Process for fabricating semiconductor integrated circuit device having polycide line and impurity region respectively exposed to contact holes different in depth

[0002] This invention relates to a process for fabricating a semiconductor integrated circuit device and, more particularly, to a process for fabricating a semiconductor device having contact holes to a polycide signal line and an impurity region.

[0003] The integration density of semiconductor integrated circuit has been increased through a scaling down of circuit components/signal lines. The signal lines of a semiconductor integrated circuit device are getting narrow. The narrow signal line increases the resistance against an electric signal, and the large-resistance signal line retards the signal propagation. In order to decrease the resistance, a polycide structure has been proposed for the signal line. The polycide structure is a lamination of a polysilicon layer and a refractory metal silicide. A multi-layered wiring structure is employed in the semiconductor integrated circuit device, and upper-level signal lines are connected to lower-level signal lines through contact holes. The contact holes are also miniaturized in the semiconductor integrated circuit device. If conductive metal is deposited over an inter-level insulating layer having miniature contact holes by using a sputtering technique, the step-coverage is poor, and the conductive metal does not fill the miniature contact holes. If the conductive metal layer is patterned into an upper-level signal line, the contact resistance between the upper-level signal line and a lower-level signal line is large or unstable between products. For this reason, it is not desirable to deposit the conductive metal over the inter-level signal line through the sputtering.

[0004] Polysilicon is usually deposited through a chemical vapor deposition, and the step coverage is improved. For this reason, it is appropriate to form the upper-level signal layer of polysilicon or the upper-level signal line with the polycide structure. When a manufacturer connects an upper-level polycide line through a miniature contact hole to a lower-level polycide line, the manufacturer encounters a problem in large contact resistance between the polysilicon layer of the upper-level polycide line and the refractory metal silicide layer of the lower-level polycide line.

[0005] A solution is proposed in Japanese Patent Publication of Unexamined Application No. 60-15950. According to the solution disclosed in the Japanese Patent Publication of Unexamined Application, the refractory metal silicide layer is partially removed from the lower-level polycide line, and the polysilicon layer of the upper-level polycide layer is directly connected through a contact hole to the exposed polysilicon layer of the lower-level polycide line. The direct contact between the polysilicon layers decreases the contact resistance.

[0006] When a manufacturer forms the direct contact between the polysilicon layers in a semiconductor dynamic random access memory device, the direct contact is realized through a process shown in FIGS. 1A to 1C.

[0007] The prior art process starts with preparation of a silicon substrate 1, and a field oxide layer 2 is selectively grown on the major surface of the silicon substrate 1. The field oxide layer 2 defines an active area in the major surface, and the active area is oxidized so that a gate oxide layer 3 is grown.

[0008] Polysilicon is deposited over the entire surface of the resultant structure, and refractory metal silicide is laminated on the polysilicon layer. The refractory metal silicide layer is produced through an alloying technique between the polysilicon layer and a refractory metal layer. Word lines 4a/4bare expected to be low in resistance, and the polysilicon layer and the refractory metal silicide layer are thick. In this instance, the polysilicon layer is 1000 angstroms thick, and the refractory metal silicide layer is also 1000 angstroms thick. A photo-resist etching mask is formed on the refractory metal silicide layer by using photo-lithographic techniques, and the lamination is patterned into word lines 4a/4b. The word lines 4a/4bhave the polycide structure, and a part of the word line 4aserves as a gate electrode on the gate oxide layer 3.

[0009] Subsequently, n-type dopant impurity is ion implanted into the active area, and forms n-type source/drain regions 5a/5bon both side of the channel region under the gate oxide layer 3. Insulating material is deposited over the entire surface of the resultant semiconductor structure, and forms an interlevel insulating layer 6. A photo-resist etching mask 7 is formed on the interlevel insulating layer 6 by using the lithographic techniques, and has openings over the word line 4band the n-type drain region 5a. Using the photo-resist etching mask, the inter-level insulating layer 6 is selectively etched through a reactive ion etching technique. The reactive ion etching is continued for a time long enough to reach the surface of the n-type drain region 5a, because the n-type drain region 5ais deeper than the word line 4b. The time for the reactive ion etching is, by way of example, seventy seconds. Contact holes 8a/8bare formed in the inter-level insulating layer 6, and the word line 4band the n-type drain region 5aare exposed to the contact holes 8aand 8b, respectively, as shown in FIG. 1A. Although the reactive ion etching is continued after reaching the refractory metal silicide layer of the word line 4b, the refractory metal silicide layer is resistive against the etchant, and the contact holes 8a/8bdifferent in depth are concurrently formed in the interlevel insulating layer 6.

