Patent Publication Number: US-6664174-B2

Title: Semiconductor device and method for fabricating the same

Description:
This application is a division of prior application Ser. No. 09/238,171 filed Jan. 28, 1999, issued as U.S. Pat. No. 6,297,541. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device including a fuse circuit which can be disconnected by laser ablation and a method for fabricating the semiconductor device, and a laser system suitable to disconnect a fuse of the semiconductor device. 
     Semiconductor devices, such as memory devices of DRAMs, SRAMs, etc., logic devices, etc., are constituted by a very large number of elements, and a part of the circuit or of the memory cells are often inoperative due to various cause in their fabrication processes. In this case, when semiconductor devices partially defective circuits or memory cells are generally regarded as defective devices, the semiconductor devices have low fabrication yields, which might lead to fabrication cost increase. In view of this, recently such defective semiconductor devices have defective circuits or defective memory cells replaced by redundant circuits or redundant memory cells which have been prepared in advance, to create properly functioning devices. In some semiconductor devices, a plurality of circuits having functions different from each other are formed integrated and later those of certain functions are replaced, and in other semiconductor devices prescribed circuits are formed, and later characteristics of the semiconductor devices are adjusted. In such reconstruction of semiconductor devices, usually a fuse circuit having a plurality of fuses is formed on the semiconductor devices, and after operation tests, etc., the fuses are disconnected by laser beam irradiation. 
     A conventional semiconductor device including a fuse circuit and a method for fabricating the same will be explained with reference to FIGS. 11A-11C. FIG. 11A is a diagrammatic sectional view of the conventional semiconductor device, which shows a structure thereof. FIG. 11B is a plan view of the conventional semiconductor device, which shows the structure thereof. FIG. 11C is a diagrammatic sectional view of the conventional semiconductor device with a fuse disconnected, which shows the structure thereof. 
     A fuse  202  is formed on a substrate  200 , connected to a prescribed circuit for replacing the circuit. An inter-layer insulation film  204  for covering the fuse  202  is formed thereon. An interconnection layer  206  is formed on the inter-layer insulation film  204 , connected to the fuse  202  therethrough. A passivation film  211  is formed on the interconnection layer  206 . A part of the passivation film  211  on the fuse  292  is removed. A plurality of the fuses  202  are formed on the substrate  200  at a prescribed pitch (FIGS.  11 A and  11 B). 
     To disconnect the fuse  202  in such fuse circuit, a laser beam  208  is irradiated to a region where the fuse is formed, whereby the fuse  202  is rapidly heated by its absorbed energy to a high temperature and undergoes laser explosion (FIG.  11 C). 
     Here to further micronize the semiconductor device, it is necessary to further decrease a pitch between the fuses  202 , but a pitch P of the fuses  202  is determined by a spot size  210  of the laser beam  208  and alignment accuracy of the laser beam  208 . 
     A spot size of the laser beam  208  has a lower limit which is determined by a wavelength of the laser beam  208 , and the spot size  208  can be decreased as the laser beam has a shorter wavelength. However, when a wavelength of the laser beam is too short, there is a risk that the laser beam may pass through a region where the fuse  202  is not formed, arrives at the base semiconductor substrate and is absorbed therein, and cause thermal laser explosion. In a case that the semiconductor substrate is silicon, the laser beam has an about 1 μm wavelength, at which silicon substrates absorb small amounts of laser beams. That is, a lower limit is about 1.5-2.0 μm in spot size. 
     On the other hand, alignment accuracy is required for the prevention of a disadvantage that the base silicon substrate is damaged if the laser explosion regions overlap each other in blowing both fuses  202  adjacent to each other and also for the prevention of a disadvantage that in disconnecting one of fuses  202  adjacent to each other, the other is damaged or blown. Usually a lower limit of the alignment accuracy is about 0.5 μm. 
     Thus, a lower limit of the fuse pitch of the above-described conventional fuse disconnecting method is about 2.0-2.5 μm. 
     As a method for narrowing a pitch P of the fuses, a party of the applicants of the present application has proposed a method using a photoresist. 
     In the method using a photoresist, a photoresist  212  is formed on a semiconductor device shown in FIG. 11A (FIG.  12 A), a laser beam  208  whose power is low enough not to cause laser explosion is irradiated to expose the photoresist  212  (FIG.  12 B), the exposed photoresist  212  is developed to remove the photoresist  212  in the exposed region  214  (FIG. 12C, and a fuse  202  is removed by the usual etching process with the photoresist  212  as a mask (FIG.  12 D). 
