Method of forming a polysilicon buried contact

Abstract: A method is provided for forming a polysilicon buried contact of an integrated circuit, and an integrated circuit formed according to the same. A field oxide region is formed over a portion of a substrate leaving an exposed active region. An oxide layer is formed over the active region. A first photoresist layer is formed and patterned over the first silicon layer. The first silicon layer is then etched to form an opening therethrough to expose a portion of the oxide layer. The oxide layer is etched through the opening to expose a portion of the substrate. a conductive etch stop layer is formed over the exposed portion of the substrate and the first photoresist layer. The first photoresist layer and the etch stop layer overlying the first photoresist layer are then removed. A second silicon layer is formed over the first silicon layer and the remaining etch stop layer. A second photoresist layer is formed and patterned over the second silicon layer. The first and second silicon layers are then etched to form a conductive structure contacting the exposed portion of the substrate through the etch stop layer.

Description: The present invention relates generally to semiconductor integrated circuit processing, and more specifically to a method of forming a polysilicon buried contact. To create a transistor, a polysilicon layer is deposited on top of a thin oxide layer. To form a resistor, a polysilicon layer is deposited on a field oxide layer. Both of these semiconductor devices may be made from the same polysilicon layer during the same process steps. The polysilicon layer, however, must be electrically isolated from the silicon substrate in order for these devices to operate. A buried contact may also be made from the same polysilicon layer as the gate electrode of a transistor and a resistor. The buried contact, however, requires an electrical connection between the polysilicon layer and the silicon substrate, generally where polysilicon and diffusion wires interconnect. In the case of the buried contact, an opening is made in the oxide layer overlying the silicon substrate to expose a portion of the silicon. If a split polysilicon layer, or two polysilicon layers, are used to form the gate electrodes and resistors, the second polysilicon layer may be used to form the buried contacts. IBM Technical Disclosure Bulletin, Vol. 25, No. 8, January 1983, pp.4425-26 discloses a buried contact/depletion device process which eliminates possible contamination of the gate silicon dioxide dielectric by the resist layer in the depletion device. This process incudes the steps recited in the pre-characterizing portion of claim 1 and provides a structure having the features of the pre-characterizing portion of claims 22 and 23. Hewlett-Packard Journal, vol. 33, No. 10, pp. 21-27, 1982, discloses other method of forming buried contacts. IBM Technical Disclosure Bulletin, Vol. 25, No. 8, January 1983, pp.4067-68 discloses a process for fabricating buried contacts in which palladium silicide is formed in the upper surface of the buried diffusion region so as to provide a contact having more uniform characteristics. A split polysilicon process is generally used in SRAM applications. After the formation of a field oxide layer in the case of a resistor or diode, and a gate oxide layer in the case of a gate electrode, the first amorphous or polysilicon layer is deposited. The buried contact openings to the silicon substrate are patterned, etched and doped, if necessary. The second amorphous or polysilicon layer is then deposited, patterned and etched to form the upper portion of the split polysilicon of the gate electrodes and resistors as well as the buried contact in the contact opening which connects directly to the silicon substrate. During the gate electrode and resistor split polysilicon etch, both amorphous silicon or polysilicon layers are etched over the gate oxide and field oxide regions. With the buried contact, however, since the second amorphous or polysilicon layer forms the contact, only the second layer is etched during the etch step since the first layer has been removed previously. The thickness of the second polysilicon layer is obviously less than the thickness of the total split polysilicon stack forming the gate electrodes or resistors from the first and second polysilicon layers. In other words, the distance to etch the second polysilicon layer from the top of the resistors to the substrate is substantially less than the distance to etch the layer from the top of the gate electrode or resistor to the gate oxide or the field oxide.. At the location of the buried contact, there is no natural etch stop at the surface of the substrate such as oxide. In order to completely dry etch the gate electrodes and resistors, an overetching situation occurs while etching the buried contact due to the lesser height of polysilicon over the buried contact in relation to the height of the gates and resistors. This overetching condition results in an undesirable trenching of the silicon substrate. The depth of the trench is a function of the relative thicknesses of the first and second split polysilicon layers, as well as the surface topography. These factors dictate the amount of overetch required to clear the polysilicon away from the edge of the active areas around the gate and resistor devices. In order to prevent the trenches from forming in the substrate during the dry etch process of the second split polysilicon layer, the present invention uses an etch stop layer disposed over the silicon substrate. This additional layer will act as an etch stop in any subsequent polysilicon etch to prevent trenching the silicon substrate around the buried contact area. This layer will also improve the contact resistance between the buried contact and the substrate. The invention may be incorporated into a method according to claim 1 for forming a semiconductor device structure, and the semiconductor device structure formed thereby, by forming a buried contact. According to the present invention there is provided a. method of forming a buried contact of an integrated circuit comprising the steps of: forming an oxide layer on a substrate; forming a first silicon layer over the oxide layer; forming and patterning a first photoresist layer over the first silicon layer; etching the first silicon layer to form an opening therethrough which exposes a portion of the oxide layer; wherein an edge is formed in the first silicon layer; etching the oxide layer through the opening to expose a portion of the underlying substrate; depositing a conductive etch stop layer over the exposed portion of the substrate and the first photoresist layer; wherein the edge of the first silicon layer is covered by the conductive etch stop layer; removing the first photoresist