[0010] Subsequently, the resultant semiconductor structure is subjected to a dry etching or a plasma etching, and the refractory metal silicide layer of the word line 4bis partially etched away. As a result, the polysilicon layer of the word line 4bis exposed to the contact hole 8aas shown in FIG. 1B. The photo-resist etching mask 7 is stripped off.

[0011] Polysilicon is deposited over the entire surface of the semiconductor structure, and the polysilicon layer is held in contact with the polysilicon layer of the word line 4band the n-type drain region 5ain the contact holes 8a/8b. Subsequently, refractory metal is deposited over the polysilicon layer. The refractory metal is converted to refractory metal silicide, and the refractory metal silicide layer is laminated on the polysilicon layer. A photo-resist etching mask (not shown) is formed on the refractory metal silicide layer, and the refractory metal layer and the polysilicon layer are selectively etched away. Upper-level signal lines 9aand 9bare formed on the inter-level insulating layer 6, and have the polycide structure as shown in FIG. 1C.

[0012] The polysilicon layers of the upper-level signal lines 9a/9bare directly held in contact with the polysilicon layer of the word line 4band the n-type drain region 5aof the single crystal silicon. For this reason, low contact resistance is achieved through the prior art fabrication process. However, the manufacturer encounters a problem in that leakage current flows between the upper-level signal line 9band the silicon substrate 1.

[0013] It is therefore an important object of the present invention to provide a fabrication process through which contact holes different in depth are concurrently formed without increase of contact resistance and the serious leakage current.

[0014] The present inventor contemplated the problem inherent in the prior art structure, and noticed that the drain region had been depressed as shown in FIG. 2. The present inventor thought that the etchant had removed the surface portion of the drain region 5aduring the dry etching for the refractory metal silicide. As described hereinbefore, the word lines 4a/4bhad the refractory metal silicide layer of the order of 1000 angstroms thick, and the dry etching was continued for the long time. Even though the manufacturer used the dry etchant large in selectivity, the drain region 5awas partially etched, and the depression was unavoidably formed in the drain region 5a. The source/drain regions 5a/5bwere getting shallow, and the reduction in depth seriously affected the electrical isolation between the drain region 5aand the silicon substrate 1. The present inventor concluded that the dry etching for the refractory metal silicide was never ignorable in the semiconductor integrated circuit device of the next generation. If the deep contact hole 8bwere covered with a photo-resist etching mask, the drain region 5awould be prevented from the dry etchant. However, the photo-resist etching mask made the process complicated. Moreover, it was difficult to remove the photo resist from the deep contact hole 8bafter the dry etching.

[0015] To accomplish the object, the present invention proposes to remove a refractory metal silicide layer from a lower polycide line before completion of a deep contact hole.

[0016] In accordance with one aspect of the present invention, there is provided a process for fabricating a semiconductor device comprising the steps of preparing a semiconductor structure including a first lower-level conductive line having a semiconductor layer and a refractory metal silicide layer laminated on the semiconductor layer and a second lower-level conductive line without a refractory metal silicide layer, forming an inter-level insulating layer over the semiconductor structure having a first portion over the first lower-level conductive line and a second portion over the second lower-level conductive line, etching the first portion and the second portion until the refractory metal silicide layer of the first lower-level conductive line is exposed to a first contact hole, the second lower-level conductive line being still covered with a remaining second portion, removing a part of the refractory metal silicide layer exposed to the first contact hole from the first lower-level conductive line and etching the remaining second portion for exposing the second lower-level conductive line to a second contact hole.

