Abstract:
An improved etch sequence and an improved integration scheme of plasma doping in the fabrication of a DRAM integrated circuit device are described. Semiconductor device structures are provided in and on a substrate wherein the substrate is divided into an array area and a periphery area. The semiconductor device structures are covered with a dielectric layer. The dielectric layer is concurrently etched through in the array area to form bit line contact openings and in the periphery area to form substrate contact openings. Doped regions are formed in the substrate exposed within the bit line contact openings and the substrate contact openings using a plasma doping process. Next, the dielectric layer is etched through to form a gate contact opening. Thereafter, the bit line contact openings, the substrate contact openings, and the gate contact opening are filled with a conducting layer to complete forming contacts in the fabrication of a DRAM integrated circuit.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of fabricating deep trench DRAM devices in the fabrication of integrated circuits. 
     (2) Description of the Prior Art 
     In the fabrication of integrated circuit devices, deep trench (DT)-based dynamic random access memory (DRAM) devices require certain integration practices. It has been customary to separate the contact etches in the array area from the contact etches in the periphery. Different fill materials (polysilicon in the contact area and tungsten silicide in the periphery, for example) and different contact methods (diffusion contact in the array area and implant contact in the periphery) are reasons for the separation. However, in DRAM devices with a design rule of less than 150 nm, a low resistivity material is required for a contact, especially for those using a deep trench as a storage node. Thus, polysilicon is no longer an attractive option for the contact in the array area because of its high resistivity. 
     It has also been customary to combine the contact to junctions of support (CS) etch with the contact to gate (CG) etch because of their close proximity in the support (or periphery) area. For example, a self-aligned contact (SAC) process has been used for the bit line contact (CB) etch in the array area while the contact to substrate and the gate contact etch in the array area have been etched together with a moderate selectivity (&lt;3:1) etch method. The moderate etch selectivity recipe has been chosen partly because it must etch through a nitride capping layer on top of the gate. However, this moderate etching selectivity, especially of oxide to nitride, puts the future manufacturing process in jeopardy due to insufficient overlay control between the gate and the contact to the substrate (CS) in the periphery. Overlay control becomes more difficult as the ground rule (or critical dimension of the gate) shrinks especially for those devices having a ground rule of less than 0.17 æm. The protection becomes even weaker with insufficient selectivity. A proximity of the contact to substrate (especially an NMOS contact) to the gate contact is a dangerous event. Deleterious effects such as shorting of the gate contact to the metal line, threshold voltage roll-off (lowering of the threshold voltage as gate length decreases), junction leakage, and lowering of the effective saturation current can result. This is especially true for implanted junctions. 
     A number of patents have addressed aspects of DRAM fabrication. U.S. Pat. No. 5,292,677 to Dennison discloses a single etch stop layer for all contacts wherein all contacts are opened together. U.S. Pat. No. 5,718,800 to Juengling teaches selective contact etching using two masks. U.S. Pat. No. 6,136,643 to Jeng et al etches contact openings using nitride/oxide selectivity. U.S. Pat. No. 6,008,104 to Schrems shows a DRAM process with several selective etches. Self-aligned contact etches are taught, for example, in U.S. Pat. Nos. 6,133,153 to Marquez et al and 5,965,035 to Hung et al. U.S. Pat. No. 4,912,065 to Mizuno et al shows a plasma doping process. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide an effective and very manufacturable method of DRAM formation in the fabrication of integrated circuits. 
     It is a further object of the invention to provide an improved etch sequence for DRAM device fabrication. 
     Another object of the invention is to provide an improved integration scheme of plasma doping in the fabrication of a DRAM integrated circuit device. 
     Yet another object of the invention is to provide an improved etch sequence and an improved integration scheme of plasma doping in the fabrication of a DRAM integrated circuit device. 
     In accordance with the objects of the invention, an improved etch sequence and an improved integration scheme of plasma doping in the fabrication of a DRAM integrated circuit device are achieved. Semiconductor device structures are provided in and on a substrate wherein the substrate is divided into an array area and a periphery area. The semiconductor device structures are covered with a dielectric layer. The dielectric layer is concurrently etched through in the array area to form bit line contact openings and in the periphery area to form substrate contact openings. Doped regions are formed in the substrate exposed within the bit line contact openings and the substrate contact openings using a plasma doping process with a separate blocking mask, respectively. Next, the dielectric layer is etched through to form a gate contact opening. Thereafter, the bit line contact openings, the substrate contact openings, and the gate contact opening are filled with a conducting layer to complete forming contacts in the fabrication of a DRAM integrated circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings forming a material part of this description, there is shown: 
     FIGS. 1 through 10 are cross-sectional representations of a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process of the present invention provides an improved etch sequence and an improved integration scheme of plasma doping in the fabrication of DRAM integrated circuit devices. The method of the present invention is particularly useful for deep trench DRAM devices. However, it will be understood by those skilled in the art that the process of the present invention should not be limited to the application herein illustrated, but can be applied and extended to other applications, including, for example, ferro-electric RAM (FeRAM) or magnetic RAM (MRAM). 
