# Patent

## Title
Technique for fabricating logic elements using multiple gate layers

## Abstract

Various techniques are described which utilize multiple poly-silicon layers in the design and fabrication of various logic elements that are used in semiconductor devices. According to a specific implementation of the present invention, logic gate cell sizes and memory array cell sizes may be reduced by fabricating various transistor gates using multiple poly-silicon layers. The techniques of the present invention of using multiple layers of poly-silicon to form transistor gates of logic elements provides extra degrees of freedom in fine tuning transistor parameters such as, for example, oxide thickness, threshold voltage, maximum allowed gate voltage, etc.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims benefit of priority from U.S. Provisional Application No. 60/421,115 filed Jul. 2, 2002, and entitled “TECHNIQUE FOR FABRICATING LOGIC ELEMENTS USING MULTIPLE GATE LAYERS,” which is hereby incorporated herein by reference.


BACKGROUND OF THE INVENTION
The present invention relates generally to integrated circuit chip fabrication, and more specifically to a technique for fabricating logical elements using a multiple gate layer technique.
The fabrication of an integrated circuit typically involves a variety of steps including a design phase, multiple simulation phases, and a fabrication phase. During the design phase, the various logical components of the integrated circuit (IC) are incorporated into a unified design layout, typically with the help of integrated circuit design software. Thereafter, during a simulation phase, the integrated circuit design is tested using conventional circuit simulation software such as, for example, spice parameter extraction software. Examples of spice parameter extraction software include BSIMPRO (licensed by Celestry Design Technologies, Inc., of San Jose, Calif.), and Aurora (licensed by Synopsys, Inc., of Mountain View, Calif.). During the fabrication stage of the integrated circuit, a variety of IC structures may be formed on a silicon wafer by forming layers on and removing various layered regions from the silicon wafer.
Generally, it is desirable to reduce the costs and expenses associated with integrated circuit (IC) chip fabrication. A conventional technique for reducing IC chip fabrication costs involves reducing the number of deposition and etching steps which are performed during the IC chip fabrication process. For this reason, it is the conventional practice in the industry to fabricate integrated circuits using only a single layer of deposited poly-silicon. Such a technique provides for a relatively less complex and cheaper fabrication process. In fact, the use of a single poly-silicon layer in the fabrication of logic elements (which form part of an integrated circuit) is so widely accepted that most conventional circuit simulation software currently available on the market are primarily designed to be compatible with standardized layout and fabrication techniques which use a single poly-silicon layer.
Examples of a portion of a conventional integrated circuit design are illustrated in FIGS. 1A–C of the drawings. FIG. 1A shows a schematic diagram of a circuit 100 which includes two transistors that are connected in series (herein referred to as “series transistor circuit”). When incorporated as part of an integrated circuit using conventional IC fabrication techniques, the series transistor circuit 100 of FIG. 1A may be fabricated as illustrated in FIG. 1B. As illustrated in FIG. 1B, the circuit portion 150 includes two serially connected transistors which have been fabricated using a single poly-silicon layer. More specifically, as shown in FIG. 1B, the circuit portion 150 includes two gate portions 102a, 102b which have both been fabricated using a single poly-silicon layer. Additionally, circuit portion 150 also includes two oxide layer portions 104a, 104b, which have both been fabricated using a single oxide layer. The circuit portion 150 further includes a substrate 110 (e.g., silicon substrate), which includes three doped regions 105a, 105b, 105c formed within a doped well region 108. In the example of FIG. 1B, the circuit portion 150 has been configured as two serially connected NMOS transistors, which include P-well region 108, and N+ doped regions 105a–c. Such a circuit may be used, for example, in the formation of a variety of conventional logic elements such as NOR gates, NAND gates, etc.
FIG. 1C shows an example of a conventional IC design layout 170 of the series transistor circuit 100 of FIG. 1A. As illustrated in FIG. 1C, the conventional technique for fabricating the series transistor circuit 100 is performed using a single poly-silicon layering technique, wherein gates 102a and 102b are formed over an active region 115 of the transistor circuit. Each of the gates 102a, 102b is formed from the same poly-silicon layer. Using conventional terminology, gates 102a and 102b may each be described as being composed of “poly1” material since each of these gates are formed from the same first layer of deposited polysilicon (i.e., poly-1). According to conventional design rules, each of the gates 102a and 102b are required to be separated by a minimum distance 117 in order to ensure proper operation of the fabricated circuit.
