Abstract:
A small-size (w&lt;0.5 micrometers) alignment mark in combination with a “k1 process” is proposed, which is particularly suited for the fabrication of trench-capacitor DRAM devices which requires highly accurate AA-DT alignment. The “k1 process” is utilized to etch away polysilicon studded in the alignment mark trenches and refresh the trench profile prior to AA pattern transferring, thereby improving wafer alignment accuracy and precision.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the field of semiconductor fabrication and, more particularly, to an alignment mark and alignment method for the fabrication of trench-capacitor dynamic random access memory (DRAM) devices. 
   2. Description of the Prior Art 
   Semiconductor integrated circuits undergo a variety of processing steps during manufacture, such as masking, resist coating, etching, and deposition. In many of these steps, material is overlaid or removed from the existing layer at specific locations in order to form the desired elements of the integrated circuit. Proper alignment of the various process layers is therefore critical. The shrinking dimensions of modern integrated circuits require increasingly stringent overlay alignment accuracy during pattern transfer. If the proper alignment tolerance is not achieved, the result is a device that is defective or has reliability problems. 
   Reduction type projection printing has been known as an apparatus of projection exposure for transferring a pattern drawn on a reticle to a resist. In reduction type projection printing process, a step and repeat method is used. The step and repeat method refers to a method of transferring a reticle pattern to a resist in which exposure is performed every time a wafer on a two-dimensionally movable x-y stage is moved in any given direction. A beam such as i-line or KrF laser is directed from a light source through a condenser lens to a reticle. The beam which passes through the reticle is projected on a photoresist on a wafer fixed on a wafer x-y stage through a reduction projection lens. Position of the wafer is automatically moved successively in x, y directions by the wafer x-y stage and the wafer is exposed shot-by-shot. 
   Registration is typically used to measure the accuracy of a process layer alignment performed using an alignment mark. Registration involves comparing the position of a subsequent layer to that of an existing layer by overlaying a distinct pattern on a matching pattern previously formed on the existing layer. The deviation in position of the overlay from the original provides a measure of accuracy of the alignment. Currently available registration structures include box-in-box visual verniers to determine the amount of alignment offset. 
   Trench-capacitor DRAM or deep-trench (DT) capacitor DRAM devices are known in the art. Typically, a trench-storage capacitor consists of a very-high-aspect-ratio contact-style hole pattern etched into the substrate, a thin storage-node dielectric insulator (capacitor dielectric), a doped low-pressure chemical vapor deposition (LPCVD) polysilicon fill, and buried-plate diffusion in the substrate. The doped LPCVD silicon fill and the buried plate serve as the electrodes of the capacitor. A dielectric isolation collar (collar oxide) disposed in the upper region of the trench prevents leakage of the signal charge from the storage-node diffusion to the buried-plate diffusion of the capacitor. After forming the trench capacitors, shallow trench isolation (STI) and active area (AA) regions are formed on the substrate between the trench capacitors. At the stage of forming the AA region, a very high accuracy of the AA-DT alignment is required. 
   SUMMARY OF INVENTION 
   It is the primary object of the present invention to provide an improved method of fabricating microelectronic device such as trench-capacitor DRAM devices with improved alignment accuracy. 