[0017] The features and advantages of the fabrication process will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which:

[0018] FIGS. 1A to 1C are cross sectional views showing the prior art process for forming the contact holes disclosed in Japanese Patent Publication of Unexamined Application No. 60-15950;

[0019] FIG. 2 is a cross sectional view showing, in a large scale, the structure around the drain region of the prior art dynamic random access memory device;

[0020] FIG. 3 is a flow chart showing essential steps of a process for fabricating a semiconductor integrated circuit device according to the present invention;

[0021] FIGS. 4A to 4H are cross sectional views showing a process for fabricating a semiconductor dynamic random access memory device; and

[0022] FIG. 5 is a cross sectional view showing the structure of another semiconductor integrated circuit device.

[0023] FIG. 3 illustrates essential steps of a process embodying the present invention. The process contains the step of forming lower-level conductive lines over a semiconductor substrate, and the step is labeled with SP1. In this instance, the lower-level conductive lines have the polycide structure, i.e., the laminated structure of a polysilicon layer and a refractory metal silicide layer. The lower-level conductive lines with the polycide structure are simply referred to as “polycide lines” hereinbelow.

[0024] The step SP1 is followed by step SP2. In the step SP2, impurity regions are formed in the semiconductor substrate. The polycide lines extend on a level higher than the impurity regions. In this instance, the impurity regions are self-aligned with selected one or ones of the polycide lines, and serve as other lower-level conductive lines. The self-alignment is not indispensable feature of the present invention, and the impurity regions may be formed in the semiconductor substrate independently of the polycide lines.

[0025] The step SP2 is followed by step SP3. The polycide lines and the impurity regions are covered with an inter-level insulating layer in the step SP3. The inter-level insulating layer is different in thickness between a portion over the impurity region and another portion over the polycide line. When insulating material is deposited over a non-flat surface, the non-flat configuration may be transferred to the upper surface of the inter-level insulating layer. In order to make the inter-level insulating layer over the polycide lines thinner than the inter-level insulating layer over the impurity regions, the inter-level insulating layer may be polished.

[0026] The step SP3 is followed by step SP4. In the step SP4, contact holes are formed in the inter-level insulating layer. The inter-level insulating layer is different in thickness, and, accordingly, the contact hole over the impurity region is deeper than the contact hole over the polycide line. The contact hole over the polycide line and the contact hole over the impurity region are referred to as “shallow contact hole” and “deep contact hole”, respectively. The step SP4 is broken down into sub-steps SP41, SP42 and SP43.

[0027] In the sub-step SP41, the inter-level insulating layer is selectively etched away until the refractory metal silicide is exposed to the shallow contact hole. For the selective etching, an appropriate etching mask may be formed on the inter-level insulating layer. The shallow contact hole is firstly completed in the process according to the present invention. A part of the refractory metal silicide layer is exposed to the shallow contact hole, and the deep contact hole is incomplete.

[0028] Subsequently, the part of the refractory metal silicide layer is etched away from the polycide line in the sub-step SP42. The impurity region is still covered with the insulating layer, and the impurity region is never etched away in the sub-step SP42.

[0029] After the removal of the refractory metal silicide, the inter-level insulating layer is selectively etched away in the sub-step SP43 until the impurity region is exposed to the deep contact hole. The etching mask may prevent the other portion of the inter-level insulating layer from the etchant. When the impurity region is exposed, the deep contact hole is completed, and the etching is stopped. As a result, any depression is formed in the surface portion of the impurity region.

[0030] Though not shown in FIG. 3, upper-level conductive lines are formed on the inter-level insulating layer, and are held in contact with the polycide line and the impurity region, respectively. The upper-level conductive line held in contact with the polycide line may have the polycide structure, and the polysilicon layer of the upper-level conductive line is directly held in contact with the polysilicon layer of the lower-level conductive line. This results in reduction of contact resistance. Moreover, the impurity region is not substantially etched away, and remains thick enough to prevent the semiconductor structure from leakage current.