While the use of a single poly-silicon layer conforms with standardized IC layout and fabrication techniques, such standardized techniques necessitate specific design and layout requirements which may result in an inefficient utilization of space on the silicon wafer or substrate. Accordingly, it will be appreciated that there exists a continual need to improve upon integrated circuit chip fabrication techniques in order to accommodate and take advantage of new and emerging technologies.
SUMMARY OF THE INVENTION
According to different embodiments of the present invention, various methods, devices and systems are described for forming a logic element on an integrated circuit fabricated on a substrate. A first gate structure, comprised of a first poly-silicon layer, is formed over the substrate. A second gate structure, comprised of a second poly-silicon layer, is then formed over the substrate in a manner such that a portion of the second gate structure is formed over a portion of the first gate structure, thereby forming a overlapping gate region. According to specific embodiments, the logic element may correspond to a variety of logic elements or devices such as, for example, NAND gates, AND gates, NOR gates, OR gates, XOR gates, SRAM cells, latches, etc.
According to specific embodiments, the logic element of the present invention includes a circuit portion which is designed to exhibit performance characteristics substantially similar to performance characteristics of two serially connected transistors or, alternatively, two transistors connected in parallel. In such embodiments, the first gate structure and second gate structure may correspond to transistor gates of the circuit portion. Additionally; the logic element may also include source and drain doped regions formed in the substrate, and an active region formed in the substrate between the source and drain regions, wherein the active region is designed to enable current flow between the source and drain regions.
It will be appreciated that the various techniques of the present invention for using multiple layers of poly-silicon to form transistor gates of logic elements provides extra degrees of freedom in fine tuning transistor parameters such as, for example, oxide thickness, threshold voltage, maximum allowed gate voltage, etc.
Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A–C show examples of conventional integrated circuit design and fabrication techniques for a series transistor circuit.
FIG. 2A shows a portion 200 of a logic element which has been fabricated in accordance with a specific embodiment of the present invention.
FIGS. 2B–2J illustrate one technique for fabricating a logic element in accordance with a specific embodiment of the present invention.
FIG. 2K shows an alternate embodiment of a portion 280 of a logic element which has been fabricated in accordance with a specific embodiment of the present invention.
FIGS. 3A–D illustrate different embodiments of a series transistor circuit which may be fabricated in accordance with the techniques of the present invention.
FIG. 4 shows a top view of a conventional design layout for fabricating an SRAM memory cell.
FIG. 5 shows an example of an SRAM memory cell design layout which may be fabricated using the technique of the present invention.
FIGS. 6A–C show examples of conventional integrated circuit design and fabrication techniques for a parallel transistor circuit.
FIGS. 7A–D illustrate different embodiments of a parallel transistor circuit which may be fabricated in accordance with the techniques of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention describes various techniques which utilize multiple poly-silicon layers in the design and fabrication of various logic elements (e.g., NAND gates, AND gates, NOR gates, OR gates, XOR gates, SRAM cells, latches, etc.) that are used in semiconductor devices. According to a specific implementation of the present invention, logic gate cell sizes and memory array cell sizes may be reduced by fabricating various transistor gates using multiple poly-silicon layers. In this way, area reduction of the integrated circuit chip may be achieved by reducing the standardized design rules corresponding to minimum poly-1 to poly-1 spacing. Thus, for example, the technique of the present invention may allow for the layout and/or design of overlapping poly-silicon pieces which do not short to one another because the different poly-silicon pieces may be formed using a multi-layer poly-silicon fabrication technique. According to a specific embodiment, such overlapping pieces may be composed of at least two different poly-silicon layers which are vertically separated by at least one insulating layer such as, for example, an oxide layer. In this way, electrical shorting of the overlapping poly-silicon pieces may be prevented. Moreover, the technique of the present invention of using multiple layers of poly-silicon to form transistor gates of logic elements provides extra degrees of freedom in fine tuning transistor parameters such as, for example, oxide thickness, threshold voltage, maximum allowed gate voltage, etc.
