1.  Field
This patent specification describes a semiconductor apparatus and method of making the same, and more particularly a semiconductor apparatus with a stable contact resistance and a method of making the semiconductor apparatus. 
2.  Discussion of The Background
A highly integrated SRAM (static random access memory) and a hybrid LSI (large-scale integration) circuit with an analog circuit, typically use a relatively high resistive element made of a polysilicon film. 
A resistive value of a resistive element is generally expressed by an equation;
R=(xcfx81/t)xc3x97(L/W),
wherein R represents a resistive value, xcfx81 represents a resistivity, t represents a film thickness, L represents a length of the resistive element, and W represents a width of the resistive element. 
From the above equation, it is understood that an increase of one or both of the resistivity xcfx81 and the length L or a decrease of one or both of the film thickness t and the width W increases the resistive value R. A decrease of the film thickness t is useful as a method of stably increasing the resistive value R. However, it also causes problems such as those discussed below. 
FIG.  1 is a schematic diagram showing a cross section view of a polysilicon thin film resistive element with a contact hole for connecting the thin film resistive element to a metal wiring.  In FIG.  1, a resistive element pattern 41 is formed on a field oxide film 3 formed on a semiconductor substrate.  The resistive element pattern 41 is made of a polysilicon film including an N-type or P-type impurity.  On the resistive element pattern 41, an inter-layer insulating film 43 is formed.  To form a contact to the resistive element 41, a relatively low resistive region 45 including a relatively high concentration impurity is normally formed to both sides of the resistive element pattern 41 to have a better electrical conductivity.  A contact hole 47 is formed in the inter-layer insulating film 43 to the low-resistive region 45.  A metal wiring layer 49 is formed on the inter-layered film 43.  A conducting filler 51 fills in the contact hole 47 to electrically connect the low-resistive region 45 and the metal wiring layer 49. 
When the polysilicon film constituting the resistive element 41 and the low-resistive region 45 are made thin to increase a resistance value of the resistive element 41, the low-resistive region 45 may unexpectedly be etched through by the dry etching used to form the contact hole 47.  As a result, the bottom of the contact hole 47 comes through the low-resistive region 45 and reaches the base, the field oxide film 3, as shown in FIG.  2.  In this case, a contact area between the conducting filler 51 filled in the contact hole 47 and the low-resistive region 45 is limited to a side portion 47a of the contact hole 47 and a contact resistance consequently increases.  As a result, an attempt to make a desired resistive value by determining the length and width of the resistive element 41 fails and such a desired resistive value is not obtained. 
One attempt to solve the above-mentioned problem is to make thicker the polysilicon film in the region forming the contact hole.  This attempt is described, for example, in Japanese Laid-Open Patent Application Publication No.  05-055520, Japanese Laid-Open Patent Application Publication No.  06-069207, Japanese Laid-Open Patent Application Publication No.  10-032246, and Japanese Laid-Open Patent Application Publication No.  10-163430. 
For example, a first background method described in Japanese Laid-Open Patent Application Publication No.  05-055520 is shown in FIG.  3.  This method forms a thick polysilicon film 155 for a low-resistant portion on a first insulating film 103 (i.e., the field oxide film 3).  The polysilicon film 155 includes an additive of a high concentration impurity to make an electrical connection in a wiring region via a metal wiring layer 149 and a contact portion 147 (i.e., similar in function to contact hole 47) for an electrode.  After that, the polysilicon film 155 is etched to have an opening 153 for a high-resistive portion, reaching the first insulating film 103.  Then, a thin polysilicon film 141 (i.e., similar in function to resistive element pattern 41) is formed to cover the entire surfaces of the first insulating film 103 including the opening 153 and the polysilicon film 155.  The thin polysilicon film 141 is for a high-resistive portion and includes no impurity additives or a relatively low concentration impurity additive.  After that, a second insulating film 143 (i.e., the inter-layer insulating film 43) is formed on the entire surface of the thin polysilicon film 141 and, in the second insulating film 143, the contact portion 147 for an electrode is formed. 
In this way, the first background method attempts to avoid variations of the value of contact resistance by having the polysilicon film 155 underneath the contact portion 147 which may include a contact opening portion 157 coming through the thin polysilicon film 141 that constitutes a resistive element. 