     According to this method, the laser beam  208  may have a power which is sufficient only to expose the photoresist  212 , and it is not necessary that the power is high enough to laser explode the fuse  202  or the semiconductor substrate. Accordingly, the laser beam  208  can easily have a shorter wavelength and can have a spot size  210  which is decreased in accordance with a wavelength of the laser beam  208 . Accordingly, a fuse pitch P, which is determined by a spot size  210  of the laser beam can be decreased. 
     However, the method using a photoresist must additionally include a photoresist application step and a photoresist development step, a fuse etching step and a photoresist releasing step. Conventionally, it has caused no trouble that the test process following completion of the wafer process has lower cleanliness in comparison with that in the wafer process clean room, but in a case that a process, such as etching or others, is performed after the test, it is necessary to perform the test process in a clean room of high cleanliness so that dust on wafers does not pollute the etching system, or an etching system which is exclusively used for the fuse disconnection is installed, which leads to higher fabrication costs rather than simple increase of fabrication steps. 
     As described above, in the conventional fuse disconnecting method, it is difficult to narrow a fuse pitch corresponding to increased integration of a semiconductor device while depressing increase of fabrication steps and fabrication costs. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a structure of a semiconductor device including a fuse circuit which is easily higher integrated and does not add to fabrication costs and a method for fabricating the semiconductor device, and a laser system suitable to disconnect the fuses. 
     The present invention provides a semiconductor device and a method for fabricating the same for disconnecting a fuse by laser ablation, and a laser system suitable to disconnect fuses of the semiconductor device. Laser ablation is a phenomena that a laser beam of high intensity is irradiated to an object-to-be-irradiated to disconnect bonds of substances by energy of the irradiated laser beam and instantaneously sublimate the object-to-be-irradiated. 
     The conventional fuse disconnecting method using laser explosion due to absorption of a laser beam converts optical energy to vibrations of stretches, etc. of bonds of substances, i.e., to thermal energy for laser explosion, while laser ablation dissociates bonds of substances directly by optical energy, and is based on the phenomena which is quite different from laser explosion. 
     Due to such mechanism difference, in the laser ablation, a part a laser beam irradiated to vanishes with a boundary with respect to a part the laser beam has not been irradiated to remain in a beautiful facet. On the other hand, in the conventional laser explosion, the laser explosion takes place up to the vicinity of a part a laser beam is irradiated to, generating a number of particles and blurring the boundary between the laser beam irradiated part and a non-laser beam irradiated part. The cutting edge formed by the laser ablation is different from that formed by the laser explosion, so that the fuse disconnecting method can be distinguished by observing the cutting edge. 
     The laser ablation can thus beautifully remove a laser beam irradiated part but has a disadvantage that substantially all material is instantaneously removed without good controllability, with a result that not only a fuse but also a part of the semiconductor substrate therebelow are removed. 
     In view of this, the inventors of the present invention made earnest studies and found a material which is difficult to be sublimated by laser ablation. The inventors of the present invention are the first to have made it clear that a blocking layer of the material which is difficult to be sublimated by laser ablation is provided below the fuses to thereby stop the laser ablation on the blocking layer with good controllability. 
     Even in disconnecting fuses by the laser ablation, if the laser ablation can be controlled by the blocking layer, there is no risk that even with laser beams of short wavelengths, semiconductor substrates will not be damaged, as they are damaged by the conventional laser explosion. Accordingly a laser beam can have a small spot size corresponding to a wavelength of the laser beam. 
     In disconnecting two fuses adjacent to each other, even when both laser spots overlap each other, the blocking layer which is sufficiently thick can keep semiconductor substrates from damage. That is, a fuse pitch can be made smaller in accordance with decrease of a wavelength of the laser beam. 
     The laser ablation requires only a laser system to disconnect fuses and requires no additional etching system, etc., and increases neither fabrication steps and fabrication costs. 
     As the blocking layer for controlling the laser ablation, W (tungsten) film, for example, can be used. 
     That is, the above-described object is achieved by a semiconductor device comprising: a blocking layer formed on a substrate; an insulation film formed on the blocking layer; and a fuse formed on the insulation film, whereby a fuses can be disconnected by laser ablation with good controllability without damaging the base substrate. The fuses to be disconnected can be arranged at a very small pitch, which can improve integration of the fuse circuit. 
