. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices, and more specifically, to a semiconductor device having a connecting hole not larger than 0.4 μm□ in size and an overlay mark. The present invention also relates to a method of manufacturing such a semiconductor device. 
     2. Description of the Background Art 
     In the manufacture of a semiconductor device, the higher integration and accompanying scaling down of the semiconductor device are making the width of a pattern line as well as the space between pattern lines smaller. In addition, strict overlay-accuracy for high density integration is required due to complication of the longitudinal structure of a device. 
     FIG. 11 is an illustration showing a conventional overlay technique. Overlapping is accomplished when a pattern on a photomask  20  is transferred to a wafer  21 . More specifically, in the overlaying, the position of a wafer overlay mark  23  in diffraction grating form formed on the wafer is measured using alignment light  24  through photomask  20 . The displacement between the position thus measured and a stage is corrected by moving the stage, and a chip pattern  25  on photomask  20  is transferred onto wafer  21  as a chip pattern  26   a . It is noted that the wafer alignment pattern to be used for overlapping the next layer is also transferred at the same time. 
     There are at least two types of such overlay marks  22 , one for the alignment in the X direction and the other in the Y direction. 
     For the high density integration and accompanying scaling down of a semiconductor device, a technique for forming a fine pattern using a halftone phase shift mask (hereinafter referred to as a halftone mask) as photomask  20  has been proposed. 
     With reference to FIG. 12, photomasks in general includes a usual mask and a phase shift mask. A halftone mask is known as an example of the phase shift mask. The usual mask is a glass plate on which a pattern formed of metal such as Cr or MoSi is formed. The halftone mask is a glass plate on which a metal pattern of MoSiON, CrON or the like is formed. 
     The halftone mask is provided with a material which inverts the phase of light passing through non-shading portions in the location corresponding to shading portions formed on the usual mask. The halftone mask enhances the light contrast of the pattern and forms a fine pattern as compared with the usual mask. 
     FIGS. 13A and 13B show the differences between the usual mask and the halftone mask. As for the halftone mask, the phase of light is inverted in the non-shading portion. The use of the halftone mask allows a pattern  26  and a peak  27  of light intensity to be clearly distinguished, thereby increasing resolution. A peak  28  of light intensity is however formed that can cause a ghost pattern as will be later described. 
     The problem associated with the manufacture of a semiconductor device by means of lithography technique using a conventional halftone mask will now be described. 
     With reference to FIG. 14, a connecting hole portion  29  and an overlay mark portion  30  are formed on a semiconductor substrate  9 . A first oxide film  10 , a barrier metal  11 , an aluminum film  12 , a titanium nitride film  13  and a second oxide film  14  are formed on semiconductor substrate  9  in connecting hole portion  29 . First and second oxide films  10  and  14  are formed on semiconductor substrate  9  in overlay mark portion  30 . Resist  15  for forming a connecting hole is provided in connecting hole portion  29 . Resist  15   b  for forming an overlay mark is provided in overlay mark portion  30 . A halftone mask  31  having non-shading portions in the positions to have a connecting hole and an overlay mark, respectively, is prepared. Halftone mask  31  has shading and non-shading portions  32  and  33  in overlay mark portion  30 . Resist  15  is irradiated with light  34  using halftone mask  31 . At this time, portions  35  and  36  to have a connecting hole and an overlay mark, respectively, are also exposed to the light. Further, a ghost pattern  37  is produced in the non-shading portion at the time. Ghost pattern  37  is formed by the phase-inverted light (corresponding to peak  28  in the light intensity) reflected by the surface of substrate  9  and directed upon resist  15   b.    
     The formation of ghost pattern  37  will now be described in further detail. FIG. 22 shows changes in the reflectivity of the surface of an oxide film relative to changes in thickness when the oxide film is provided on a highly reflective substrate such as a silicon substrate. As is apparent from FIG. 22, the amplitude of the reflectivity caused by the change in the thickness of the oxide film is large. The change in the diameter of the opening portion of resist is accordingly large as shown in FIG.  23 . The amplitude period of reflectivity corresponds to about 1240 Å for a wavelength of 365 nm, and therefore the maximum and minimum values of reflectivity are within the range of the amplitude if the thickness of the oxide film changes by 620 Å. Thus, the diameter of the opening portion of the resist largely changes. When an oxide film having a thickness around 10000 Å is provided, the resist is inevitably exposed to light reflected from the silicon substrate due to the above mentioned change in the diameter if the thickness of the oxide film has a variation of 10% in its surface. 
     FIG. 24 is a graph showing the optimum exposure amount relative to the size of a connecting hole to be formed on the highly reflective substrate. The exposure amount allowing formation of a ghost pattern is also shown in FIG.  24 . Herein, the abscissa represents the size of the connecting hole, and the optimum exposure amount given in FIG. 24 also applies to an overlay mark having a diameter of at least 1 μm, which can be regarded as the same in terms of size to the connecting hole having a diameter of 1 μm. 