As noted previously, conventional wisdom teaches the desirability to reduce or minimize the cost associated with fabricating integrated circuits. Typically, one technique for reducing or minimizing such costs is to minimize the number of poly-silicon layers which are used to form the logic elements of an integrated circuit. More recently, fabrication techniques used for fabricating certain types of memory such as, for example, flash memory, utilize a double poly-silicon layer process whereby different layers of poly-silicon are deposited onto the silicon wafer at different times in order to form the control gates and floating gates of the flash memory cells. In specific applications where an integrated circuit chip design is to include both flash memory and conventional logic elements, fabrication of the integrated circuit chip may involve a double poly-silicon layering process in order, for example, to form the flash memory cells. However, during fabrication of such integrated circuits, it remains the conventional practice to fabricate the logic elements of the integrated circuit using a single poly-silicon layer. One reason why it is desirable to design and fabricate the logic elements of an integrated circuit using a single poly-silicon layer (even in situations where the integrated circuit includes flash memory) is because single poly layer circuits are more simplistic in design and therefore are typically easier to fabricate, and are less subject to fabrication errors. Additionally, as noted previously, most conventional circuit simulation software currently available on the market are designed only to be compatible with standardized layout and fabrication techniques which use a single poly-silicon layer. Such circuit simulation software is typically not compatible with multi-poly-silicon layer designs.
Contrary to conventional wisdom and practice, however, the present invention teaches the desirability to fabricate logic elements using multiple poly-silicon layers, particularly in applications where memory elements (e.g., flash memory, DRAM) and logic elements are to be fabricated on the same integrated circuit chip. In such applications, one is able to take advantage of the multiple poly-layer process (e.g., used for fabricating the memory elements) by designing logic elements which also utilize multiple poly-layers.
FIG. 2A shows a portion 200 of a logic element which has been fabricated in accordance with a specific embodiment of the present invention. More specifically, the circuit portion 200 illustrated in FIG. 2A is an example of how a series transistor circuit (such as that shown in FIG. 1A) may be fabricated in accordance with a specific embodiment present invention. According to specific embodiments, the circuit portion 200 may be used in the fabrication a variety of logic elements such as, for example, NAND gates, AND gates, NOR gates, OR gates, XOR gates, SRAM cells, latches, etc.
A specific embodiment for fabricating the circuit portion 200 is illustrated in FIGS. 2B–2I of the drawings. FIGS. 2B–2I illustrate one technique for fabricating a logic element in accordance with a specific embodiment of the present invention. In the example illustrated in FIGS. 2B–2I, it is assumed that a silicon wafer is being used for the fabrication of an integrated circuit chip. In preparation for the IC chip fabrication process, portions of the silicon wafer may be doped with an p-type material, thereby forming a P-well 208.
As shown in FIG. 2B, a first oxide layer 204a′ is formed on the surface of the silicon wafer 210. After formation of the first oxide layer 204a′, a first poly-silicon (poly-1) layer 202a′ may be deposited over the first oxide layer. Regions of the poly-1 layer 202a′ may then be removed or etched to thereby form first poly-silicon layer portion 202a, as illustrated in FIG. 2C. After formation of the first poly-silicon layer portion 202a, regions of a first oxide layer 204a′ may be removed to thereby form a first oxide layer portion 204a as illustrated in FIG. 2D. According to a specific embodiment, the formation and/or removal of the poly-silicon layers and oxide layers may be achieved using conventional IC chip fabrication techniques commonly known to one having ordinary skill in the art.
As illustrated in FIGS. 2E and 2F, a second oxide layer 204b′ and a second poly-silicon (poly-2) layer 202b′ may then be formed and/or deposited over portions of the silicon wafer, including, portions 202a and 204a. As shown in FIG. 2G, regions of the poly-2 layer may be removed to thereby form second poly-silicon layer portion 202b. Thereafter, selected regions of the second oxide layer may be removed to thereby form second oxide layer portion 204b as illustrated in FIG. 2H. As illustrated in FIG. 2I, doped regions 205a and 205b may then be formed, for example, using conventional ion implementation techniques. According to specific embodiments, the doped regions 205a and 205b may be doped with an n-type material such as, for example, arsenic. Alternatively, region 208 may be doped with an n-type material, thereby forming an N-well, and regions 205a and 205b may be doped with a p-type material.
When implemented as two serially connected NMOS transistors, the circuit portion 200 will correspond to the circuit portion 275 of FIG. 2J, which may be schematically represented by the schematic diagram 100 of FIG. 1A. As shown in FIG. 2J, the two gates (e.g., gate B 202b and gate A 202a) of the series transistor circuit 275 are used to control the flow of current from source 205a to drain 205b. 
A comparison between circuit portion 150 of FIG. 1B (which represents a serial transistor circuit fabricated using conventional techniques) and circuit portion 275 of FIG. 2J reveals a number of differences. For example, the distance separating gate A and gate B is much smaller in FIG. 2J as compared to FIG. 1B. More specifically, as illustrated in FIG. 2J, the distance separating gate 202b and gate 202a is approximately equal to the thickness of the second oxide layer portion 204b. Additionally, as illustrated in FIG. 2J, the poly-2 layer portion 204b is positioned in a continuous manner both over and adjacent to the poly-1 layer portion 202a and first oxide layer portion 204a. The poly-2 layer portion 202b also overlaps with a region of the poly-1 layer portion 202a. According to different embodiments, this amount of overlap may vary within a range of 0% of overlap (e.g., abutting gate regions) to about 100% overlap (e.g., completely overlapping gate regions).