[0031] The process described hereinbefore is applicable to fabrication of a semiconductor dynamic random access memory device as shown in FIGS. 4A to 4H. The process starts with preparation of a p-type silicon substrate 21. The major surface of the silicon substrate 21 is thermally oxidized, and the major surface is covered with a silicon oxide layer 22. Silicon nitride is deposited over the silicon oxide layer 22, and a silicon nitride layer 23 is laminated on the silicon oxide layer 22. Photo-resist solution is spread over the silicon nitride layer 23, and is baked so that a photo-resist layer (not shown) is laminated on the silicon nitride layer 23. A pattern image for active areas is transferred from a photo-mask to the photo-resist layer, and a latent image for active areas is formed in the photo-resist layer. The latent image is developed, and a photo-resist etching mask (not shown) is formed on the silicon nitride layer 23. In other words, the photo-resist etching mask is formed by using photo-lithographic techniques.

[0032] Using the photo-resist etching mask, the silicon nitride layer 23 is selectively etched away, and the silicon oxide layer 22 is also selectively etched. As a result, the active areas are covered with the silicon oxide layer/silicon nitride layer 23, and a remaining area is exposed. The photo-resist etching mask is stripped off.

[0033] Using the silicon nitride layer 23 as a mask, the silicon substrate 21 is selectively oxidized, and a field oxide layer 24 is selectively grown to 4000 angstroms thick as shown in FIG. 4A.

[0034] The silicon nitride layer 23 and the silicon oxide layer 22 are etched away, and the active areas are exposed to the openings defined by the field oxide layer 24. Only one active area 25 is shown in FIGS. 4A to 4H, and description is focused on a semiconductor structure fabricated on the active area and adjacent field oxide layer.

[0035] The active area is thermally oxidized, and a thin silicon oxide layer is grown to 100 angstroms thick. Polysilicon is deposited to 1000 angstroms thick over the entire surface of the resultant semiconductor structure, and phosphorous is doped into the polysilicon layer. Tungsten silicide is deposited to 1000 angstroms thick by using a chemical vapor deposition technique, and a tungsten silicon layer is laminated on the n-type doped polysilicon layer.

[0036] A photo-resist etching mask (not shown) is formed on the tungsten silicide layer by using the lithographic techniques, and the tungsten silicide layer, the n-type doped polysilicon and the silicon oxide layer are selectively etched. A gate oxide layer 26 is formed from the silicon oxide layer, and word lines 27a/27bare formed from the lamination of the n-type doped polysilicon layer and the tungsten silicide layer. FIG. 4B shows the word line 27aextending over the field oxide layer 24 and the other word line 27bextending over the gate oxide layer 26. Though not shown, the word lines 27a/27bfurther extend over other gate oxide layers (not shown) and the field oxide layer 24. The word line 27aconsists of an n-type doped polysilicon layer 27cand a tungsten silicide layer 27d, and the other word line 27balso consists of an n-type doped polysilicon layer 27eand a tungsten silicide layer 27f. The word line 27bon the gate oxide layer 26 serves as a gate electrode of an n-channel enhancement type access transistor of a memory cell.

[0037] Using the gate electrode 27bas an ion-implantation mask, phosphorous is ion implanted into the active area 25 at dosage of 1×10¹⁹atom/square-cm, and forms source/drain regions 28a/28bon both sides of a channel region under the gate oxide layer 26.

[0038] Subsequently, silicon oxide is deposited to 1000 angstroms thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and forms a silicon oxide layer 29. Boro-phosphosilicate glass is deposited to 8000 angstroms thick, and forms a boro-phosphosilicate glass layer 30. The boro-phosphosilicate glass layer 30 is subjected to a chemical mechanical polishing, and a flat surface is created as shown in FIG. 4C. The silicon oxide layer 29 and the boro-phosphosilicate glass layer 30 as a whole constitute an inter-level insulating layer 31, and the inter-level insulating layer 31 over the drain region 28ais thicker than the inter-level insulating layer 31 over the word line 27a. In fact, the inter-level insulating layer 31 over the word line 27ais of the order of 3000 angstroms thick, and is increased to 7000 angstroms thick over the drain region 28a.

[0039] Subsequently, a photo-resist etching mask 32 is formed on the inter-level insulating layer 31 by using the photo-lithographic techniques. The photoresist etching mask 32 has openings 32a/32bas shown in FIG. 4D. The opening 32ais located over the word line 27a, and the other opening 32bis located over the drain region 28a.