     Assuming that the optimum exposure amount in the case of a connecting hole of 1 μm□ is normalized as 1, 1.5 times of the optimum exposure amount is required for a connecting hole of 0.4 μm□. Then, the optimum exposure amount allowing formation of a ghost pattern is sufficiently between the normalized 1.5 and 1. With reference to FIG. 14, ghost pattern  37  is consequently formed in the overlay mark portion in forming connecting hole  35 . 
     It is noted that the overlay mark can be well or poorly formed because of the variation in reflectivity as is apparent from FIGS. 22 and 23. This variation is the problem. 
     Returning to FIGS. 14,  15  and  16 , development of resist  15  to form resist patterns  15   a  and  15   b  actually results in resist patterns  15   a  and  15   b  having an undesired void portion  38  caused by the light for forming a ghost pattern peculiar to a halftone mask as shown in FIG. 16 rather than those free from a void in a resist as shown in FIG.  15 . 
     It is noted that the overlay mark is in a striped pattern having a width of 1 μm and the size of the connecting hole is 0.4 μm□. 
     With reference to FIGS. 16 and 17, etching oxide film  14  using resist patterns  15   a  and  15   b  as masks forms oxide films  16   a  and  16   b  having a connecting hole  39  and a pattern  40  of an oxide film to be an overlay mark, respectively. A poorly shaped resist pattern causes a void  141  to be formed in pattern  40  of the oxide film, that is, in the overlay mark. 
     With reference to FIG. 18, a second interconnection layer  41  is formed to contact with a titanium nitride film  13  though connecting hole  39 . At the time, the component of the second interconnection layer is formed also in overlay mark portion  30 . Resist  42  is applied to cover second interconnection  41 . 
     Then, resist  42  is selectively exposed to light through a halftone mask using an overlay mark  40  as a reference for alignment to form a resist pattern  43 . Although resist pattern  43  is a portion for patterning second interconnection layer  41 , it is formed offset due to the poorly shaped overlay mark  40  as shown in FIG.  18 . 
     With reference to FIGS. 18 and 19, resist  42  is developed to form resist pattern  43 . 
     With reference to FIGS. 19 and 20, patterning of second interconnection layer  41  using resist pattern  43  as a mask forms second interconnection layer  41  disconnected from aluminum film  12 , the first interconnection layer. It is is noted that FIG. 21 is a cross section of a semiconductor device where the steps have ideally proceeded without having the above mentioned offset. In this case, second interconnection layer  41  is tightly connected to aluminum film  12  having titanium nitride film  13  interposed. 
     The above mentioned disconnection causes yield to decrease in the manufacture of a semiconductor device. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to solve the above problem and it is an object to provide a method of manufacturing an improved semiconductor device to enhance overlay accuracy using a halftone mask. 
     It is another object of the present invention to provide a semiconductor device manufactured by such a method. 
     In a semiconductor device according to a first aspect of the invention, a first interconnection layer and a second interconnection layer provided thereabove are connected to each other through a connecting hole. The device is provided with a semiconductor substrate. A connecting hole portion having the connecting hole and an overlay mark portion having an overlay mark are provided on the semiconductor substrate. The overlay mark portion includes a pattern of oxide film to be an overlay mark and an antireflection film underlying the pattern of the oxide film. 
     In the semiconductor device according to a second aspect of the invention, the antireflection film is provided on a metal film formed on the semiconductor substrate. 
     In the semiconductor device according to a third aspect of the invention, the metal film is formed of a material mainly including aluminum, aluminum silicon, aluminum copper, copper or tungsten. 
     In the semiconductor device according to a fourth aspect of the invention, the antireflection film is formed of titanium, titanium nitride, amorphous silicon or silicon nitride. 
     In the semiconductor device according to a fifth aspect of the invention, the size of the connecting hole is not larger than 0.4 μm□. 
     In a method of manufacturing a semiconductor device according to a sixth aspect of the invention, a first interconnection layer and a second interconnection layer provided thereabove are connected through a connecting hole. A metal film for the first interconnection layer is formed on a semiconductor substrate. A conductive antireflection film is formed on the first metal film. An oxide film is formed on the antireflection film. A resist layer is formed on the oxide film. The resist layer is selectively irradiated with light using a halftone phase shift mask. Then, it is developed to form resist patterns for forming the connecting hole and an overlay mark. The oxide film is etched using the resist patterns for the connecting hole and the overlay mark as masks to form the connecting hole in the oxide film as well as a pattern of oxide film for the overlay mark. The second interconnection layer is formed to be electrically connected to the first interconnection layer through the connecting hole using the overlay mark as a reference for alignment by means of lithography technique. 
