Method of making a conductive contact on a semiconductor body

Abstract: A method of forming a capped and borderless contact of polysilicon on a body of a semiconductor material includes depositing a layer of undoped polysilicon (24) on the surface of the body (14) and forming an opening (30) therethrough to the surface of the body (14). The side walls of the opening (30) are then coated with a layer of silicon nitride (32) and the opening is then filled with doped polysilicon which forms the contact (34). The doped and undoped polysilicon (34,24) are heated in an oxidizing atmosphere to grow a layer of silicon dioxide (38) thereon having a thicker portion (40) over the doped polysilicon then over the undoped polysilicon. The silicon dioxide layer (38) is etched to remove the thinner portion (42) leaving the thicker portion (40) over the doped polysilicon as a capping layer. The undoped polysilicon is then etched away and a layer of a dielectric material is deposited on the body and surrounding the doped polysilicon contact (34).

Description: Field of the Invention The present invention relates to a method of making a conductive contact for a semiconductor device, and more particularly, to a method of making a capped and borderless contacts for semiconductor devices, such as integrated circuits. Background of the Invention Contacts for semiconductor devices are conductive regions on a surface of a semiconductor body which make electrical contact to the body or conductive regions in the body. Capped contacts are those which are completely surrounded by insulating material. Borderless contacts are those which can overlap isolation or gate areas of semiconductor devices. For high density integrated circuits, such as dynamic random access memories (DRAMs), forming such capped and borderless contacts is very difficult, particularly in a full scale manufacturing process. Such contacts are, however, required for high density types of integrated circuits such as DRAMs. Especially the inner node contact from transfer transistor to storage electrode requires these capped and borderless contacts. Capping is necessary to allow other conducting layers to cross over the contact area as is e.g. required by bitlines in DRAM memories. One technique for forming capped and borderless contacts is to form the contacts of doped polysilicon. The doped polysilicon contacts regions are formed in openings in undoped (intrinsic) polysilicon. The undoped polysilicon can easily be etched selective to gate encapsulations or isolations to allow overlap or borderlessness. However, the undoped polysilicon must then be removed and replaced with an insulating material of low dielectric constant, such as silicon dioxide. Standard masking systems for removing the undoped polysilicon without damaging the doped polysilicon contact regions are difficult to control or they require special selective etchants. In Patents Abstracts of Japan, vol.4, no.18 (E-171), Feb.13, 1980, & JP-A-54 158 167 a polysilicon contact connecting a doped region in a semiconductor substrate is formed by the following process : via a contact hole the doped region is connected to an undoped polysilicon layer which is deposited surfacewide. A part of the undoped polysilicon layer outside the contact hole is implanted with n-type dopants, and an oxidation is carried out to convert this part into silicon oxide completely. On the undoped polysilicon only a thin oxide layer is formed due to the different oxidation rates. The thin oxide layer is removed and the undoped polysilicon is subjected to n-type-implantation to form an electric contact to the doped region. With this process a capped contact cannot be produced. Therefore, it is desirable to have a method of forming capped borderless doped polysilicon contacts which is relatively simple to carry out and can be readily used in a manufacturing process. SUMMARY OF THE INVENTION An aspect of the present invention is directed to a method of making borderless contacts of doped polysilicon in openings in undoped polysilicon in which a self-aligned masking layer of silicon dioxide is formed over the doped polysilicon. This is achieved by thermally growing the silicon dioxide over both the doped and undoped polysilicon with the thickness of the silicon dioxide layer being greater over the doped polysilicon than over the undoped polysilicon. The thinner portions of the silicon dioxide layer over the undoped polysilicon can then be easily removed by etching while still leaving a portion of silicon dioxide over the doped polysilicon. The method of the present invention for making a conductive contact to a body of semiconductor material is set out in claim 1. The method includes forming a layer of undoped polysilicon on a surface of the body. An opening is formed in the undoped polysilicon layer and the opening is filled with doped polysilicon. A layer of silicon dioxide is grown over both the undoped and doped polysilicon with the silicon dioxide layer over the doped polysilicon being thicker than over the undoped polysilicon. The thinner portions of the silicon dioxide layer over the undoped polysilicon are then removed leaving the silicon dioxide only over the doped polysilicon. The undoped polysilicon is then removed. The invention will be better understood from the following more detailed description taken with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGs. 1-6 are cross-sectional view of a semiconductor device, illustrating the various steps of the method of the present invention for forming a device with a borderless contact. It should be understood that the drawings are