Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the field of semiconductor integrated-circuit (IC) manufacturing, and in particular a technique of providing uniform etch depth by using ion implantation and a timed etch. 
         [0003]    2. Description of the Related Art 
         [0004]    The vertical dimensions of semiconductor devices (e.g. capacitors, transistors) with non-planar surfaces are often defined simply by a timed etch of a homogeneous dielectric (e.g. oxide) material that serves to electrically isolate semiconductor device regions. For example,  FIG. 1A  illustrates a cross-sectional view of a patterned silicon layer  101  and dielectric areas  102  that can provide electrical isolation. During the process of forming devices in patterned silicon layer  101 , dielectric areas  102  can be etched. Generally, this etch has high selectivity between materials, thereby etching dielectric areas  102  much more quickly than patterned silicon layer  101 . In some embodiments, a mask can be used to further protect patterned silicon layer  101  during the etching of dielectric areas  102 . 
         [0005]    Unfortunately, the etch rate across a wafer is typically not perfectly uniform due to equipment imperfections, variations in IC pattern density, and/or inherent conditions associated with the etch process being used. For example, it is not uncommon to have an etch rate variation of 10% across a wafer. The etch rate variation causes non-uniform etching of dielectric areas  102 . Non-uniformity of etched dielectric areas  102 A, as shown in  FIG. 1B , results in variations of vertical device dimensions across a chip, within a wafer, or even from wafer to wafer. In turn, these vertical device dimension variations may result in significant device and circuit performance variations, which is highly undesirable. 
         [0006]    Therefore, a need arises for a technique for providing uniform etch depth to minimize device and circuit performance variations. 
       SUMMARY OF THE INVENTION 
       [0007]    A method of performing a timed etch of a material to a precise depth is provided. In this method, ion implantation of the material is performed before the timed etch. This ion implantation process substantially enhances the etch rate of the material within a precisely controlled depth range corresponding to the range of implantation-induced damage. That is, the damaged material etches significantly faster than the undamaged material. Note that the range of the implantation-induced damage can be tailored by adjusting the implanted ion species, dose, and energy. Generally, the larger the ion mass, the lower the dose required to induce a threshold level of damage required to enhance the etch rate. For example, if argon (Ar) is the implanted ion species, a dose of 1E14 per square centimeter can adequate to significantly enhance the etch rate of silicon dioxide. The depth to which a threshold level of damage is achieved increases approximately logarithmically with the implanted dose, whereas it increases linearly with the projected ion range that is determined by the implant energy. 
         [0008]    Advantageously, by performing the ion implantation prior to etching, the variation in vertical etch depth can be reduced by a factor approximately equal to the etch rate of the damaged material divided by the etch rate of the undamaged material. The vertical etch depth can be used to provide a vertical dimension of a non-planar semiconductor device. Minimizing vertical device dimension variations on a wafer can reduce device and circuit performance variations, which is highly desirable. 
     