     In the method of manufacturing a semiconductor device according to a seventh aspect of the invention, the metal film is formed of a material mainly including aluminum, aluminum silicon, aluminum copper, copper or tungsten. 
     In the method of manufacturing a semiconductor device according to an eighth aspect of the invention, the antireflection film is formed of titanium, titanium nitride, amorphous silicon or silicon nitride. 
     In the method of manufacturing a semiconductor device according to a ninth aspect of the invention, the size of the connecting hole is not larger than 0.4 μm□. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing a semiconductor device in the first step of a method of manufacturing a semiconductor device according to an embodiment of the present invention. 
     FIG. 2 is a plan view of the semiconductor device shown in FIG.  1 . 
     FIGS. 3 to  7  are cross sectional views of the semiconductor device in the second to sixth steps of the method of manufacturing a semiconductor device according to the embodiment of the present invention. 
     FIG. 8 is a diagram showing a relation between the thickness of an oxide film and reflectivity when a low reflective substrate is used. 
     FIG. 9 is a diagram showing a relation between the thickness of an oxide film and the diameter of the opening portion in a resist when a low reflective substrate is used. 
     FIG. 10 is a diagram showing a relation between the size of a connecting hole and the optimum exposure amount when a low reflective substrate is used. 
     FIG. 11 is a view showing a conventional overlay technique. 
     FIG. 12 is a diagram showing the types of conventional photo masks. 
     FIGS. 13A and 13B are diagrams showing the functions of conventional usual and halftone type masks. 
     FIGS. 14 to  20  are cross sectional views of the semiconductor device in the first to seventh steps of a conventional method of manufacturing a semiconductor device. 
     FIG. 21 is a cross sectional view of an imaginary semiconductor device if the steps can proceed ideally in the conventional method of manufacturing a semiconductor device. 
     FIG. 22 is a diagram showing a relation between the thickness of an oxide film and reflectivity when a highly reflective substrate is used. 
     FIG. 23 is a diagram showing a relation between the thickness of an oxide film and the diameter of the opening portion of a resist when a highly reflective substrate is used. 
     FIG. 24 is a diagram showing a relation between the size of a connecting hole and the optimum exposure amount when a highly reflective substrate is used. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method of manufacturing a semiconductor device according to the present invention will now be described with reference to the drawings. 
     With reference to FIG. 1, a semiconductor substrate  1  having a connecting hole portion  29  and an overlay mark portion  30  is prepared. Connecting hole portion  29  and overlay mark portion  30  both include a substrate  1 , a first oxide film  2 , a barrier metal  3 , an aluminum film  4 , a titanium nitride film  5  and a second oxide film  6 . Resist films  7   a  and  7   b  are provided on second oxide film  6 . A halftone mask  31  having non-shading portions at the positions to have a connecting hole and an overlay mark, respectively, is prepared. Resist films  7   a  and  7   b  are irradiated by light  34  using halftone mask  31 . 
     FIG. 2 is a plan view of the semiconductor device shown in FIG.  1 . With reference to FIG. 2, the size of connecting hole  39  is 0.4 μm□. The present invention is effective in forming a connecting hole not larger than this size. An overlay mark  22  is a striped pattern having a width of 1 μm. 
     Titanium nitride film  5  is not only essential to maintaining the reliability of aluminum interconnection  4  but also serves as an antireflection film for aluminum film  4 . 
     With reference to FIGS. 1 and 3, resist films  7   a  and  7   b  are developed. As titanium nitride film  5  serves as an antireflection film for aluminum film  4  in the overlay mark portion, the resulting resist pattern  70   b  forming an overlay mark is free from a ghost pattern and a suitable shape is attained. 
     With reference to FIGS. 3 and 4, a second oxide film  6  is etched using resist patterns  70   a  and  70   b  as masks to form an oxide film  8   a  having connecting hole  39  as well as a pattern  8   b  formed of an oxide film to have an overlay mark  40 . 
     With reference to FIGS. 4 and 5, a second interconnection layer  41  is formed on semiconductor substrate  1  to contact with titanium nitride film  5  through connecting hole  39 . A resist film  42  is formed on second interconnection layer  41 . Resist film  42  is selectively exposed to light using overlay mark  40  as a reference for alignment. 
     With reference to FIGS. 5 and 6, resist film  42  is developed to form a resist pattern  43 . 
     With reference to FIGS. 6 and 7, second interconnection layer  41  is etched using resist pattern  43  as a mask to form a pattern for second interconnection layer  41 . The suitable shape of overlay mark  40  allows formation of the pattern for second interconnection layer  41  in a prescribed position without being offset. 
     Next, the reason why the overlay mark can be suitably formed will be described. 