layer and the portion of the etch stop layer disposed over the first photoresist layer; forming a second silicon layer over the first silicon layer and the remaining conductive etch stop layer; and etching the first and second silicon layers to expose a portion of the conductive etch stop layer and to form a conductive structure contacting the exposed portion of the substrate through the conductive etch stop layer. The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: Figures 1-4 and 6 are cross-sectional views of the fabrication of a semiconductor device structure according to the present invention. Figure 5 is a top plan view of a portion of a semiconductor device structure according to the present invention. The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. Referring to Figure 1 , an integrated circuit device is to be formed in a silicon substrate 10 . A field oxide layer 12 is formed over a portion of the substrate 10 by methods known in the art leaving an active region exposed. An oxide layer 14 is then formed over the exposed active region which is a portion of the substrate 10 not covered by the field oxide 12 . A first silicon layer 16 is deposited over the oxide layer 14 and the field oxide layer 12 . The first silicon layer may be amorphous silicon or polysilicon. Photoresist layer 18 is then formed and patterned, as known in the art, over the first silicon layer 16 . Referring to Figure 2 , the first silicon layer 16 is etched to form an opening 20 to expose a portion of the oxide layer 14 . The oxide layer 14 is then etched in the opening 20 to expose a portion of the substrate 10 . At this stage, the substrate 10 may be doped through opening 20 to form an active layer 22 which is oppositely doped from the underlying substrate 10 . Depending upon the nature of the doping material used, the dopant may also penetrate the amorphous or polysilicon layer 16 along each sidewall in opening 20 . Before any subsequent deposition steps, an in-situ predeposition cleaning of any native oxide may need to be performed. A conductive etch stop layer 24 , such as a refractory metal silicide, is deposited over the first photoresist layer 18 and the exposed portion of substrate 10 in opening 20 by methods known in the art. The thickness of the etch stop layer will be typically between approximately 100 and 1000 angstroms (10 angstroms = 1 nm). The etch stop layer is deposited at a temperature which is low enough so that the first photoresist layer 18 does not flow or decompose. The actual temperature for deposition of the conductive etch stop layer will depend upon the chemical structure of the photoresist. The conductive etch stop layer 24 may also be a refractory metal, refractory metal nitride or a composite layer such as a refractory metal/refractory metal nitride, refractory metal/refractory metal silicide or refractory metal nitride/refractory metal silicide. The conductive etch stop layer will act as an etch stop for subsequent etching processes as well as to reduce contact resistance. If a refractory metal or refractory metal nitride, for example, is used, the etch stop layer is annealed after the deposition process. Annealing, by rapid thermal annealing, will form a metal silicide in the interface between the substrate 10 and the conductive etch stop layer 24 . Any refractory metal or refractory metal nitride remaining after the anneal step may be removed to clean the surface of the etch stop layer. Referring to Figure 3 , the first photoresist layer 18 is removed using a lift-off process as known in the art. During the lift-off process, the first photoresist layer 18 and the portion of the etch stop layer 24 covering the first photoresist layer 18 are removed. A second silicon layer 26 is next deposited over the first silicon layer 16 and the remaining conductive etch stop layer 24 by methods known in the art. Layer 26 may be comprised of amorphous silicon or polysilicon. A second photoresist layer 28 is formed and patterned over the second polysilicon layer 26 as known in the art. At this stage, the second polysilicon layer 26 may be selectively doped by known methods to achieve the doping level required for the various devices to be made from this layer. Referring to Figure 4 , the first and second silicon layers 16 and 26 are then etched to form a conductive structure which contacts the exposed portion of the substrate 10 through the conductive etch stop layer 24 . Lightly doped source/drain (LDD) regions 32 are formed in substrate 10 . Oxide sidewall spacers 34 are then formed on the sidewalls of the first and second silicon layers 16 and 26 . A heavily doped source/drain region 36 is then formed by methods known in the art. As will be shown by the plan view in Figure 5 , the first and second silicon layers 16 and 26 form a resistor 38 and a gate 40 . Active layer 22 and the source/drain region 36 connect outside the plane of the drawing as will be shown in Figure 5 . Referring to Figure 5 , the oxide layer 14 covers the substrate 10 not covered by the field oxide 12 except for the portion exposed in the opening 20 . The oxide sidewall spacers 34 surround the resistor 38 and the gate 40 . The conductive etch stop layer 24 is shown through the opening 20 and is continuous under resistor 38 . The etch stop layer 24 overlies the doped active layer 22 (shown in Figure 4 ) which connects to the source/drain region 36 underlying the oxide layer 14 . Figure 6 is a cross sectional view along the line B-B as shown in the plan view of Figure 5 . Opening 20 shows the etch stop layer 24 as filling the opening 20 . Source/drain regions 36 underlie the oxide layer 14 except under the resistor 38 and the gate 40 (not shown). The source/drain regions 36 and the active layer 22 are both formed in those regions in the opening 20 not covered by the resistor 38 . This forms a continuous conductive region in the substrate 10 , allowing the resistor 38 to connect to the source/drain region 36 of the gate 40 . As will be understood by those skilled in the art, oxide layer 14 protects the underlying substrate 10 during the etching of the gate 40 and resistor 38 . However, there is no oxide within the opening 20 to protect the underlying substrate. The etch stop layer 24 within the opening 20 protects the underlying substrate during the etch step which forms gate 40 and resistor 38 . The etch stop layer prevents trenching of the substrate 10 within the opening 20 which are not covered by the resistor 38 . As will be appreciated by those skilled in the art, prevention of such trenching improves the quality of the buried contact. In addition to preventing trenching, the conductive etch stop layer 24 reduces contact resistance as known in the art. As will be appreciated by those skilled in the art, the process steps described above can be used with nearly any conventional process flow.