     The above-described object is also achieved by a semiconductor device including a memory cell region where a plurality of memory cells are formed, and a fuse circuit region where a fuse circuit for replacing a defective memory cell by a redundant memory cell is formed, the semiconductor device comprising: a blocking layer formed in the fuse circuit region; an insulation film formed on the blocking layer; and a fuse formed on the insulation film and formed of the same conducting layer as a conducting layer forming the memory cells or an interconnection layer formed in the memory cell region. The semiconductor device having this structure allows the fuses which can be disconnected by the laser ablation with good controllability to a replacement circuit to a redundant circuit of a memory device. The fuses to be disconnected by the laser ablation can be arranged at a very small pitch, which can improve integration of the memory device. 
     In the above-described semiconductor device, it is preferable that the fuse is formed of the same conducting layer as a metal interconnection layer formed in the memory cell region. The fuses can be formed of the same conducting layer as any of the metal interconnection layers forming the semiconductor device. 
     In the above-described semiconductor device, it is preferable that each of the memory cells includes a transfer transistor and a capacitor; and the fuse is formed of the same conducting layer as a gate electrode of the transfer transistor, a storage electrode of the capacitor, an opposed electrode of the capacitor or a bit line. The fuses may be formed of not only the metal interconnection layer but also of the same conducting layer as the above-described conducting layer forming the memory cells. 
     In the above-described semiconductor device, it is preferable that the device further comprises a cover film formed on the fuse. In the above-described semiconductor device the fuses can be disconnected by laser ablation, so that even in a case that the cover film is formed on the fuses, the fuses can be disconnected from above the cover film. 
     In the above-described semiconductor device, it is preferable that the device further comprises a polyimide film formed on the cover film for relaxing a stress in assembly process. In the above-described semiconductor device the fuses can be disconnected by laser ablation, so that even in a case that the polyimide film is formed on the cover film, the fuses can be disconnected from above the polyimide film. 
     In the above-described semiconductor device, it is preferable that the blocking layer is formed of a film including a tungsten film. Because tungsten film is difficult to be sublimated by the laser ablation, the blocking layer if formed of a film including a tungsten film, whereby laser ablation can be stopped by the blocking layer. 
     In the above-described semiconductor device, it is preferable that the fuse is formed of a film including a polycrystalline silicon film, an aluminum film or an aluminum alloy. These conducting materials are very easily sublimated by laser ablation, and can be used as fuses to be disconnected by laser ablation. 
     In the above-described semiconductor device, it is preferable that the device includes the fuse disconnected by laser ablation. 
     The above-described object is also achieved by a semiconductor device including a memory cell region where a plurality of memory cells are formed, and a fuse circuit region where a fuse circuit for replacing a defective memory cell by a redundant memory cell is formed, the semiconductor device comprising: a base semiconductor substrate; a layer or layers formed on the base semiconductor substrate; and a fuse formed on the layer or the layers in the fuse circuit region and formed of the same conducting layer as a conducting layer forming the memory cells or an interconnection layer formed in the memory cell region and disconnected by laser ablation, wherein a thickness of the layer or the layers is much thicker than a thickness of the fuse. 
     The above-described object is also achieved by a method for fabricating a semiconductor device comprising the steps of: forming a blocking layer on a substrate; forming an insulation film on the blocking layer; and forming a fuse on the insulation film. The above-described method for fabricating the semiconductor device can fabricate a semiconductor device which can disconnect the fuses by laser ablation. The fuses to be disconnected can be arranged at a very small pitch, which can improve integration of the fuse circuit. 
     In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises after the fuse forming step, a step of disconnecting the fuse by laser ablation. Fuses are disconnected by the laser ablation, whereby the fabrication process is not complicated, and no additional fabrication system is required. As a result, without increasing fabrication costs, the fuses can be arranged at a smaller pitch. 
     In the above-described method for fabricating the semiconductor device, it is preferable that in the step of disconnecting the fuse, the laser ablation is stopped by the blocking layer. The blocking layer is formed of a material which is difficult to be sublimated by laser ablation below the fuses, whereby the laser ablation can be stopped by the blocking layer with good controllability. 
     In the above-described method for fabricating the semiconductor device, it is preferable that in the step of disconnecting the fuse, the fuse is disconnected by a laser beam having a wavelength of not more than 500 nm. It is not necessary that the laser ablation considers absorption of a laser beam by the base substrate, so that laser beams of a below 1 μm-wavelength range, which is the absorption range of the substrate, can be used. By using such laser beams of short wavelengths the laser beams can have reduced spot sizes, whereby the fuses can be arranged at a small fuse pitch. 