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1A  illustrates a cross-sectional view of a patterned silicon layer and dielectric areas therein that can provide electrical isolation. 
           [0010]      FIG. 1B  illustrates the patterned silicon layer and dielectric areas of  FIG. 1A  after a timed dielectric etch process with etch rate variations has been applied. 
           [0011]      FIG. 2  illustrates exemplary steps for improving the uniformity and controllability of a timed etch. 
           [0012]      FIG. 3  illustrates the patterned silicon layer and dielectric areas of  FIG. 1A  after an ion implantation process that can advantageously enhance the dielectric etch rate. 
           [0013]      FIG. 4  illustrates the patterned silicon layer and dielectric areas of  FIG. 1A  after an ion implantation process and a timed dielectric etch. 
           [0014]      FIGS. 5A ,  5 B,  5 C,  5 D are various views of a transistor including a segmented channel region, wherein the ridge isolation material in the segmented channel region can be uniformly etched after an ion implantation process. 
           [0015]      FIG. 6  illustrates the cross-section of a portion of an inverted-T-channel field-effect transistor (ITFET) for which an ion implantation process can be used to form uniformly thin horizontal portions of the channel. 
           [0016]      FIG. 7  illustrates an exemplary dynamic random access memory (DRAM) cell including a capacitor, wherein an ion implantation process can be used to accurately and uniformly form the sidewall portion of the capacitor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    In accordance with one aspect of the invention, ion implantation can be used to improve the uniformity and controllability of a timed etch. As described below, this ion implantation can advantageously enhance the etch rate within a depth range that reaches down to a desired etch depth. 
         [0018]      FIG. 2  illustrates exemplary steps for improving the uniformity and controllability of a timed etch. In step  201 , a wafer having patterned silicon and dielectric areas can be positioned for an etching process. This wafer could have a simplified cross-section similar to that shown in  FIG. 1A . 
         [0019]    In step  202 , an ion implantation process can be performed. The implantation effectively “damages” the upper portions of the silicon and dielectric areas. In other words, much like spraying a plaster wall with bullets can facilitate the subsequent removal of the plaster, ion implantation can accelerate the subsequent removal of the implanted material during a subsequent timed etch. As shown in  FIG. 3 , damaged region  301  (shown by the dotted, semi-transparent area) has a depth  302  within the dielectric areas. Note that the depth of the damaged region  301  within the silicon areas may be different than depth  302 . Depth  302  is determined by the implant species (e.g. Argon), the dose of the implant (e.g. 1E14 per square centimeter), and the energy of the implant. Notably, ion implantation is an extremely uniform process (e.g. dose and energy of an implant species can be controlled within 1% across a wafer and from wafer to wafer). As a result, depth  302  can be controlled to a precise depth range. 
         [0020]    Therefore, referring back to  FIG. 2 , when a timed etch (e.g. a wet etch such as a diluted hydrofluoric acid solution or a dry etch such as a plasma etch including fluorine radicals) is performed in step  203 , the etch depth of the dielectric areas can be precisely controlled. Specifically, as shown in  FIG. 4 , the etched dielectric areas  401  have an etch depth  402  that is substantially identical to the depth  302  ( FIG. 3 ) of damaged region  301  within the dielectric areas. This is because the etch rate of the damaged dielectric is significantly enhanced as compared with the etch rate of the underlying undamaged dielectric. Thus, once the timed etch reaches the undamaged dielectric, i.e. the dielectric areas below depth  302 , the etching rate dramatically slows. 