     Referring now more particularly to FIG. 1, there is shown a semiconductor substrate  10 . P-well  12 , N-wells  16  and  18 , and buried plate connector  14  have been formed within the substrate. Deep trench capacitor  24  has been formed partially underlying shallow trench isolation  22 . Gate electrodes and interconnection lines  30  have been formed overlying the semiconductor substrate. A capping layer  34  covers the gate electrodes and interconnection lines and the substrate between the lines. The capping layer may be a nitride layer such as silicon nitride or silicon oxynitride having a thickness of between about 300 and 500 Angstroms. 
     Inter-layer dielectric layer  40  is blanket deposited over the semiconductor device structures. This layer may comprise silicon dioxide, borophospho-tetraethoxysilane (BP-TEOS) oxide, borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), or a combination of BPSG and silicon dioxide, and so on, and may be one or more layers. The total thickness of the layer  40  is between about 6000 and 10,000 Angstroms. The top of the inter-layer dielectric layer  20  may be planarized, for example by reflowing of the dielectric material, etchback, or chemical mechanical polishing (CMP), or the like. 
     The improved etch sequence of the present invention involves first etching the bit line contacts (CB) in the array area and the contacts to substrate (CS) in the periphery area together using a SAC process. Next, the gate contact etch is performed. With this method, both CB and CS will be protected from misalignment between the contact holes and the gate, making the device more robust after junction and contact implantation. The dedicated contact to gate (CG) etch allows this etching to be fully optimized by preventing excessive etching through the gate silicide layer and by allowing a wider contact area. The new scheme does not require new etching tools or processes, but does need a new mask design and may require an additional i-line blocking mask for contact implantation for an implanted contact formation scheme for independently optimized CB, CSN, and CSP contacts. The new mask design cost will be compensated for by a higher yield, device performance, and functionality. The resulting device will have a steady threshold voltage control, a lower sub-threshold voltage current, and a higher saturation current. The new method will also extend the current design lifetime by effectively making the junction stable in a controlled manner. 
     Array area A is shown on the left side of FIG. 1, and periphery area P is shown on the right side. In the process of the present invention, the bit line contacts  42  in the array area and the contacts to the substrate  44  in the periphery area are opened together using a self-aligned contact etch that is selective to oxide with respect to nitride. 
     FIG. 2 is an enlarged illustration of a portion of the integrated circuit device. Two gate electrodes  30  in the array area A are shown on the left side of the figure and two gate electrodes  30  in the periphery area P are shown on the right side. For example, the gate electrodes  30  are formed overlying a gate oxide layer  26 . The gate electrodes may comprise a first polysilicon layer  27  having a thickness of between about 800 and 1200 Angstroms, a second silicide layer  28  such as tungsten silicide having a thickness of between about 650 and 800 Angstroms, and a third nitride layer  29  such as silicon nitride having a thickness of between about 1600 and 2000 Angstroms. Capping nitride layer  34  covers the top side surfaces of the gates  30  and the gate oxide surfaces. Source/drain regions  70  are formed by doping with arsenic or phosphorus at a dosage of greater than 5 E 14 ions/cm 2  and an energy of less than 25 KeV. The interlayer dielectric layer  40  has a thickness over the gates  30  of between about 3250 and 6000 Angstroms. 
     Now, an anti-reflective coating (ARC) layer  50  may be deposited over the planarized interlayer dielectric layer  40 . For example, the ARC layer may comprise an organic ARC or a dielectric ARC such as silicon oxynitride having a thickness of between about 600 and 1200 Angstroms. 
     Now, a photoresist mask  55  is formed over the surface of the wafer. The mask has openings for the CB and CS contact openings. A SAC etch is performed concurrently for the bit line contact openings in the array area A and the contacts to the substrate in the periphery area P. 
     FIG. 3 illustrates a CB contact opening  42  and a CS contact opening  44 . The preferred SAC etching recipe for the combined CB and CS etch consists of etching with C 4 F 8  and CO gases or with C 5 F 8  gas with a selectivity of oxide with respect to nitride of more than 10 to 1. The second etching step is an over etch step that etches through the nitride layer  34  overlying the substrate within the contact opening. This etch should be performed with a corner selectivity of oxide with respect to nitride of more than 15:1. For example, a low oxygen partial pressure over etch may be used with low power and low pressure with gas chemistry optimized to achieve the desired corner selectivity. 