     In the above-described method for fabricating the semiconductor device, it is preferable that the laser beam is third or more harmonics of a Nd:YAG laser or third or more harmonics of a Nd:YLF laser. 
     In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises after the fuse forming step, a step of forming a cover film for covering the fuse. In the laser ablation the layers are sublimated sequentially from above, so that even in a case that the cover film is formed on the fuses, the fuses can be disconnected from above the cover film. 
     In the above-described method for fabricating the semiconductor device, it is preferable that the method further comprises a step of forming a polyimide film for relaxing a stress in assembly process. Even in a case that the polyimide film is formed on the cover film, the fuses can be disconnected also from above the polyimide film. 
     In the above-described method for fabricating the semiconductor device, it is preferable that in the step of forming the blocking layer, the blocking layer including a tungsten film is formed. 
     The above-described object is also achieved by a laser system for disconnecting by laser ablation a fuse of the semiconductor device including a blocking layer formed on a substrate, an insulation film formed on the blocking layer and the fuse formed on the insulation film, the laser system comprising: a laser resonator for oscillating a laser beam having an oscillation wavelength of not more than 500 nm and an energy density sufficient to disconnect the fuse by laser ablation; a lens mechanism for condensing the laser beam emitted by the laser resonator into a required spot size; and an alignment mechanism for irradiating the laser beam outputted by the laser resonator to a required position on the semiconductor device. The laser system having this structure applies a laser beam at an arbitrary position on a wafer to disconnect the fuses by laser ablation. 
     In the above-described laser system it is preferable that the laser resonator outputs third or more harmonics of a Nd:YAG laser or third or more harmonics of a Nd:YLF laser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of a semiconductor device according to a first embodiment of the present invention, which shows a structure thereof. 
     FIG. 1B is a sectional view of the semiconductor device according to the first embodiment of the present invention, which shows a structure thereof. 
     FIG. 2 is a flow chart of the method for fabricating the semiconductor device according to the first embodiment of the present invention. 
     FIGS. 3A-3C and  4 A- 4 B are sectional views of the semiconductor device according to the first embodiment of the present invention in the steps of the method for fabricating the same, which explain the method. 
     FIG. 5 is a flow chart of the method for fabricating the semiconductor device according to a modification of the first embodiment (Part. 1). 
     FIG. 6 is a flow chart of the method for fabricating the semiconductor device according to a modification of the first embodiment (Part. 2). 
     FIG. 7 is a diagrammatic sectional view of the semiconductor device according to a second embodiment of the present invention, which shows a structure thereof. 
     FIG. 8 is a diagrammatic sectional view of the semiconductor device according to a modification of the second embodiment of the present invention, which shows a structure thereof (Part 1). 
     FIG. 9 is a diagrammatic sectional view of the semiconductor device according to a modification of the second embodiment of the present invention, which shows a structure thereof (Part 2). 
     FIG. 10 is a diagrammatic view of the laser system according to a third embodiment of the present invention. 
     FIGS. 11A-11C are diagrammatic views of the conventional semiconductor device, which show a structure thereof and the method fabricating the same (Part 1). 
     FIGS. 12A-12D are diagrammatic views of the conventional semiconductor device, which show the structure thereof and the method fabricating the same (Part 2). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     [A First Embodiment] 
     The semiconductor device and the method for fabricating the same according to a first embodiment of the present invention will be explained with reference to FIGS. 1A-1B,  2 ,  3 A- 3 C and  4 A- 4 B. 
     FIG. 1A is a plan view of the semiconductor device according to the present embodiment, which shows a structure thereof. FIG. 1B is a sectional view of the semiconductor device according to the present embodiment, which shows a structure thereof. FIG. 2 is a flow chart of the method for fabricating the semiconductor device according to the present embodiment. FIGS. 3A-3C and  4 A- 4 B are sectional views of the semiconductor device in the steps of the method for fabricating the semiconductor device, which show the method. 
     First, the structure of the semiconductor device according to the present embodiment will be explained with reference to FIGS. 1A and 1B. FIG. 1A is a plan view of the semiconductor device according to the present embodiment, which shows the structure thereof. FIG. 1B is a sectional view along the line  1 - 1 ′ in FIG.  1 A. 
     A blocking layer  12  for restraining laser ablation is formed on a substrate  10 . An inter-layer insulation film  14  is formed on the substrate  10  and the blocking layer  12 . A plurality of fuses  22  of, e.g., a conducting film, such as aluminum or polycrystal line silicon, are formed on the inter-layer insulation film  14 . A cover film  30  is formed on the fuses  22 . 