   According to the claimed invention, a method of fabricating a microelectronic device is disclosed. The present invention method comprises the steps of: 
   (1) providing a semiconductor substrate having thereon a device region and an alignment mark region; 
   (2) simultaneously etching capacitor trenches into the semiconductor substrate within the device region, and forming alignment mark within the alignment mark region, wherein the alignment mark comprises a plurality of trench lines, and each of the trench lines has a width of smaller than 0.5 microns; 
   (3) fabricating a trench capacitor within each of the capacitor trenches at least comprising depositing polysilicon layer into the capacitor trenches and simultaneously into the trench lines; 
   (4) forming a first photoresist layer over the semiconductor substrate, the first photoresist layer having an opening that merely exposing the alignment mark region and the alignment mark; 
   (5) performing a first etching process to etch the polysilicon layer within the exposed trench lines of the alignment mark; 
   (6) stripping the first photoresist layer; 
   (7) forming a second photoresist layer over the semiconductor substrate; 
   (8) performing a wafer alignment process comprising irradiating the alignment mark using an alignment light beam that passes through the second photoresist layer, thereby acquiring wafer alignment information; and 
   (9) performing an exposure process employing a photomask defining avtive area pattern thereon, thereby transferring the avtive area pattern to the second photoresist layer. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
       FIGS. 1-7  are schematic, cross-sectional diagrams showing a method of fabricating alignment mark and alignment method for the fabrication of trench-capacitor DRAM devices in accordance with the preferred embodiment of this invention; 
       FIG. 8  is a schematic top view of the alignment mark in accordance with the preferred embodiment of this invention; and 
       FIG. 9  is a flowchart  90  depicting the method of fabricating deep trench capacitor DRAM device involving the present invention K1 process in accordance with the preferred embodiment of this invention. 
   

   DETAILED DESCRIPTION 
   The present invention pertains to an alignment mark and alignment method for the fabrication of trench-capacitor DRAM devices. The present invention also addresses a method of reliably forming deep trench capacitor DRAM devices. In describing the preferred embodiment of the present invention, reference will be made herein to  FIGS. 1-8  of the drawings. Features of the invention are not drawn to scale in the drawings. 
   Briefly, the method for fabricating a trench capacitor of a DRAM device can be summarized as follows: 
   Phase 1: deep trench etching. (“Mask One”) 
   Phase 2: buried plate and capacitor dielectric formation. 
   Phase 3: first polysilicon deep trench fill and first recess etching. 
   Phase 4: collar oxide formation. 
   Phase 5: second polysilicon deposition and second recess etching. 
   Phase 6: collar oxide wet etching. 
   Phase 7: third polysilicon deposition and third recess etching. 
   Phase 8: STI process and active area definition. (“Mask Two”; or “AA mask”) 
   Please refer to  FIGS. 1-7 .  FIGS. 1-7  are schematic, cross-sectional diagrams showing a method of fabricating alignment mark and alignment method for the fabrication of trench-capacitor DRAM devices in accordance with the preferred embodiment of this invention. As shown in  FIG. 1 , semiconductor substrate  12  has thereon a device region  101  and an alignment mark region  102 , in which memory devices and alignment mark are to be formed, respectively. A pad oxide layer  14  and a pad nitride layer  16  are formed on the main surface of the semiconductor substrate  12 . 
   As shown in  FIG. 2 , a conventional lithography process (employing “Mask One”) and dry etching process are carried out to etch deep trenches  22  and  24  within the device region  101  and within the alignment region  102 , respectively, into the pad nitride layer  16 , the pad oxide layer  14  and the semiconductor substrate  12 . The plurality of deep trenches  24  constitute an alignment mark  20 . The top view of the alignment mark  20  is illustrated in  FIG. 8 . Preferably, the width W of each deep trench  24  of the alignment mark  20  is smaller than 0.5 microns (small sized), more preferably between 0.2 and 0.3 microns. Since the deep trenches  22  and  24  are formed simultaneously, the depth of the deep trenches  22  and  24  are substantially the same, preferably a depth of about 8000 angstroms, but not limiting thereto. 
   After the formation of deep trenches  22  and  24 , a series of deep trench capacitor fabrication processes from Phase 2 to Phase 7 as previously described are performed to form the trench capacitor structure  30  within the device region  101 , as depicted in  FIG. 3 . The trench capacitor structure  30  comprises buried plate electrode  32 , polysilicon electrode  34 , capacitor dielectric between the buried plate electrode  32  and the polysilicon electrode  34 , and collar oxide  36 . Simultaneously, layers of polysilicon  34  are deposited into the deep trenches  24  within the alignment mark region  102 . 