When the first background method is applied to a manufacturing process for a SRAM (static random access memory) or a hybrid LSI (large scale integration) circuit, for example, it is needed to form a polysilicon-made gate electrode for a MOS (metal oxide semiconductor) transistor in addition to the resistive element pattern which is the thin polysilicon film 141.  This patent specification uses the term MOS for FET (field effect transistors) using a gate made of any conductive material such as a metal, a polysilicon, etc.  When an electrode of the MOS transistor is formed with the thick polysilicon film 155, a residue film may be formed from the thin polysilicon film 141 on the side of the polysilicon electrode made from the thick polysilicon film 155.  Such residue film on the electrically conductive film can significantly affect the characteristic of the transistor.  To avoid the generation of such residue, the process may become excessively complex. 
A second background method is described in Japanese Laid-Open Patent Application Publication No.  06-069207, and its procedure is shown in FIGS.  4 and 5.  As shown in FIG.  4, a first polycrystalline silicon layer 255 (i.e., similar in function to thick polysilicon film 155) is formed on the insulating film 3 formed on the semiconductor substrate 1.  The upper surface of the first polycrystalline silicon layer 255 is covered with a silicon dioxide film 259 serving as an insulating film, and a second polycrystalline silicon layer 241 (i.e., similar in function to resistive element pattern 41) is formed on the silicon dioxide 259 and the insulating film 3 of the substrate surface such that the second polycrystalline silicon layer 241 contacts by its side the silicon dioxide 259 and the insulating film 3. 
With this arrangement, a multi-layered structure of the first and second polycrystalline silicon layers 255 and 241 is applied to the region of the wiring and the contact holes, and only the second polycrystalline silicon layer 241 is applied to the high resistive region.  Therefore, as in the case of the first background method, the contact resistive value would not be affected by the second polycrystalline silicon layer 241 even if the second polycrystalline silicon layer 241 comes through around the contact opening area because there is the first polycrystalline silicon layer 255 underneath the second polycrystalline silicon layer 241. 
As shown in FIG.  5, the second background method forms a gate insulating film 261 on the semiconductor substrate 1 as it forms the resistive element.  Then, the first polycrystalline silicon layer 255 and the silicon dioxide film 259 are formed on the gate insulating film 261.  After that, the second polycrystalline silicon layer 241 is formed to cover the first polycrystalline silicon layer 255 and the silicon dioxide film 259.  Then, from above, an impurity ion is implanted to the semiconductor substrate 1 so as to form a diffusion region 263 serving as a source or a drain. 
However, when the first polycrystalline silicon layer 255 is used as a gate electrode of the MOS transistor, the second polycrystalline silicon layer 241 which is of high resistivity directly contacts the side of the gate electrode, just as in the case of the first background method, causing similar problems. 
Another attempt to solve the above-mentioned problem of the variations in the contact resistive value is to form a metal silicide layer only on a region in an upper surface of a thin polysilicon film where a contact hole is formed in order to avoid an etching leakage during the formation of the contact hole.  This attempt is described, for example, in Japanese Laid-Open Patent Application Publication No.  05-029346 and Japanese Laid-Open Patent Application Publication No.  07-183526. 
This attempt uses a fact that the etching rate of the metal silicide layer is lower than that of the silicon dioxide film and the inter-layer insulating film so that the etching of the metal silicide layer is completed during the etching process to form the contact hole. 
For example, a third background method described in Japanese Laid-Open Patent Application Publication No.  07-183526 relates to a method of forming a contact on a thin film transistor and is applied to a process of forming a resistive element made of a thin film polysilicon.  However, this document does not describes a method of forming a resistive element made of a thin film polysilicon and a MOS transistor on the same substrate.  Therefore, when this method is applied to a process of forming a resistive element made of a thin film polysilicon and a MOS transistor on the same substrate, the same problem as with the first background method may occur.  That is, a residue of the thin polysilicon film for the resistive element may remain on the side of the polysilicon side. 