     As described above, the semiconductor device according to the present embodiment is characterized in that the blocking layer  12  for restraining laser ablation is formed below a region where the fuses  22  are formed. Owing to the blocking layer  12  thus provided, in disconnecting the fuses  22  by laser ablation, which lacks controllability, the laser ablation can be stopped by the blocking layer  12  with good controllability. 
     It is preferable that the blocking layer  12  is formed of a material which is difficult to be sublimated by laser ablation, and the blocking layer  12  can be provided specifically by a W (tungsten) film, or a laminated film of W/TiN/Ti or others. These film and laminated films have been conventionally widely used in the fabrication of semiconductor devices and have good processing alignment. 
     The blocking layer may be provided by all layers of the semiconductor device arranged below the fuses  22  because unessentially the blocking layer specially has the blocking function and preferably the layer disposed between the fuses  22  and the base semiconductor substrate is thicker. It is preferable that the thickness of the layer disposed between the fuse  22  and the base semiconductor substrate is much thicker than the thickness of the fuse  22 . 
     The fuses  22  may be formed of any material as long as the material is sufficiently susceptible to the laser ablation in comparison with the blocking layer  12 . Materials actually used in semiconductor devices are, e.g., polycrystalline silicon, metal suicide, Al, Al alloys, such as Al—Si—Cu, Al—Cu—Ti, etc., and Ti, TiN and their laminated films. 
     A pitch of the fuses  22  is determined by a spot size of a laser beam and an alignment allowance. A minimum spot size of a laser beam can be approximated to be about twice a wavelength of the laser beam. For example, when a wavelength of a laser beam is 0.355 μm, and an alignment allowance is 0.5 μm, a pitch of the fuses  22  is about 1.2 μm. Accordingly, in comparison with conventional semiconductor devices, the fuses  22  can have a very small pitch. 
     The blocking layer has a film thickness which can endure twice laser ablation, whereby there is no risk that even when spots of laser beams for disconnecting adjacent fuses overlap each other, the base substrate will not be damaged, and it is not necessary to ensure an alignment allowance. The fuses  22  can have accordingly further smaller pitches. 
     It is unnecessary that the laser ablation considers the absorption by the base substrate, which allows a laser beam to have a shorter wavelength and accordingly allows a fuse pitch to be further smaller. Accordingly the semiconductor device can be further micronized. 
     As described above, according to the present embodiment, the blocking layer of a material which is difficult to be sublimated by the laser ablation is provided below the fuses, whereby the semiconductor device can have a structure which allows the fuses to be disconnected by the laser ablation with good controllability. 
     The laser ablation does not affect the base substrate even when a laser beam has a short wavelength, which permits a spot size of a laser beam to be very small corresponding to a wavelength of the laser beam. This allows a fuse pitch to be smaller, which enables the semiconductor device to be more highly integrated. 
     Next, the method for fabricating the semiconductor device according to the present embodiment will be explained with reference to FIGS. 2,  3 A- 3 C and  4 A- 4 B. 
     The method for fabricating the semiconductor device according to the present embodiment is characterized, as shown in FIG. 2, by comprising the step of forming the blocking layer for restraining laser ablation on a substrate (Step S 11 ), the step of forming fuses on the blocking layer through an insulation film (Step S 12 ), the step of forming a cover film for covering the fuses (Step S 13 ), the test step of making an operation test on a circuit (Step S 14 ), and the step of disconnecting a fuse by the laser ablation (Step S 15 ). 
     The method for fabricating the semiconductor device according to the present embodiment will be detailed by means of a specific structure thereof. 
     First, the blocking layer  12  for restraining laser ablation is formed on the substrate  10  with a prescribed device formed on (Step S 11 ). The blocking layer  12  is formed, e.g., by depositing an about 350 nm-thick W film on the substrate  10  by CVD method and patterning the W film in a prescribed pattern. 
     Then, the inter-layer insulation film  14  for covering the blocking layer  12  is formed. The inter-layer insulation film  14  is formed by, e.g., depositing an about 1 μm-thick silicon oxide film by CVD (FIG.  3 B). 
     Subsequently, a conducting film to be the fuses is deposited on the inter-layer insulation film  14 . For example, an about 100 nm-thick TiN film  16 , an about 1 μm-thick Al—Cu—Ti film  18  and an about 50 nm-thick TiN film  20  are deposited. 
     The laminated film of the TiN film  20 /Al—Cu—Ti film  18 /the TiN film  16  is patterned to form the fuses  22  of the laminated film over the blocking layer  12  (Step S 12 , FIG.  3 C). The fuses  22  are formed over the blocking layer  12 , for example, in an about 0.7 μm-width and at a 2 μm-pitch. 