Further, as illustrated in FIG. 2J, an entire doped region has been eliminated from the silicon substrate, as compared to FIG. 1B. For example, as illustrated in FIG. 2J, the circuit portion 275 includes two N+ doped regions 205a, 205b. In contrast, the circuit portion 150 of FIG. 1B includes 3 N+ doped regions, namely 105a, 105b, and 105c. A comparison of FIGS. 1B and 2J reveals that the doped region 105b which exists between gate A and gate B of FIG. 1B has been eliminated in the structure of FIG. 2J This reduces the area of the logic element on the wafer, which results in a reduction of the die size and associated fabrication costs.
It will be appreciated that alternate embodiments of the present invention may include features different than those illustrated in the circuit portion 275 of FIG. 2J. For example, FIG. 2K shows an alternate embodiment of a circuit portion 280 which has been fabricated in accordance with a specific embodiment of the present invention. As shown in the embodiment of FIG. 2K, the circuit portion 280 includes two overlapping poly-silicon layers 282a, 282b, which are formed over substrate 210. In this particular embodiment, the substrate 210 is comprised of N-type material, and the (p+) doped regions 285a, 285b are formed of P-type material. One of the notable differences between the circuit portions 275 and 280 is that, the P-well region 208 of circuit portion 275 (FIG. 2J) functions as a local substrate for the transistors of circuit portion 275, whereas circuit portion 280 does not include a separate well region that is distinct from the substrate 210. Rather, in the circuit portion 280 (FIG. 2H), the substrate 210 functions as the local substrate for the transistors of circuit portion 280.
FIGS. 3A–D illustrate different embodiments of a series transistor circuit which may be fabricated in accordance with the techniques of the present invention. FIG. 3A shows a perspective view of the circuit portion 200 of FIG. 2A. FIG. 3B shows a perspective view of an alternate embodiment of a circuit portion 350 which may be used to implement the series transistor circuit 100 of FIG. 1A.
Referring to FIG. 3A, it is noted that the design of circuit portion 300 differs from conventional circuit designs in several aspects. For example, as noted previously, different poly-silicon layers are used to form the transistor gates 202a, 202b. Additionally, the location and configuration of the gate structures differs from that of conventional circuit designs, such as that illustrated in FIG. 1B. For example, as shown in FIG. 3A, gate 202b overlaps a portion of the gate 202a in a manner which results in a portion of the gate 202a being interposed between gate 202b and the active region of the substrate 208. Additionally, unlike the conventional gate configurations of FIG. 1B (which are generally flat), the configuration of gate 202b is not flat, but rather is of a stepped configuration which includes portions that extend both horizontally and vertically. As a result, neither the top nor bottom surfaces of gate 202b are substantially planar.
It is also noted that the design of circuit portion 300 differs from conventional non-volatile memory structures in several aspects. For example, in conventional non-volatile memory cell structures, two layers of poly-silicon may be used to form a conventional split gate cell which includes a control gate and a floating gate. Conventionally, the floating gate is designed as an electrically isolated region which is used as a storage node to store a charge for a single non-volatile memory cell. It is important for the floating gate to be electrically isolated from all other structures in the memory cell in order to properly store the charge. In contrast, neither of the gate structures 202a, 202b of FIG. 3A are implemented as electrically isolated regions in a manner similar to the floating gate structure of a non-volatile memory. Rather, the gate structures 202a, 202b may each be electrically coupled to other portions of the integrated circuit, via one or more contact regions, in order to allow desired gate voltage to be applied to the transistor circuit 300. This is illustrated, for example, in the embodiment of FIG. 3C.
FIG. 3C shows a top view of a circuit layout 360 which has been designed using a specific embodiment of the technique of the present invention. More specifically, the embodiment of FIG. 3C represents a specific implementation of a design layout for series transistor circuit such as that illustrated in FIG. 3A. A conventional schematic illustration of a series transistor circuit is illustrated in FIG. 1A. However, FIG. 3D shows an example of a schematic diagram 370 which may be used to schematically represent the circuit 360 of FIG. 3C.