not necessarily to scale. DETAILED DESCRIPTION Referring now to FIG. 1, there is shown a cross-sectional view of a portion of a metal-oxide-semiconductor (MOS) field effect transistor (FET) 10 as it appears at the start of the method of the present invention for forming a borderless contact. The transistor 10 comprises a substrate body 12 of a semiconductor material, such as single crystalline silicon, having a surface 14. On a portion of the surface 14 is a gate 16 of a conductive material, such as conductive polysilicon. The gate 16 is insulated from the surface 14 by a thin gate dielectric layer 18 which is typically of silicon dioxide. The gate 16 is covered with an encapsulation layer 17, typically of silicon dioxide, and has sidewall spacers 19 along each side, also typically of silicon dioxide. In the body 12 and at the surface 14 are source and drain regions 20 and 22 of a desired conductivity type. The source and drain regions 20 and 22 are at opposite sides of the gate 16. For illustrative purposes the transistor 10 is an n-channel insulated gated field effect transistor (IGFET) with the body being of p-type conductivity and the source and drain regions 20 and 22, respectively, being of n-type conductivity. The method of the present invention will be described with regard to forming a contact to the region 22. However, it should be understood that the method of the present invention can be used to form contacts to both of the regions 20 and 22 or to other regions formed in the body 12, such as buried capacitors of the type shown in US-A-4 927 779 (S. H. Dhong, issued May 22, 1990). The first step in the method of the present invention is to deposit a layer 24 of undoped (intrinsic) polysilicon over the body surface 14 and the gate 16. This can be achieved by the well known chemical vapor deposition technique in which the body 12 is exposed to a gas containing silicon, such as silane, and heated to decompose the gas and deposit polysilicon on the surface 14. A masking layer 26, such as of a photoresist, is coated over the polysilicon layer 24. Using standard photolithographic techniques, the masking layer 26 is provided with an opening 28 therein over the area of the body surface 14 where the contact is to be formed. The exposed portion of the polysilicon layer 24 is then removed using a suitable etchant to form an opening 30 therethrough down to the portion of surface 14 which includes a portion of the region 22. Referring now to FIG. 2, there is shown a cross-sectional view of the transistor 10 during the next steps of the method of the present invention. The masking layer 26 is removed with a suitable solvent and then side walls of the opening 30 are coated with a layer 32 of silicon nitride or another insulating material. This is achieved by depositing a layer of silicon nitride over the polysilicon layer 24, the sidewalls of the opening 30 and the exposed area of the body surface 14 at the bottom of the opening 30. The portions of the silicon nitride layer which are on the polysilicon layer 24 and the body surface 14 at the bottom of the opening 30 are then removed with an anisotropic etch, such as a plasma etch. This leaves the silicon nitride layer 32 on the sidewalls of the opening 30. Referring now to FIG. 3, there is shown a cross-sectional view of the transistor 10 after the next step in the method of the present invention. The opening 30 is then filled with doped polysilicon to form an electrical contact 34. This is achieved by depositing a layer of doped polysilicon over the undoped polysilicon layer 24 and within the opening 30. The portion of the doped polysilicon layer over the undoped layer 24 is then removed with a suitable etchant leaving the doped polysilicon contact 34 in the opening 30. During this etching step, the contact 34 may be provided with a shallow cavity 36 in its surface. The contact 34 is doped with any suitable dopant of the desired conductivity type, such as phosphorus for n-type conductivity. Also, for reasons which will be explained, the contact 34 is preferably heavily doped to a concentration of about 5x10 20 impurities/cm 3 . The doped polysilicon may be deposited by any well known chemical vapor deposition technique in which the body 12 is exposed to vapors of a gas containing silicon, such a silane, and a gas containing the desired dopant. The body 12 is heated to a temperature at which the gases decompose and deposit doped polysilicon. Referring now to FIG. 4, there is shown a cross-sectional view of the transistor 10 after the next step in the method of the present invention. The polysilicon layer 24 and contact 34 are then heated in an oxidizing atmosphere at a temperature at which a silicon dioxide layer 38 is formed on the surfaces of the layer 24 and the contact 34. Typically, this can be carried out in steam at a temperature of between 700°C and 900°C. When polysilicon is oxidized in this manner, the silicon dioxide film formed on doped polysilicon is thicker than the silicon dioxide film formed on undoped polysilicon. Thus, a portion 40 of the silicon dioxide layer 36 directly over the contact 34, which is of doped polysilicon, is thicker than a portion 42 of the silicon dioxide layer 38 which is over the undoped polysilicon layer 24. By highly doping the contact 34 as stated above, the thickness of the thicker portion 40 of the silicon dioxide layer 36 can be made more than twice the thickness of the thinner portion 42. Referring now to FIG. 5, there is shown a cross-sectional view of the transistor 10 after the next step of the method of the present invention. The silicon dioxide layer 38 is then