     Then, the cover film  30  is formed on the substrate with the fuses  22  formed on (Step S 13 ). The cover film  30  comprising, e.g., an SiON film  24 , an SOG film  26  and an SiN film  28  is formed, e.g., by depositing the SiON film  24  in an about 100 nm-thick, next applying the SOG film  26  in an about 1 μm-thick and planarizing the surface thereof, then depositing the SiN film  28  by CVD in an about 1 μm-thick (FIG.  4 A). 
     Subsequently, openings for exposing bonding pads (not shown) are formed in the cover film by the usual lithography and etching. 
     In conventional methods for fabricating semiconductor devices in which fuses are disconnected by laser explosion, it is necessary to cause the laser explosion so that the cover film is removed or is thinned, and usually etching for forming fuse windows concurrently with forming openings for bonding pads is performed. However, in the method for fabricating the semiconductor device according to the present embodiment using laser ablation, it is not necessary to form fuse windows in the cover film  30  because the films are sublimated sequentially from above. Accordingly, in the step of patterning the cover film  30  it is sufficient to form only the bonding pad openings of a 100 μm-order, and it is unnecessary to form micronized fuse windows. This can simplify the lithography step. This does not mean that the fuse windows should not be formed as in the conventional methods. Formation of the fuse windows has a merit that the films on the fuses can be decreased, which allows a total power of the laser ablation to be decreased, and furthermore, merits of increased throughputs and laser ablation control with higher accuracy. 
     Then, a prescribed circuit test is performed to locate a defective memory cell to locate a fuse to be disconnected for replacement of the defective memory cell by a redundant memory (Step S 14 ). At this time, coordinates of a location of the fuse to be disconnected on the wafer are stored in advance for use in disconnecting the fuse. 
     Then, the fuse  22  located by the circuit test is disconnected by the laser ablation (Step S 15 , FIG.  4 B). In the above-described structure of the semiconductor device, the fuse  22  could be disconnected, the laser ablation being stopped by the blocking layer  12  under conditions of a 355 nm-oscillation wavelength, a 40 nsec pulse width, a 100 μj power and a 2.3 μm spot size. As for the spot size of the laser beam, an about 0.71 μm spot size can be also used. The spot size can be reduced until about twice a wavelength of the laser beam. 
     Then, the circuit test is repeated as required, and proper products are assembled. 
     As described above, according to the present embodiment, a fuse  22  is disconnected by the laser ablation, which makes it unnecessary to form fuse windows in the cover film  30 . This simplifies the lithography step for patterning the cover film  30 . To disconnect a fuse  22  neither a photoresist nor an etching system is necessary, which needs no additional etching system exclusively used in the fuse disconnection. 
     In the laser ablation a laser system can independently shorten a wavelength, and can have a very small spot size. Accordingly, a region for the fuse circuit can have a drastically reduced area, whereby the semiconductor device can have higher integration. 
     In some cases, a polyimide film is formed on the cover film  30  as cushioning material for restraining influence due to thermal expansion difference between the semiconductor chip and a plastic package when the former is loaded in the latter. The method for fabricating the semiconductor device according to the present embodiment is effective in these cases. That is, in the method for fabricating the semiconductor device according to the present embodiment using the laser ablation, a target is sublimated sequentially from the surface thereof, and even in the case that a polyimide film is formed on the cover film, a fuse can be disconnected through the polyimide film. 
     As exemplified in FIG. 5, the blocking layer is formed on the substrate (Step S 21 ), the fuses are formed over the blocking layer through the inter-layer insulation film (Step S 22 ), the cover film for covering the fuses is formed (Step S 23 ), the polyimide film for covering the cover film is formed (Step S 24 ), a prescribed operation test is conducted (Step S 25 ), and a fuse is disconnected by the laser ablation (Step S 25 ). 
     In the present embodiment, the operation test is conducted after the cover film is formed, but the operation test can be conducted if at least the final interconnection layer is formed. Fuses can be disconnected after the operation test. Accordingly, in the method for fabricating the semiconductor device as exemplified in FIG. 6, which comprises the step of forming the blocking layer on a substrate (Step S 31 ), the step of forming fuses over the blocking layer through the insulation film (Step S 32 ), the step for forming the cover film for covering the fuses (Step S 33 ), and the step of forming the polyimide film for covering the cover film (Step S 34 ), the operation test step (Step S 35 ) may be performed before the cover film forming step or after the polyimide film forming step. The fuse disconnecting step (Step S 36 ) may be performed before the cover film forming step or the polyimide film forming step if the fuse disconnecting step is after the operation test step (Step S 35 ). 