As shown in FIG. 3C, the series transistor circuit 360 includes two gates 382a, 382b having overlapping portions over the active region 365. According to a specific embodiment, the circuit 360 may be fabricated using a multiple poly-silicon laying technique of the present invention, wherein a first gate (e.g. 382a) is formed with the poly-1 layer, and a second gate (e.g. 382b) is formed with the poly-2 layer. A portion of the two gates overlap as shown at region 367. According to a specific embodiment, the width W1 of the gate overlap region 367 is at least equal to or greater than the width W2 of the active region 365. As illustrated in FIG. 3C, each of the gates 382a, 382b includes a respective contact region 362a, 362b for providing electrical contact to each of the gates. According to a specific embodiment, the poly-1 and poly-2 layers may be made of poly-silicon or other appropriate conductive material commonly known to one having ordinary skill in the art.
Additionally, as shown in FIG. 3C, the active region 365 may include a source contact region 364 and a drain contact region 366. In the example of FIG. 3C, the gate contact regions 362a, 362b are placed on opposite sides of the active region 365 in order to ensure that design constraints are met relating to minimum spacing between contact points. However, it will be appreciated that there are a variety of different ways to implement the circuit 360 of FIG. 3C. For example, in an alternate implementation (not shown), the contact points on each of the gates 382a, 382b may be located on the same side of the active region 365. It will be appreciated, however, that one commonality among each of the different implementations is that portions of the gate 382a, 382b will overlap or abut each other over the active region 365. It will be appreciated that the circuit portion 300 of FIG. 3A illustrates one type of multi-poly overlay gate structure which may be used in the fabrication of logic elements of an integrated circuit chip. One issue related to the structure of circuit portion 300 is that the variable gate length of gate 202b may be subject to misalignment relative to gate 202a. One embodiment for resolving the problems of gate misalignment due to variable gate length is illustrated in FIG. 3B of the drawings.
FIG. 3B shows a perspective view of an alternate embodiment of a logic element circuit portion 350 which has been implemented in accordance with a specific embodiment of the present invention. As illustrated in FIG. 3B, the circuit portion 350 includes a first oxide layer 304a, a poly-1 gate 302a, a second oxide layer 304b, and a poly-2 gate 302b. As illustrated in FIG. 3B, the poly-2 gate 302b is adjacent to both sides of the poly-1 gate 302a. In addition, the poly-2 gate 302b overlaps the poly-1 gate 302a over the active region 308 of the substrate. Thus, as illustrated in FIG. 3B, at least a portion of the gate 302a is interposed between gate 302b and the active portion 308 of the silicon substrate. Additionally, as shown in FIG. 3B, gate 302a is separated from gate 302b by a distance which is approximately equal to the thickness of the second oxide layer 304b. 
One of the advantages of the gate structure configuration of FIG. 3B is that it may reduce or eliminate the variable gate length issue described above with respect to FIG. 3A. For example, according to a specific implementation, the overall width W of gate 302b may remain constant since, for example, the width of gate 302b is determined by a mask edge which is able to be properly aligned. Accordingly, the gate configuration of FIG. 3B may be used to mitigate misalignment issues between the poly-1 gate 302a and the poly-2 gate 302b. 
It is noted that the circuit portions 300 and 350 are only intended to illustrate the structures over the active regions (e.g., 208, 308) of each circuit, and do not necessarily reflect all of the features of each circuit. Thus, it will be appreciated that the circuit portions 300 and 350 may include other features not illustrated in FIGS. 3A and 3B. For example, one such feature relates to the contact points used for contacting the gate structures 202a, 202b, 302a, 302b. Another feature relates to the configuration of the gate structures 202a, 202b, 302a, 302b. For example, in one implementation each of the gate structures may be implemented as a line of poly-silicon which may extend in any direction in the X-Z plane. Another feature relates to the addition of other transistors which may be used to build desired logic elements.
It will be appreciated that the technique of the present invention of using multiple layers of poly-silicon to form logic elements provides extra degrees of freedom in fine tuning various transistor parameters such as, for example, oxide thickness, threshold voltage, maximum allowed gate voltage, etc. For example, according to different embodiments, the poly-1 and poly-2 gate oxides may each be fabricated with different thickness in order to fine tune various transistor parameters. According to a specific implementation, two logic transistors of the same size (e.g., width and length) may benefit from having 2 different threshold voltages because their respective gate oxides may be made of 2 different oxide layers. Additionally, it will be appreciated that in conventional MOS transistors, the drain and the source junctions both diffuse laterally under the gate region, thereby reducing the effective gate length and exacerbating short channel effects. However, using the series transistor circuit configurations of the present invention, one junction (with its corresponding lateral diffusion) may be eliminated, for example, for each pair of transistors that are connected in series, thereby improving the short channel effects of the series transistor circuit(s).