etched with a suitable etchant until the thinner portion 42 of the silicon dioxide layer 38 is completely removed from over the undoped polysilicon layer 24. Although the thicker portion 42 of the silicon dioxide layer 38 is reduced in thickness, a portion still remains to provide a capping layer over the contact 34 which is self-aligned with the contact 34. The exposed portion of the undoped polysilicon layer 24 is then removed with a suitable etchant. During this etching step, the contact 34 is protected from being etched by the capping portion 40 of the layer 38 and by the sidewall silicon nitride layers 32. Referring now to FIG. 6, there is shown a cross-sectional view of the transistor 10 after the next step of the method of the present invention. A layer 44 of a dielectric material, such as silicon dioxide, is then deposited over the gate 16 and the exposed area of the surface 14 so as to completely encircle the contact 34. This is achieved by depositing a layer of silicon dioxide over the surface 14, the gate 16 and the contact 34 using any well known chemical vapor deposition technique. The silicon dioxide layer is then planarized, using any well known planarization technique, to form the layer 44 which encircles the contact 34 and has a surface which is planar with the surface of the contact 34. This provides the transistor 10 with a conductive contact 34 which can provide lateral contact to other areas such as e.g. trench capacitors (not shown in the drawing). The contact is borderless and capped so that the contact is isolated from crossing conductor lines. This is important since the same technique can be used to form other contacts which contact to these crossing conductor lines. Bitlines of DRAM cells are an example. In the method of the present invention, by using the difference in the oxidation rate of doped and undoped polysilicon, a self-aligned capping layer 40 of silicon dioxide is easily formed over the contact 34. The self-aligned capping layer 40, along with the sidewall spacers silicon nitride layers 30 around the contact 34, protect the contact 34 during the removal of the undoped polysilicon layer 24 and its replacement with the dielectric layer 44. This allows the removal of the undoped polysilicon layer 24 with the use of any standard etchant which simplifies the removal of the undoped polysilicon layer. Thus, the method of the present invention provides a simple method of forming borderless contacts which can be used as part of a manufacturing process for integrated circuits containing the contacts. It is to be appreciated and understood that the specific embodiments of the invention are merely illustrative of the invention. Various modifications may be made. For example, the contact can be made to any device formed in the substrate body. Also, although only one contact has been shown as being made to the body, the device can include any desired number of the contacts as well as conductive strips connecting the contacts. These and similar changes are within the scope of the invention which is defined by the claims following.

Claims: A method of making a conductive contact on a body (12) of a semiconductor material comprising the successive steps of: forming a layer of undoped polysilicon (24) on a surface of the body; forming an opening (30) through the undoped polysilicon layer to the surface of the body; filling said opening with doped polysilicon (34); heating the doped and undoped polysilicon in an oxidizing atmosphere so as to grow a layer of silicon dioxide (38,40,42) on the doped and undoped polysilicon with the silicon dioxide layer being thicker over the doped polysilicon than over the undoped polysilicon; removing all of the silicon dioxide layer from over the undoped polysilicon while leaving a portion (40) of the silicon dioxide layer over the doped polysilicon; and removing the undoped polysilicon layer, using the portion of the silicon dioxide layer left over the doped polysilicon as a masking layer. The method of claim 1 further comprising the step of coating the side walls of the opening in the undoped polysilicon layer with a layer of silicon nitride (32) prior to filling the opening with the doped polysilicon. The method of any one of claims 1 - 2 in which the doped polysilicon contains a concentration of dopant of about 5x10 20 impurities/cm 3 . The method of any one of claims 1 - 3 in which the silicon dioxide layer is grown at a temperature of between 700°C and 900°C in steam. The method of any one of claims 1- 4 in which the opening is formed in the undoped polysilicon by forming a masking layer (26) on the undoped polysilicon with the masking layer having an opening (28) therethrough where the opening in the undoped polysilicon layer is to be provided, and then etching the undoped polysilicon layer through the opening in the masking layer. The method of claim 5 in which the undoped polysilicon layer is removed by etching. The method of any one of claims 1 - 6 further comprising the step of depositing a layer of a dielectric material (44) on the surface of the body encircling the doped polysilicon after the undoped polysilicon is removed. The method of claim 7 further comprising the step of planarizing the surface of said layer of a dielectric material so as to be essentially planar with the oxidized surface of the doped polysilicon. The method of any one of claims 7 - 8 in which said layer of a dielectric material is a layer of silicon dioxide.

Inventors: ROEHL SIEGFRIED DR, ROEHL, SIEGFRIED, DR.
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1993
Main classification: H01L21
IPC codes: H01L23, H01L21