     Returning now to the larger portion of the wafer shown in FIG. 4, the CB  42  and CS  44  have been opened as described above. Now contact implantation is to be performed. A deeper junction is required at the contact than for the source/drain junction. For example, twice the energy and five times the dosage may be required for the contact over the S/D junction implantation. For low resistivity, a high dosage implantation is required to form an Ohmic contact. Lateral diffusion is minimized so as not to adversely impact drain engineering. High energy will jeopardize the device, so in this contact implantation step, a low energy implantation is desirable. However, the low energy implantation will reduce throughput requiring a long process time to achieve the high dose. 
     The process of the present invention includes a new integration scheme of plasma doping for DRAM contact formation. In plasma doping, the throughput at a low energy implantation is not limited by space charge limitation, but it is rather controlled by duty cycle (pulse length times repetition rate) and plasma density. The independent control of throughput (time averaged current density) from the energy makes the plasma doping work for low energy and high dosage application. Pulse engineering can be used rather than continuous; segment voltage with pulses for better controllability. No channeling is found in plasma doping, resulting in shallow junctions. 
     Now the contact implantation of the present invention will be described in reference to FIG.  4 . In a first step, a mask  75  covers the periphery area and exposes the array area A. Plasma doping  80  is performed. A plasma of a gas species containing dopant ions is generated. The dosage is controlled by pulse engineering with a duty factor of between about 0.01% and 10%. Low energy of between about 100 eV and 10 KeV is used with an independent substrate bias. The plasma doping process is a low temperature and low power process; therefore, no extra cooling of the silicon substrate is needed in most operating ranges. The temperature is between about 25 and 100 oC and time averaged power on the substrate is between about 10 and 300 watts. A very high surface concentration of dopant is found after plasma doping, higher than the solid solubility limit which is ideal for ultra shallow junctions. The plasma doping process of the present invention has a high throughput of between about 60 and 100 wafers per hour as compared with a conventional ion implantation process throughput of between about 1 and 10 wafers per hour. Plasma doping with arsenic or phosphorus at a dosage of greater than 1 E 15 ions/cm 2  and an energy of less than 10 KeV is performed to fabricate NFET contact junctions  58 . 
     Referring now to FIG. 5, the mask  75  is removed and another mask  85  is formed to expose a portion of the periphery area P. Plasma doping  90  is performed. A plasma of dopant gases is generated. NFET contact junctions  60  are formed by doping with arsenic or phosphorus at a dosage of greater than 3 E 15 ions/cm 2  and an energy of less than 10 KeV. The other plasma doping process parameters are the same as in the first step above. 
     Referring now to FIG. 6, the mask  85  is removed and another mask  95  is formed to expose the PFET portion of the periphery area P. Plasma doping  100  is performed. A plasma of dopant gases is generated. PFET contact junctions  65  are formed by doping with boron at a dosage of greater than 1 E 15 ions/cm 2  and an energy of less than 10 KeV. The other plasma doping process parameters are the same as in the first step above. In many cases, the PFET implantation step (CSP) is not done to prevent excess boron outdiffusion. However, future devices will require CSP. Either an implant junction (as described above) or a diffusion junction can be used to form PFET contact junctions  65 . 
     Referring now to FIG. 7, the mask  95  is removed. Annealing is performed such as by a rapid thermal process (RTP) and furnace annealing to drive in the contact junctions  58 ,  60 , and  65 . The wafer is cleaned, for example, using a conventional wet cleaning process. 
     Now, the separate gate contact etch is to be performed. Referring now to FIG. 8 in the close-up view, the gate contact opening  46  is etched having a high selectivity to the silicide material in layer  29 . The contact to gate etch is now a less critical etch. CF 4 , CHF 3 ,  02  and Ar gases may be used for the oxide etch. Endpoint detection can now be used for its homogeneous structure to stop on top of the cap nitride layer  34 . Then, the cap nitride etch can be optimized to render a high selectivity to tungsten silicide (layer  28 ) by using CH 2 F 2  or CHF 3  gases. 
     Referring now to FIG. 9, the contacts CS, CB, and CG are completed by filling the contact openings  42 ,  44 , and  46  (shown in FIG. 8) with a metal layer. Typically, a barrier metal layer, not shown, such as titanium/titanium nitride or other glue layer and barrier layer, is deposited within the contact openings. Then, a metal layer such as tungsten is deposited and planarized to leave metal plugs  102 ,  104 , and  106 . The completed contacts are shown in expanded view in FIG.  10 . 
     Processing continues as is conventional in the art with higher levels of metallization. The process of the present invention provides a combined etching scheme which makes it possible to optimize both CS/CB and CG without adversely interfering with each other. It can be implemented simply without additional tools or process development. In addition, the plasma doping scheme of the present invention, in combination with the new etching scheme, results in low resistance shallow junctions and increased throughput. The process of the present invention is extendible to metal gates or dual work function gates. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.