Another common circuit which is used in the design of conventional logic elements is illustrated in FIG. 6A. FIG. 6A shows a circuit portion 600 which includes 2 transistors connected in parallel (herein referred to as parallel transistor circuit 600). A conventional design layout for fabricating the parallel transistor circuit 600 is illustrated in FIG. 6C. As illustrated in FIG. 6C, the conventional parallel transistor circuit layout 670 includes the formation of 2 poly-1 gates 652a, 652b over an active region 681 of the silicon substrate. The gates 652a, 652b are formed using a single poly-silicon layer. According to conventional design rules, the gates are required to be separated from each other by a minimum distance 679. In the embodiment shown in FIG. 6C, the source regions 672a, 672b of the parallel transistor circuit are electrically coupled together via an electrical connection 677.
FIG. 6B illustrates a sectional view of a parallel transistor circuit portion 650 which has been fabricated using conventional IC fabrication techniques. The circuit portion 650 illustrated in FIG. 6B has been fabricated using a single poly-silicon layer by employing a technique similar to the technique described previously for fabricating series transistor circuit portion 150 of FIG. 1B. As illustrated in FIG. 6B, circuit portion 650 includes first oxide layer portions 604a, 604b (which are both formed from the same, first oxide layer), poly-1 gate portions 602a, 602b (which are formed from a single poly-silicon layer), and 3 distinct doped regions 605a, 605b, 605c. 
FIG. 7A shows a perspective view of a parallel transistor circuit portion 700 which has been fabricated in accordance with a specific embodiment of the present invention. As illustrated in FIG. 7A, the circuit portion 700 includes a first oxide layer portion 704a, a poly-1 gate 702a, a second oxide layer portion 704b, a poly-2 gate 702b, and 2 doped regions 705a, 705b, which may function as the source and drain regions of the parallel transistor circuit. According to a specific implementation, the technique for fabricating the parallel transistor circuit portion 700 of FIG. 7A is similar to the technique described previously with respect to FIGS. 2B–2I of the drawings. Thus, for example, the poly-1 gate 702a may be formed from a first poly-silicon layer, and the poly-2 gate 702b may be formed from a second poly-silicon layer which is different than the first poly-silicon layer. Additionally, oxide layer portion 704a may be formed from a first oxide layer, and oxide layer portion 704b may be formed from a second oxide layer which is different than the first oxide layer.
FIG. 7C shows a top view of a design layout 760 which may be used for fabricating a parallel transistor circuit such as that illustrated in FIG. 7A. FIG. 7D shows a schematic diagram 770 which provides a schematic representation of the parallel transistor circuit design 760 of FIG. 7C. As illustrated in FIG. 7C, the parallel transistor circuit design 760 includes a poly-1 gate 782a and a poly-2 gate 782b. Each of the gates 782a, 782b are positioned at least partially over active region 765 between the source and the drain. Each of the gates includes a respective contact region 762a, 762b. A portion of the two gates overlap as shown at region 767. According to a specific embodiment, the width W2 of the gate overlap region 767 is less than the width W1 of the active region 760.
One difference between the parallel transistor circuit design of FIG. 7C and the conventional parallel circuit design of FIG. 6C is that the circuit of FIG. 6C includes two source regions 672a, 672b which are electrically coupled together via an electrical connection 677. In contrast, as illustrated in FIG. 7C, the parallel transistor circuit 760 includes a single source region 764 and a single drain region 766.
FIG. 7B shows a perspective view of an alternate embodiment of a parallel transistor circuit portion 750 which has been fabricated in accordance with the techniques of the present invention. As illustrated in FIG. 7B, the circuit portion 750 includes a first oxide layer portion 754a, a poly-1 gate 752a, a second oxide layer portion 754b, a poly-2 gate 752b, and 2 doped regions 755a, 755b, which may function as the source and drain regions of the parallel transistor circuit. According to a specific implementation, the technique for fabricating the parallel transistor circuit portion 750 of FIG. 7B is similar to the technique described previously with respect to FIGS. 2B–2I of the drawings. Thus, for example, the poly-1 gate 752a may be formed from a first poly-silicon layer, and the poly-2 gate 752b may be formed from a second poly-silicon layer which is different than the first poly-silicon layer. Additionally, oxide layer portion 754a may be formed from a first oxide layer, and oxide layer portion 754b may be formed from a second oxide layer which is different than the first oxide layer.
It is noted that one of the structural differences between the series transistor design of FIG. 3C and the parallel transistor design of FIG. 7C is that, as illustrated in FIG. 3C, either of the gates 382a, 382b has the ability to cut off the flow of current from source 364 to drain 366. As illustrated in FIG. 7C, however, neither of the gates 782a, 782b has complete control over the flow of current from source 764 to drain 766. Rather, each of the gates has control over a portion of the current flow through the active region. Yet, according to the embodiment as illustrated in FIG. 7C, appropriate control voltage is preferably applied to both gates 782a, 782b in order to stop the flow of current from source to drain, for example.