         [0021]    Advantageously, by accelerating the etch rate by a factor of X (greater than 1), the amount of time needed to etch the damaged dielectric areas can be reduced by a factor of 1/X. Therefore, the over-etch time needed to ensure that all dielectric areas on the wafer are etched to at least the desired etch depth can also be reduced by a factor of 1/X. Reducing the over-etch time can significantly minimize etch depth variations across the wafer. Thus, by enhancing the etch rate within a precise depth range that reaches down to the desired etch depth, the uniformity and controllability of a timed dielectric etch process can be significantly improved. 
         [0022]    Note that damaged region  301  also includes portions of patterned silicon layer  101 . The damage within the silicon can be easily repaired by thermal annealing at a temperature greater than approximately 500 degrees Celsius to re-crystallize the silicon epitaxially from the underlying undamaged portions. Such a thermal annealing process, which is called solid phase epitaxial re-crystallization (SPER), is well known to those in the integrated-circuit manufacturing industry. It can be performed either before or after the timed dielectric etch, preferably the latter to avoid partial healing of the intentional damage in the dielectric areas. The SPER temperature and duration should ideally be sufficiently low so as to minimize diffusion of any dopant atoms (e.g. Boron, Indium, Phosphorus, Arsenic, Antimony) within the silicon. 
         [0023]    Exemplary Application: Corrugated-Channel MOSFET 
         [0024]    By forming metal-oxide-semiconductor field effect transistors (MOSFETs) over a substrate having precisely-formed and regularly-spaced stripes (ridges of semiconductor material(s)), both high performance (i.e. high on-current) and low static power consumption (i.e. low source-to-drain leakage current) can be achieved with good uniformity. The stripes, which can be formed with the aid of an ion implantation process as described below, enable the formation of segmented channel regions that accommodate a wide range of gate-electrode configuration options and also provide greater performance consistency between devices. 
         [0025]      FIG. 5A  shows a top view of an exemplary transistor  500  that includes a segmented channel region. Transistor  500  is formed on a substrate  590  and is surrounded by device isolation material  593  (e.g. shallow trench isolation), and includes a source  510 , a drain  530 , a gate  550 , sidewall spacers  561  and  562 , a source contact region  571 , and a drain contact region  572 . Gate  550  (with a gate length LG) is located between source  510  and drain  530  and is formed over a channel region  520  in substrate  590 . Sidewall spacers  561  and  562  lie over at least a portion of source  510  and drain  530 , respectively, and serve to offset the gate  550  from source contact region  571  and drain contact region  572 , respectively. As indicated by the dotted lines, channel region  520  includes multiple ridges  591  that run between source  510  and drain  530 . Ridges  591  are formed from at least one semiconductor material and may be homogenous structures (e.g. silicon). 
         [0026]    Ridges  591  are formed on an elevated base region  595  that rises from substrate  590 , as shown in  FIG. 1B .  FIG. 1B  is a cross-sectional view of transistor  500  through view location A-A (rotated 90° for clarity). Each ridge  591  has a width W, and is spaced from adjacent ridges by a spacing SP. Furthermore, each ridge  591  extends a height HR above elevated base region  595 , which itself rises a height HB from the adjacent surfaces  590 -S of substrate  590 . 
         [0027]    Note that because ridges  591  are identified relative to substrate  590 , ridges  591  continue to exist as “ridges” even covered with other materials (e.g. even though ridge isolation material  592 , device isolation material  593 , gate dielectrics  540 , and gate  550  completely cover ridges  591 , ridges  591  are still considered to be ridges.) As described in greater detail below, each of ridges  191  is a highly precise structure that therefore provides highly quantifiable performance measures. 