     FIG. 8 is a diagram showing a relation between the thickness of an oxide film and the reflectivity in the surface of the oxide film when the oxide film is formed on a low reflective substrate. As shown, the amplitude of reflectivity resulting from the change in thickness of the oxide film becomes smaller on the low reflective substrate. Accordingly, with reference to FIG. 9, the variation of the diameter of an opening portion for the resist also becomes smaller. FIG. 9 shows that even if there is a variation in the thickness of the oxide film, the change in the diameter of the opening portion in the resist can be restrained is not large when the change in the reflectivity is small. 
     FIG. 10 shows a relation between the size of a connecting hole and the optimum exposure amount on the low reflective substrate. The exposure amount allowing formation of a ghost pattern is also shown in this figure. Assuming that the optimum exposure amount for forming a connecting hole having a diameter of 1 μm is normalized as  1 , the optimum exposure amount for forming a connecting hole having a diameter 0.4 μm is 1.2. As is shown in FIG. 10, the optimum exposure amount allowing formation of a ghost pattern is above the (normalized) optimum exposure amount, 1.2. Thus, a ghost pattern is not formed with the optimum exposure amount for forming a connecting hole having a diameter of 0.4 μm. 
     It is noted that the titanium nitride film underlying the overlay mark functions as an antireflection film for the aluminum film in the present embodiment. As a result, according to the principles described in conjunction with FIGS. 8 to  10 , a ghost pattern is not formed in the overlay mark portion in forming a connecting hole, and therefore a suitable resist pattern can be obtained. 
     While the aluminum film is used as a metal film in the above embodiment, the present invention is not limited to this and other films, for example of aluminum silicon, aluminum copper, copper or tungsten can be used. 
     In addition, while the titanium nitride film is used as an example of an antireflection film, the present invention is not limited to this and any of titanium film, amorphous silicon and silicon nitride films may be used. 
     Further, although a combination of the aluminum film and titanium nitride films as a structure of an interconnection film is used in the above embodiment, the present invention is not limited to this and any film which serves as an antireflection film under an oxide film can be used. A film capable of absorbing light or buffering light may be used as an antireflection film. The titanium and titanium nitride films can prevent reflection by absorbing light, whereas the amorphous silicon and nitride silicon films can prevent reflection by means of buffering light. 
     In addition, although alignment light passes through a photo mask in the above embodiment, the present invention is not limited to this and anything can be used as long as it can determine the position of the overlay mark on a wafer even when alignment light does not pass through a photo mask or a lens. 
     In a semiconductor device according to a first aspect of the invention, the overlay portion includes a pattern of an oxide film for the overlay mark and an antireflection film underlying the pattern of the oxide film, and therefore a ghost pattern is not formed in the overlay portion. As a result, a semiconductor device not having disconnection in the connecting hole portion can be obtained. 
     In the semiconductor device according to a second aspect of the invention, the antireflection film is provided on a metal film formed on a semiconductor substrate. As a result, reflection of light by the metal film can be prevented, thereby avoiding formation of a ghost pattern. Consequently, a semiconductor device without disconnection in a connecting hole portion can be obtained. 
     In the semiconductor device according to a third aspect of the invention, as the metal film is formed of aluminum, aluminum silicon, aluminum copper, copper or tungsten, a semiconductor device including an interconnection with high conductivity can be obtained. 
     In the semiconductor device according to a fourth aspect of the invention, since the antireflection film is formed of titanium or titanium nitride, the light causing a ghost pattern can effectively be absorbed. Further, reflection can be effectively prevented by buffering of light when aluminum silicon and nitride silicon are used as an antireflection film. 
     In the semiconductor device according to the a fifth aspect of the invention, as the size of a connecting hole is not larger than 0.4 μm□, it is effectively adaptable to high density integration of semiconductor devices. 
     In a method of manufacturing a semiconductor device according to a sixth aspect of the invention, since an antireflection film is formed under the oxide film located under a resist layer, a ghost pattern is not formed in the resist layer even when the resist layer is selectively irradiated by light using a halftone mask. 
     In the method of manufacturing a semiconductor device according to a seventh aspect of the invention, aluminum, aluminum silicon, aluminum copper, copper or tungsten is used for a metal film, and therefore a semiconductor device having an interconnection with high conductivity can effectively be obtained. 
     In the method of manufacturing a semiconductor device according to an eighth aspect of the invention, titanium, titanium nitride, amorphous silicon or silicon nitride is used for an antireflection film, and therefore formation of a ghost pattern can effectively be prevented. 
     In the method of manufacturing a semiconductor device according to a ninth aspect of the invention, as the size of the connecting hole is not larger than 0.4 μm□, a semiconductor device having a fine pattern can effectively be obtained. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.