It will be appreciated that the various circuits illustrated in FIGS. 3A–3D and 7A–7D may be used to fabricate a variety of different logic elements which form part of an integrated circuit chip. Such logic elements include NAND gates, AND gates, NOR gates, OR gates, XOR gates, latches, etc. Additionally, such logic elements may include static memory cells such as, SRAM. FIG. 4 shows a top view of a conventional design layout for fabricating an SRAM memory cell 400. Conventionally, SRAM memory cells are fabricated using a standardized design which includes only a single poly-silicon layer. Such a design provides for a relatively less complex and cheaper fabrication process. Accordingly, as illustrated in FIG. 4, the conventional SRAM cell design includes three poly-1 portions 402, 404a, 404b, each of which are formed from the same poly-silicon layer.
Conventional design constraints require that the various poly-1 portions (e.g. 402, 404a, 404b) be separated from each other by a minimum distance (e.g. distance A) in order, for example, to prevent shorting. Additionally, as shown in FIG. 4, the conventional SRAM cell design includes a P+ doped region 406, and an N+ doped region 408. A portion of the SRAM cell 400 is formed over a P-well 420. In this figure, the various metal interconnections between the regions 406, 408, and 404 are not shown.
The various design constraints associated with conventional SRAM cell fabrication technology require that the various structures of the SRAM cell be designed to have at least a minimum specified amount of spacing from (or overlapping with) other structures within the SRAM cell. For this reason, the size of a conventional SRAM cell may not be reduced smaller than a minimum designated size. For example, if the minimum feature size is 100 nm, the SRAM cell size will typically have an area of at least 1 m2. However, using the fabrication technique of the present invention, memory array cell sizes may be reduced by fabricating various transistors using multiple poly-silicon layers. In this way, area reduction of the memory array cell sizes may be achieved by reducing the IC design rules corresponding to minimum poly-1 to poly-1 spacing.
FIG. 5 shows an example of an SRAM memory cell design layout which may be fabricated using the technique of the present invention. As illustrated in FIG. 5, the SRAM cell 500 includes at least one poly-1 layer 502 and multiple poly-2 layers 504a, 504b, which are formed from a different poly-silicon layer than the poly-1 layer 502. Each of the poly-silicon layers 502, 504a, 504b includes a respective gate region 530, and a respective interconnect region 532. According to a specific embodiment, the interconnect regions may correspond to portions of the poly-silicon layers which are formed over passive (or field) regions of the SRAM cell 500. In the embodiment of FIG. 5, a portion of the SRAM cell 500 is formed over a P-well 520. Additionally, in this figure, the various metal interconnections between the regions 506, 508, and 504 are not shown.
According to a specific embodiment, the technique for fabricating the various transistors which are included in the SRAM cell 500 of FIG. 5 may be fabricated using a method which is similar to the transistor fabrication technique described with respect to FIGS. 2B–II of the drawings. In one of many possible embodiments, a multiple poly-silicon layer SRAM cell may be fabricated wherein the transfer gate transistors include gates which are made of a poly-1 layer, and the pull-up and/or pull-down transistors include gates which are made of a poly-2 layer.
As illustrated in FIG. 5, the size of the SRAM cell 500 may be reduced, for example, by overlapping portions of the poly-1 layer 502 with portions of the poly-2 layer 504a, 504b as illustrated at 515. Such a design technique also helps to reduce the spacing between the poly-1 layer 502 and the N+ region 508 as indicated at B′. Additionally, as illustrated in the embodiment of FIG. 5, the overlapping of the poly-silicon regions at 515 occurs at the interconnect regions 532 of the poly-silicon layers (e.g., over the passive regions of the SRAM cell 500).
Although not illustrated in FIG. 5, the SRAM cell design 500 may also include at least two different oxide layers which help insulate the poly-silicon layers from each other and the surrounding structures. For example, the first oxide layer may be located under poly-1 portion 502, and a second oxide layer may be located under the poly-2 layers 504a, 504b, thereby electrically insulating the poly-2 layers from the poly-1 layer.
As with the circuits of FIG. 3A–D and 7A–D, the use of multiple poly-silicon and oxide layers in the SRAM cell design of FIG. 5 provides extra degrees of freedom in fine tuning transistor parameters such as, for example, oxide thickness, threshold voltage, maximum allowed gate voltage, etc.
Although several preferred embodiments of this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of spirit of the invention as defined in the appended claims. For example, according to specific embodiments, the transistor gate material used to form the logic elements of the present invention may be comprised of conductive materials (e.g, titanium), semiconductive materials (e.g., poly-silicon), or a combination of the two (e.g., titanium silicide). Additionally, the insulating layers (e.g., oxide layer) described in the various embodiments of this application may be comprised of silicon dioxide and/or other types of insulative or dielectric materials.