[0040] Using the photo-resist etching mask 32, the inter-level insulating layer 31 is selectively etched for forming contact holes 33a/33bby using a plasma etching system. The contact holes 33a/33bare 0.3 micron to 0.4 micron in diameter, which is narrower than the diameter of the prior art contact holes ranging between 0.5 micron and 0.6 micron. In the plasma etching system, gaseous mixture of CF₄and CHF₃is supplied to the reactor (not shown) at 10 pa. The flow rate of CF₄and the flow rate of CHF₃are regulated to 30 sccm and 50 sccm, respectively. The plasma etching is continued for 30 seconds, and the tungsten silicide layer 27dis exposed to the contact hole 33a. However, the other contact hole 33bis incomplete, and the drain region 28ais still covered with the remaining inter-level insulating layer 31 as shown in FIG. 4D.

[0041] Subsequently, using the photo-resist etching mask 32, again, a part of the tungsten silicide layer 27dis etched away from the word line 27aby using the plasma etching system. Gaseous mixture of HBr/SF₆is supplied to the reactor (not shown) at 60 milli-torr, and the flow rate of HBr and the flow rate of SF₆are regulated to 70 sccm and 60 sccm, respectively. The plasma etching is continued for 10 seconds, and the n-type doped polysilicon layer 27cis exposed to the contact hole 33aas shown in FIG. 4E. The gaseous etchant has a large selectivity between the tungsten silicide and the boro-phosphosilicate glass/silicon oxide, and slightly decreases the thickness of the interlevel insulating layer 31. The decrease is ignorable. For this reason, the contact hole 33bdoes not reach the drain region 28a.

[0042] The gaseous mixture of CF₄and CHF₃is supplied to the reactor (not shown) under the same conditions, and the plasma etching is continued for 40 seconds. The contact hole 33breaches the surface of the drain region 28aas shown in FIG. 4F, and is completed. The gaseous etchant has a large selectivity between the boro-phosphosilicate glass/silicon oxide and the n-type doped polysilicon, and slightly decreases the thickness of the n-type doped polysilicon layer 27c. The decrease is ignoreable. After completion of the contact holes 33a/33b, the photo-resist etching mask 32 is stripped off.

[0043] Subsequently, phosphorous-doped polysilicon is deposited to 1000 angstroms thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and the phosphorous-doped polysilicon layer is held in contact with the n-type doped polysilicon layer 27cand the n-type drain region 28a. Tungsten silicide is deposited to 1000 angstroms thick over the n-type doped polysilicon layer by using the chemical vapor deposition, and the phosphorous-doped polysilicon layer and the tungsten silicide layer form in combination the polycide structure. The phosphorous-doped polysilicon layer and the tungsten silicide layer both deposited through the chemical vapor depositions achieve good step coverage.

[0044] Subsequently, a photo-resist etching mask (not shown) is formed on the tungsten silicide layer by using the photo-lithographic techniques, and the tungsten silicide layer and the phosphorous-doped polysilicon layer are successively patterned into tungsten silicide strips 33a/33band phosphorous-doped polysilicon strips 33b/34bby using etching techniques. The tungsten silicide strip 33aand the phosphorous-doped polysilicon strip 33bform an upper-level electrode 33, and the other tungsten silicide strip 34aand the phosphorous-doped polysilicon strip 34bform a digit line 34 as shown in FIG. 4G.

[0045] The polysilicon strip 33ais directly held in contact with the n-type doped polysilicon layer 27c, and the direct contact between the doped polysilicon layers 27cand 33areduces the resistance between the upper-level electrode 33 and the word line 27a. Similarly, the phosphorous-doped polysilicon strip 34ais directly held in contact with the n-type drain region 28aof single crystal silicon, and the direct contact reduces the contact resistance between the digit line 34 and the n-type drain region 28a. Thus, the low contact resistance and the good step coverage are achieved through the process according to the present invention.

[0046] Moreover, the tungsten silicide layer 27dis partially removed from the word line 27abefore the completion of the contact hole 33b. When the contact hole 33breaches the surface of the n-type drain region 28a, the plasma etching is finished. The n-type drain region 28ais not depressed, and is thick enough to provide good electrical isolation between the digit line 34 and the silicon substrate 21. For this reason, the leakage current is drastically reduced.