   Subsequently, as shown in  FIG. 4 , a photoresist layer  40  is formed over the semiconductor substrate  12 . The photoresist layer  40  masks the device region  101  and has an opening  42  merely exposing the alignment mark region  102 . The present invention is characterized in that between the “Mask One” that is employed to define deep trenches of the trench capacitors and the “Mask Two” that is employed to define active area regions and STI regions, an additional mask referring to as “Mask K1” is employed to form the photoresist layer pattern  40  that masks the device region  101  and merely exposes the alignment mark region  102  via the opening  42 . 
   Using the photoresist layer  40  and pad nitride layer  16  as etching hard mask, a dry etching is performed to etch away the polysilicon  34  in the deep trenches  24  exposed via the opening  42 . According to the preferred embodiment of this invention, the thickness of the removed polysilicon  34  within the deep trenches  24  ranges between 3000 angstroms and 8000 angstroms. 
   The dry etching process for cleaning the polysilicon  34  within the exposed deep trenches  24  following the lithographic process employing photoresist layer  40  and Mask K1 are defined as “K1 process”. According to the preferred embodiment, the K1 process is utilized to refresh the profile of the deep trenches  24  of the alignment mark  20 , such that the alignment accuracy can be improved. After the K1 process, the photoresist layer  40  is stripped. 
   As shown in  FIG. 5 , a planarization layer  52  such as borosilicate glass (BSG) dielectric or the like. The planarization layer  52  fills the deep trenches  24  in the alignment mark region  102  and recess portions above the trench capacitors  30  in the device region  101 . 
   As shown in  FIG. 6 , a photoresist layer  60  is formed on the planarization layer  52 . The photoresist layer  60  is used to define active area regions to be formed on the surface of the semiconductor substrate  12 . An alignment device (not shown) of a stepper equipment irradiates a laser beam such He—Ne laser having a pre-selected wavelength such as 635 nm onto the alignment mark region  102  to scan the alignment mark  20  and deep trenches  24 . The diffraction light signal is detected to monitor and assess the alignment accuracy of wafer. When the alignment deviation is smaller than specific value for example, 3 sigma &lt;25 nm, the next step, i.e., exposure of the photoresist layer  60  with respect to the Mask Two (AA mask), is carried out. The active area pattern on the Mask Two is then transferred to the photoresis layer  60 , which is also referred to as “AA photo”. The exposed photoresist layer  60  is then developed using known developer. 
   As shown in  FIG. 7 , the developed photoresist layer comprises active area pattern  60   a  and opening  60   b  within the device region  101 . The opening  60   b , which defines the pattern of shallow trench isolation between the deep trench capacitors, exposes regions to be etched into the semiconductor substrate  12 . Subsequently, the conventional shallow trench isolation (STI) process is carried out. The STI process comprises etching the planarization layer  52 , pad nitride layer  16 , pad oxide layer  14 , the semiconductor substrate  12 , a portion of the polysilicon electrode  34 , a portion of the collar oxide  36 , and a portion of the capacitor dielectric via the opening  60   b , stripping photoresist layer  60 , filling the trench fill dielectric into the shallow trench, and then chemical mechanically polishing the excess trench fill dielectric. The STI process for the deep trench capacitor DRAM device is known in the art, and the details thereof is thus omitted. 
   Please refer to  FIG. 9 .  FIG. 9  is a flowchart  90  depicting the method of fabricating deep trench capacitor DRAM device involving the present invention K1 process. In Step  91 , deep trenches of the trench capacitors and alignment mark are etched. The width W of the deep trenches of the alignment mark is smaller than 0.5 microns. Trench capacitors are formed within the deep trenches in the device region. In Step  92 , K1 process is then performed to refresh the profile of the deep trenches of the alignment mark. AA photo is coated on the semiconductor substrate. From Strp  94  to Step  96 , wafer alignment process is performed. If the deviation measured by Step  94  exceeds the specific value (25 nm), a feedback signal is transmitted to a control unit of a stepper that will shift the wafer stage to compensate the deviation (Step  96 ). In Step  97 , the AA photo is exposed to light and then developed to form desired mask pattern on the wafer. In Step  98 , process of making gate conductor (GC) or word line is carried out. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.