     [A Second Embodiment] 
     A semiconductor device according to a second embodiment of the present invention will be explained with reference to FIG.  7 . 
     FIG. 7 is a diagrammatic view of the structure of the semiconductor device according to the present embodiment. 
     In the present embodiment one example of application of the semiconductor device according to the first embodiment and the method for fabricating the same to a DRAM will be explained. In a DRAM, a fuse circuit is used to replace an address circuit for designating memory cells, so that a required fuse of the fuse circuit is disconnected to replace an address of the defective memory cell by a redundant memory cell. By thus constituting the DRAM, even when a memory cell is defective, the memory cell is replaced by a redundant memory cell, whereby the DRAM is saved from becoming generally defective. 
     On a silicon substrate  40  there are formed a memory cell region  42  where a plurality of memory cells are formed, a peripheral circuit region  44  where a peripheral circuit for driving the memory cells is formed, and a fuse circuit region  46  where a plurality of fuse circuits for replacing a defective memory cell by a redundant memory cell is formed. 
     A device isolation film  48  is formed on the silicon substrate  40 . On the silicon substrate  40  in the memory cell region  42  a transfer transistor including a gate electrode  50 , and a source/drain diffused layer  52 ,  54  is formed. A bit line  56  is connected to the source/drain diffused layer  52 . A fin-shaped storage electrode  58  is connected to the source/drain diffused layer  54 . The storage electrode  58  is covered with an opposed electrode  60  through a dielectric film, and the storage electrode  58 , the dielectric film and the opposed electrode  60  constitute a capacitor. In the memory cell region  42  there are thus formed a plurality of the memory cells each including the transfer transistor and the capacitor. The structure of the memory cell shown in FIG. 7 is detailed in, e.g., Japanese Patent Publication No. 08-28476 of the applicant of the present application. In the peripheral circuit region there are formed a plurality of peripheral transistors (not shown) forming a peripheral circuit. On the silicon substrate  40  with the memory cells and the peripheral transistors formed on there is formed an insulation film  62 . A blocking layer  64  of a W film for restraining the laser ablation is formed on the inter-layer insulation film  62  in the fuse circuit region  46 . An inter-layer insulation film  66  is formed on the inter-layer insulation film  62  with the blocking layer  64  formed on. On the inter-layer insulation film  66  there are formed strapping word lines  68  of the same Al alloy layer, and an interconnection layer  70  interconnecting the peripheral transistors to constitute the peripheral circuit. An inter-layer insulation film  72  is formed on the inter-layer insulation film  66  with the strapping word lines  68  and the interconnection layer  70  formed on. A interconnection layer  74  and fuses  76  are formed of the same Al alloy on the inter-layer insulation film  72 . A cover film  78  is formed on the inter-layer insulation film  72  with the interconnection layer  74  and the fuses  76  formed on. 
     As described above, the semiconductor device according to the present embodiment is characterized in that the semiconductor device according to the first embodiment is applied to the fuse circuit of a DRAM for the replacement to the redundant circuit. That is, the fuses of the fuse circuit is formed of a second metal interconnection layer forming the DRAM, and the blocking layer  64  for restraining the laser ablation is formed below the fuses  76 . This structure of the semiconductor device enables a fuse to be disconnected by the laser ablation with good controllability. 
     The fuse disconnection can be performed in the same way as in the method for fabricating the semiconductor device according to the first embodiment. 
     As described above, according to the present embodiment, as the fuse circuit for the replacement by the redundant circuit in a DRAM, the fuse circuit including the blocking layer  64  of a material which is difficult to be sublimated by the laser ablation is applied, whereby the semiconductor device can have a structure which permits a fuse to be disconnected by the laser ablation with good controllability. 
     In the laser ablation, laser beams of a short wavelength do not affect the base substrate, which permits laser beams to have a very small spot size corresponding to their wavelength. This enables a fuse pitch to be smaller, which can improve integration of the DRAM. 