There is further another attempt to solve the above-mentioned problem of the variations in the contact resistive value.  This attempt forms an inter-layer insulating film on a high resistive pattern and, after forming a contact hole, fills the contact hole with a silicon.  This attempt is also described in Japanese Laid-Open Patent Application Publication No.  05-036624 and Japanese Laid-Open Patent Application Publication No.  05-235276, for example. 
For example, Japanese Laid-Open Patent Application Publication No.  05-036624 describes a method in which an electrical contact is arranged in a diffusion area of a thin film transistor.  According to this description, this method can form a good contact by again filling in the silicon filler even when the thin silicon film at the bottom of the contact hole is etched and is made even thinner during the forming process of the contact hole. 
However, in this method, every contact hole is required to have a common diameter in consideration of the polysilicon injection.  For example, when a contact hole has a diameter at least twice greater than the thickness of the silicon thin film, the silicon injection cannot be performed in a perfect manner and a desired contact cannot be achieved. 
This patent specification describes a novel semiconductor apparatus which makes a contact resistance value stable.  In one example, a novel semiconductor apparatus includes a semiconductor substrate, a resistive element, and a metal oxide semiconductor transistor.  The semiconductor substrate is covered with a field insulating film.  The resistive element is formed on the field insulating film.  The resistive element includes an insulative first polysilicon film, a conductive second polysilicon film, a resistive-element-aimed insulating film, and a third polysilicon film.  The insulative first polysilicon film is formed on the field insulating film.  The conductive second polysilicon film is formed in a region adjacent to each side edge of the insulative first polysilicon film in which a contact hole is formed.  The resistive-element-aimed insulating film is formed on the insulative first polysilicon film.  The third polysilicon film for determining a resistance value is formed in a continuous manner on the conductive second polysilicon film and the resistive-element-aimed insulating film.  The metal oxide semiconductor transistor is formed in an active region surrounded by the field insulating film on the semiconductor substrate.  The metal oxide semiconductor transistor includes a gate oxide film and a gate electrode which is formed on the gate oxide and includes a multi-layered film.  The multi-layered film includes a polysilicon film formed on the gate oxide as a lower layer when the conductive second polysilicon film is formed and a polysilicon film formed on the lower layer as an upper layer when the third polysilicon film is formed. 
The resistive-element-aimed insulative film of the resistive element may be made of a silicon dioxide or a silicon nitride. 
This patent specification also describes a novel method of making a semiconductor device which makes a contact resistance stable.  In one example, this novel method includes the steps of first forming, second forming, third forming, fourth forming, first patterning, fifth forming, sixth forming, second patterning, seventh forming, and eighth forming.  The first forming step forms a field insulating film for separating elements and an active region surrounded by the field insulating film on a semiconductor substrate.  The second forming step forms a gate oxide film on a surface of the active region.  The third forming step forms an insulative first polysilicon film on an entire surface of the semiconductor substrate.  The fourth forming step forms a resistive-element-aimed insulating film on the insulative first polysilicon film.  The first patterning step patterns the resistive-element-aimed insulating film so that the resistive-element-aimed insulating film remains in a region corresponding to a middle portion of a resistive element formed on the field insulating film.  The fifth forming step forms a second polysilicon film at a position adjacent to the insulating first polysilicon film formed underneath the resistive-element-aimed insulating film by diffusing or implanting an impurity to provide a conductivity to the insulating first polysilicon film with a mask of the resistive-element-aimed insulating film.  The sixth forming step forms a third polysilicon film determining a resistance value on an entire surface of the semiconductor substrate.  The second patterning step patterns the third polysilicon film, the resistive-element-aimed insulating film, the second polysilicon film, and the insulating first polysilicon film to form a resistive element on the field insulating film, including the insulating first polysilicon film, the second polysilicon film adjacent to each edge side of the insulating first polysilicon film, the resistive-element-aimed insulating film, and the third polysilicon film, and a gate electrode of a metal oxide semiconductor transistor in the active region, including the second polysilicon film and the third polysilicon film.  The seventh forming step forms an inter-layer insulating film on an entire surface of the semiconductor substrate.  The eighth forming step forms a contact hole in the inter-layer insulating film in a region in which the second polysilicon film adjacent to each side of the resistive element and the third polysilicon film are multi-laying. 
The resistive-element-aimed insulating film may be made of a silicon dioxide film or a silicon nitride film.