         [0028]    Transistor  500  is isolated from adjacent devices by device isolation material  593  (e.g. silicon dioxide), which extends down to surfaces  590 -S of substrate  590  (i.e. down to the bottom of elevated base region  595 ). Ridge isolation material  592  (which can be formed from the same material(s) or different material(s) than device isolation material  593 ) fills the inter-ridge regions to a distance HG below the top of ridges  591 . Gate  550  is formed over the top portions of ridges  591 , separated from those top portions by gate dielectric  540  (which can be formed from any dielectric material(s)). Thus, when appropriate voltages are applied to gate  550  and between source  510  and drain  530 , each of ridges  591  conducts a portion of the total on-current that flows through transistor  500 . 
         [0029]    Note that gate dielectric  540  and gate  550  “wrap” around the top portions of ridges  591  (i.e. gate dielectric  540  and gate  550  extend down the sides of ridges  591 ). This wrapping configuration allows gate  550  to more effectively control the electric potential distribution within channel region  520 , and can therefore enhance on-current while minimizing source-to-drain leakage current. 
         [0030]      FIG. 5C  shows a cross-sectional view of transistor  500  through view location B-B indicated in  FIG. 5A . View location B-B provides a cross-sectional view running parallel to the direction of current flow between source  510  and drain  530  and through one of ridges  591 , and therefore indicates the full doping profiles in and around channel region  520 . As indicated in  FIG. 5C , source  510  and drain  530  are doped regions within ridge  591 . 
         [0031]    Note that while depicted as extending below the bottom of ridges  591  for exemplary purposes, the depth D of source  510  and drain  530  below the surface of ridge  591  can alternatively be less than the overall height HR of ridge  591  (i.e. depth D is less than ridge height HR), so that ridge isolation material  592  (shown in  FIG. 5B ), which starts from the base of ridge  591 , can effectively reduce the area of the junction between the source  510  and substrate  590 , and the area of the junction between the drain  530  and substrate  590 , thereby reducing junction leakage and capacitance. 
         [0032]      FIG. 5D  shows a cross-sectional view of transistor  500  through view location C-C indicated in  FIG. 5A . View location C-C runs between two ridges  591 , parallel to the direction of current flow between source  510  and drain  530 . Therefore,  FIG. 5D  depicts gate  550  (and sidewall spacers  561  and  562 ) extending down below the top surface of ridge  591 . Specifically, gate  550  extends down to the level of ridge isolation material  592 . Note that because view location C-C runs between ridges  591 , the portions of source  510  and drain  530  on either side of gate  550  (and sidewall spacers  561  and  562 , if present) are actually fill regions  591 -F (i.e. regions formed by filling the space between the exposed portions of adjacent ridges  591 ). U.S. patent application Ser. No. 11/173,237, entitled “Segmented Channel MOS Transistor” and filed on Jul. 1, 2005 by Synopsys, Inc., describes transistor  500  in greater detail and is incorporated by reference herein. 
         [0033]    Table 1 shows sample data for comparing the performance (on-state drive current and off-state source-to-drain leakage current, each normalized to the transistor layout width) of various n-channel implementations of transistor  500  (rows 1, 2, 3, and 4) against the performance specifications with no carrier mobility enhancement as published in the International Technology Roadmap for Semiconductors (ITRS), 2003 Edition (rows 7 and 8). Each of the implementations of transistor  500  is based on a fundamental set of implementation values, including a single ridge  591  in the channel region, a 20 nm spacing between ridges  591  (i.e. the total layout width of the simulated transistor  500  is the ridge width W plus 20 nm), a thickness for gate dielectric  540  equivalent to 1.2 nm of SiO 2 , an undoped channel, a heavily p-type doped (2×10 19  cm −3  boron) pulse doped region starting precisely at the end of the gate overlap (i.e. at a distance X recess  below the surface of the ridge), no stress-based mobility enhancement, and singly doped source/drain regions. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 W 
                 LG 
                 Xrecess 
                 Ion 
                 Ioff 
               