     In the present embodiment, the interconnection layer forming the fuses  76  is the second metal interconnection layer but is not essentially the second metal interconnection layer. That is, in the laser ablation, the target is sublimated sequentially from the top layer, so that the fuses can be formed of any of the interconnection layers. It is not essential that the interconnection layer forming the fuses  76  is the uppermost interconnection layer, but as exemplified in FIG. 8, the fuses  76  may be formed of a first metal interconnection layer. Even in a case that three or more metal interconnection layers are included, the fuses  76  may be formed of any of the metal interconnection layers. However, decreasing the films on the fuses allows a total power of the laser ablation to be decreased, and furthermore, merits of increased throughputs and laser ablation control with higher accuracy. Thus, it is preferable that the fuses is formed of an uppermost conducting layer. 
     If the blocking layer  64  of a material which is difficult to be sublimated by the laser ablation is disposed below the fuses  76 , the fuses  76  may be formed of not only the metal interconnection layers but also other conducting layers. As exemplified in FIG. 9, the fuses  76  can be formed of the conducting layer forming the opposed electrode  60 , and the blocking layer  64  can be formed therebelow. The blocking layer  64  may be an additional film or may be formed of conducting layer positioned below the fuses  76 , e.g., the same conducting layer as the storage electrode  58 , the bit line  56  and the word lines  50 . 
     It is also possible that the fuses are formed of the same conducting layer as the storage electrode or the bit line. 
     In the present embodiment, the fuse circuit of the first embodiment is applied to a DRAM including fin-shaped capacitors but may be applied to DRAMs of other various structures. The fuse circuit is also applicable to other memory devices, such as SRAMs, etc. 
     [A Third Embodiment] 
     The laser system according to a third embodiment of the present invention will be explained with reference to FIG.  10 . 
     FIG. 10 is diagrammatic view of the laser system according to the present embodiment, which shows a structure thereof. 
     In the present embodiment a laser system which is applicable to the method for fabricating the semiconductor device according to the second embodiment will be explained. 
     The laser system according to the present embodiment mainly comprises a laser resonator  100  for oscillating laser beams, a laser diode  120  for optical pumping a laser light source of the laser resonator, a lens mechanism  118  for condensing a laser beam outputted by the laser resonator  110  to a required spot size, and a beam alignment mechanism  130  for irradiating the laser beam outputted by the laser resonator  100  to a required position on a wafer  142  mounted on a stage  140 . 
     A laser beam  122  emitted by the laser diode  120  is incident on the laser resonator  100  through the lens mechanism  124  to optically pump the laser light source of the laser resonator  100 . 
     The laser resonator  100  includes a laser light source  102 , two sheets of mirrors  104 ,  106  arranged with the laser light source  102  disposed therebetween for sustaining stimulated emission, control mechanisms  108 ,  110 ,  112  for converting or adjusting a frequency of a laser beam, a Q switch  114  for rapidly changing Q of the laser resonator, which are arranged along an optical axis  116  of the laser beams. The laser light source  102  can be, e.g., a 442 nm-oscillation wavelength He—Cd gas laser, a Nd:YAG solid laser of a 355 nm-third harmonics oscillation wavelength, and a Nd:YLF solid laser of a 349 nm-third harmonics oscillation wavelength. The mirror  106  preferably reflects a laser beam substantially at a 100% reflectance. The mirror  104  transmits a part of a laser beam, and the laser beam transmitted by the mirror  104  is used as an output laser beam. The control mechanisms  108 ,  110 ,  112  are not necessary in a case that a laser beam can be used as it is, but because of the control mechanisms  108 ,  110 ,  112 , a wavelength of a laser beam can be converted to a required oscillation wavelength by, e.g., optical parametric oscillation or nonlinear frequency conversion, such as doubling, tripling or quadrupling. The laser beam transmitted by the mirror  104  is incident on the beam alignment mechanism  130  through the lens mechanism  118 . 
     The beam alignment mechanism  130  includes a plurality of reflecting plates  132 ,  134 ,  136 ,  138 . The reflecting plates  132 ,  134 ,  136 ,  138  are suitably controlled to irradiate a laser beam outputted by the laser resonator  100  to a required position on the wafer  140 . A laser beam irradiated to the beam alignment mechanism  130  and the wafer  140  is formed into a required spot size. 
     The laser resonator  100 , the laser diode  120 , the beam alignment mechanism  130  and the stage  140  are concurrently controlled by laser control means  150  to irradiate a laser beam outputted under required laser oscillation conditions to a required position on the wafer  142 . Coordinate information of a position on the wafer  142  is given to the laser control means when a laser beam is irradiated in data of coordinates of a fuse-to-be-disconnected located by the operation test conducted beforehand on a chip  146 . 
     The laser system having such structure can irradiate a laser beam to an arbitrary position on a wafer to disconnect a fuse by the laser ablation.