Claims: A method of forming a buried contact of an integrated circuit comprising the steps of: forming an oxide layer (14) on a substrate (10); forming a first silicon layer (16) over the oxide layer (14); forming and patterning a first photoresist layer (28) over the first silicon layer; etching the first silicon layer (16) to form an opening (20) therethrough which exposes a portion of the oxide layer; wherein an edge of said first silicon layer forms the sidewall of said opening ; etching the oxide layer through the opening to expose a portion of the underlying substrate (10); depositing a conductive etch stop layer (24) over the exposed portion of the substrate and the first photoresist layer (18); wherein the edge of the first silicon layer (16) is covered by the conductive etch stop layer; removing the first photoresist layer (18) and the portion of the etch stop layer (24) disposed over the first photoresist layer; forming a second silicon layer (26) over the first silicon layer (16) and the remaining conductive etch stop layer (24); and etching the first and second silicon layers (16,26) to expose a portion of the conductive etch stop layer within the opening (20) and to form a conductive structure contacting the exposed portion (10) of the substrate through the conductive etch stop layer (24). A method as claimed in claim 1, comprising the step of forming a field oxide region (12) over a portion of the substrate (10) leaving an exposed active region, and wherein the oxide layer (14) is formed over the active region, and the first silicon layer (16) is formed over the oxide layer (14) and the field oxide region (12). The method of claims 1 or 2, wherein each silicon layer is amorphous silicon. The method of claim 1 or 2, wherein each silicon layer is polysilicon. The method of any preceding claim, wherein the etch stop layer is a refractory metal silicide. The method of any of claims 1 to 4, wherein the etch stop layer is a refractory metal nitride. The method of claim 6, wherein the refractory metal nitride is annealed after the step of forming an etch stop layer to form a refractory metal silicide. The method of claim 7, wherein the annealing step is performed by rapid thermal annealing. The method of claim 7 or 8, wherein any refractory metal nitride remaining after the anneal step, is removed before the step of forming a silicon layer over the etch stop layer. The method of any preceding claim, wherein the thickness of the etch stop layer is between approximately 10 and 100 nm (100 and 1000 angstroms). The method of any preceding claim, further comprising the steps of: in-situ predeposition cleaning of a native oxide before the etch stop layer step. The method of any preceding claim, further comprising the step of: cleaning the surface of the integrated circuit before the second silicon layer step. The method of any preceding claim, further comprising the step of: selectively doping portions of the second silicon layer before etching the second silicon layer step. The method of claim 2 or any of claims 3-13 when appended thereto, wherein the conductive structure extends to a resistor having a portion disposed over the field oxide region. The method of any preceding claim, wherein the conductive etch stop layer is deposited at a temperature low enough so that the first photoresist layer does not flow or decompose. The method of any preceding claim, further comprising the step of: doping the exposed portion of the substrate before forming an etch stop layer step. The method of any preceding claim, comprising the step of selectively doping the second silicon layer before the etching of the second silicon layer. The method as claimed in any preceding claim, wherein the first silicon layer and the etch stop layer overlying the first silicon layer are removed by a photoresist lift-off process. A method as claimed in claim 2 or any one of the preceding claims when appended thereto, further comprising a gate disposed over the oxide layer.

Inventors: KALNITSKY ALEXANDER, KALNITSKY, ALEXANDER
Applicants: ST MICROELECTRONICS INC, STMICROELECTRONICS, INC.
Application year: 1992
Main classification: H01L21
IPC codes: H01L21