               
                 No. 
                 Description 
                 (nm) 
                 (nm) 
                 (nm) 
                 (mA/μm) 
                 (nA/μm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Slightly 
                 20 
                 20 
                 5 
                 0.5 
                 3 × 10 −6   
               
               
                   
                 Recessed 
               
               
                 2 
                 Moderately 
                 20 
                 20 
                 10 
                 0.6 
                 0.4 
               
               
                   
                 Recessed 
               
               
                 3 
                 Very Recessed 
                 20 
                 20 
                 15 
                 0.98 
                 500 
               
               
                   
               
             
          
         
       
     
         [0034]    As indicated in Table 1, design number 1, which incorporates a ridge width of 20 nm, a gate length of 20 nm, and a recess distance X recess  of 5 nm (a “Slight Recess”), provides an on-state drive current I on  of 0.5 mA/μm and an off-state source-to-drain leakage current I off  of 3×10 −6  nA/μm. Note that I on  and I off  are listed in terms of current per micron layout width (in the direction transverse to the direction of current flow) to normalize the current values. Increasing the recess distance X recess  to 10 nm in design number 2 provides an increase in I on  to 0.6 mA/μm, at the trade-off of increasing I off  to 0.4 nA/μm. Further increasing recess distance X recess  to 15 nm in design number 3 provides a significant jump in Ion to 0.98 mA/μm, but increases I off  to 500 nA/μm. Thus, implementing transistors using values similar to design number 3 would generally be best for high performance circuits, whereas implementing transistors using values similar to design number 1 would be best for ultra-low power circuits. 
         [0035]    As indicated by the performance values provided for design numbers 1-3, the relationship between on-current I on  and off-current I off  can be adjusted via recess distance X recess , i.e. the depth of the heavily p-type doped region. In contrast, the I n /I off  relationship in conventional transistors is typically modified by adjusting the doping concentration within the channel region to achieve a particular threshold voltage. Because dimensional control (i.e. control over recess distance X recess  and depth of the heavy channel doping profile) can be more precise than dopant concentration control (i.e. control over the number of dopant atoms in the channel region), transistors having a corrugated channel made using the above-described ion implantation process and a subsequent timed etch can significantly ease the difficulties associated with achieving a particular combination of on-current I on  and off-current I off . 
       Additional Exemplary Applications 
       [0036]    Advantageously, ion implantation can be used to control etching depth in materials other than oxide. For example,  FIG. 6  illustrates a cross-sectional view of an inverted T-channel field-effect transistor (ITFET)  600 . ITFETs are described in “Inverted T channel FET (ITFET)—Fabrication and Characteristics of Vertical-Horizontal, Thin Body, Multi-Gate, Multi-Orientation Devices, ITFET SRAM Bit-cell operation. A Novel Technology for 45 nm and Beyond CMOS”, which was authored by L. Mathew et al. and published by IEEE in 2005. 
         [0037]    In ITFET  600 , fin channel  602 A and planar channel  602 B, which are formed on a buried oxide (BOX) layer  604 , provide both vertical and horizontal channel regions (hence the “inverted T” designation). After formation of fin channel  602 A and planar channel  602 B, a thin gate dielectric  603  can be formed on fin channel  602 A and planar channel  602 B. Then, a polycrystalline-silicon layer can be deposited and patterned to form a gate  601  for ITFET  600 . 
         [0038]    Notably, the thickness of planar channel  602 B (i.e. the horizontal portions of the channel) determines the threshold voltage of ITFET  600 . Therefore, the etching of the silicon layer to form fin channel  602 A and planar channel  602 B is critical to the performance of ITFET  600 . 
         [0039]    Advantageously, the above-described ion implantation process can precede the timed etch of the silicon to provide a precise silicon etch depth  605 . In one embodiment, the dopant species used in this ion implantation can include germanium at a dose greater than or equal to 1E14 per square centimeter. The depth to which a threshold level of damage can be precisely controlled by adjusting the dose and energy of the implanted species. By enhancing the silicon etch rate within a precise depth range, the uniformity and controllability of the silicon etch process can be substantially improved, thereby minimizing ITFET performance variations across a wafer. 
         [0040]    The use of an ion implantation process to improve etch depth uniformity can be applied to various types of devices having vertical surfaces. For example,  FIG. 7  illustrates an exemplary dynamic random access memory (DPAM) cell  700  including an access transistor  701  and a capacitor  702  fabricated in a semiconductor substrate  703 . Capacitor  702  is partially formed in a cavity of a shallow trench isolation (STI) region  704 , which is adjacent to a sidewall region of substrate  703 . A patterned polycrystalline-silicon layer  706  can be used to form the electrodes of access transistor  701  and capacitor  702 . The portion of capacitor  702  formed in the sidewall region can increase its capacitance with a minimum of layout area. 
         [0041]    Notably, the junction of this sidewall portion with the horizontal portion of capacitor  702  may result in some undesirable leakage. However, the depth of the sidewall portion can compensate for such leakage. A recess depth  707  of STI region  704  in which this sidewall portion of capacitor  702  is formed can significantly affect the final capacitance of capacitor  702 . Therefore, the etching of STI region  704  (e.g. a field oxide) to form capacitor  702  is critical to its performance. Advantageously, the above-described ion implantation process can precede the timed etch of STI region  704  to accurately and uniformly provide recess depth  707 . By enhancing the etch rate of STI region  704  within a precise depth range, the uniformity and controllability of the STI region etch process can be substantially improved, thereby minimizing variations of capacitances across a wafer. 
         [0042]    Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying figures, it is to be understood that the invention is not limited to those precise embodiments. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. As such, many modifications and variations will be apparent. For example, in one application, the fin height of FinFETs (transistor structures with fin-like channel regions) made on bulk silicon wafers can be determined by using the above-describe ion implantation and timed etch to provide precise control of etch depth, thereby minimizing FinFET performance variations. Accordingly, it is intended that the scope of the invention be defined by the following Claims and their equivalents.

Technology Category: 5