[0047] Subsequently, another inter-level insulating layer 35 is formed on the interlevel insulating layer 31, and a node contact hole 36 is formed in the interlevel insulating layers 31/35. The node contact hole 36 reaches the n-type source region 28b. The node contact hole 36 is plugged with a conductive column 37, and the conductive column 37 has the polycide structure. The conductive column or a contact plug 37 is held on contact with the n-type source region 28b. An accumulating electrode 38 is patterned on the interlevel insulating layer 35, and is held in contact with the conductive column 37. The accumulating electrode 38 is covered with a dielectric layer 39, and a plate electrode 40 is opposed to the accumulating electrode 38 through the dielectric layer 39 as shown in FIG. 4H. The accumulating electrode 38, the dielectric layer 39 and the plate electrode 40 as a whole constitute a storage capacitor 41.

[0048] As will be appreciated from the foregoing description, the inter-level insulating layer 31 over the n-type drain region 28ais thicker than the interlevel insulating layer 31 over the word line 27a, and the tungsten silicide layer 27dof the word line 31 is exposed to the contact hole 33abefore the exposure of the n-type drain region 28ato the other contact hole 33b. This feature is desirable, because the manufacturer can remove the part of the tungsten silicide layer 27dwithout over-etching on the n-type drain region 28a. After the removal of the part of the tungsten silicide layer 27d, the remaining inter-level insulating layer 31 is etched away in order to complete the contact hole 33b. The manufacturer can easily control the end point of the etching process, and the surface of the n-type drain region 28ais never depressed. This means that the n-type drain region 28ais thick enough to offer appropriate electrical isolation, and the leakage current is drastically reduced.

[0049] The photo-resist etching mask 32 is available for the plasma etchings on the inter-level insulating layer 31, and the process sequence is as simple as the prior art process sequence. The second plasma etching is carried out under the same conditions as the first plasma etching, and any additional component is never required for the plasma etchings.

[0050] In this instance, the word line 27aand the source/drain region 28aserve as a first lower-level conductive line and a second lower-level conductive line, respectively.

[0051] Although a particular embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.

[0052] For example, the upper-level conductive lines may be formed of polysilicon strip or polysilicon contact plugs 51/52 as shown in FIG. 5. The polysilicon contact plugs 51/52 are formed as follows. After the contact holes 33a/33bare completed, polysilicon is deposited over the entire surface of the inter-level insulating layer 31. The polysilicon fills the contact holes 33a/33b, and forms a polysilicon layer on the inter-level insulating layer 31. The polysilicon layer is uniformly etched away without an etching mask until the inter-level insulating layer 31 appears. Then, the polysilicon contact plugs 51/52 are left in the contact holes 33a/33b. In this instance, the manufacturer forms any kind of upper-level conductive line on the inter-level insulating layer 31.

[0053] Both lower-level conductive lines may be formed by conductive line different in structure. The process according to the present invention is applicable to any kind of semiconductor integrated circuit device, and is never limited to a dynamic random access memory device.

[0054] In the above-described embodiment, the impurity region is formed after the polycide structure. However, the impurity region may be formed before the polycide structure in another example.

[0055] The inter-level insulating layer over the polycide line is thinner than the inter-level insulating layer over the impurity region. However, it is not important that the polycide lines extend over the impurity regions with respect to the major surface of the semiconductor substrate.

[0056] The refractory metal is never limited to tungsten. Other kinds of refractory metal such as, for example, titanium silicide or molybdenum silicide are available for the polycide structure. The refractory metal silicide may be formed through an alloying step.

[0057] Upon completion of the inter-level insulating layer, the inter-level insulating layer over the n-type source region 28amay be thicker than the interlevel insulating layer 31 over the word line 27a. If so, the chemical mechanical polishing is not carried out.

[0058] If a time lug takes place between the exposure of the refractory metal silicide layer and the exposure of the impurity region, the difference in thickness of the inter-level insulating layer is not an indispensable feature.