## Claims

1. A method of forming a logic element of an integrated circuit, the logic element including a circuit portion configured or designed opexfonn at least one logic operation, the integrated circuit being fabricated on a substrate, the method comprising:
forming a first gate over the substrate, the first gate being comprised La first transistor gate layer;
forming a second gate over the substrate, the second gate being comprised of a second transistor gate layer;
wherein a portion of the second gate is formed ova a first portion of the first gate, thereby forming a first overlapping gate region;
forming a first electrical contact region on the first gate;
forming a second electrical contact region on the second gate;
wherein the first gate corresponds to a completed first transistor gate and wherein the second gate correspond to a completed second transistor gate of the circuit portion
forming a first doped region in the substrate which functions as a source region of the circuit portion;
forming a second doped region in the substrate which functions as a region of the circuit portion;
forming an active region in the substrate between said source and regions, said active region being designed to enable current flow between said source and drain regions;
wherein the first transistor gate includes an active region portion and a contact region portion, the active region portion corresponding to that portion of the first transistor gate which is over the active region:
wherein the forming of the completed second transistor gate include removing portions of the second transistor gate layer in a manner which results in formation of the completed second transistor gate such that at least the portion of the completed second transistor gate is formed over the entirety of the active region portion of the first transistor gate; and
forming the logic element in a manner which enables the logic element to perform at least one logic operation.

2. The method of claim 1 further comprising:
forming the first transistor gate layer over the substrate; and
thereafter forming the second transistor gate layer over the substrate.

3. The method of claim 1 wherein the gate is formed having a first thickness, and
wherein the second gate is formed having a second thickness; and
wherein the first thickness is different than the second thickness.

4. The method of claim 1 further comprising:
forming a first dielectric layer over said substrate;
thereafter forming the first transistor gate layer over the substrate;
thereafter forming a second dielectric layer over said substrate; and
thereafter forming the second transistor gate layer over the substrate.

5. The method of claim 1 wherein the first gate layer and the second gate layer each include poly-silicon material.

6. The method of claim 4 wherein the first and second dielectric layers each include silicon dioxide.

7. The method of claim 4 wherein the dielectric layer is formed having a first thickness, and
wherein the second dielectric layer is formed having a second thickness; and
wherein the first thickness is different than the second thickness.

8. The method of claim 1 further comprising:
forming a dielectric layer between the first gate and the second gate, the dielectric layer having a first thickness;
wherein a spacing between said second portion of the second gate and aid first portion of the first gate is substantially equal to the first thickness of the dielectric layer.

9. The method of claim 1 wherein the circuit portion is designed to exhibit performance characteristics substantially similar to performance characteristics of two serially connected transistors including a first transistor and a second transistor;
wherein the first transistor has associated therewith a first threshold voltage value; and
wherein the second transistor has associated therewith a second threshold voltage value which is different than the first threshold voltage value.

10. The method of claim 1 wherein said circuit portion is devoid of third doped region having doping characteristics substantially similar to doping characteristics of the first doped region.

11. The method of claim 1 wherein said circuit portion is devoid of a floating gate.

12. The method of claim 1 wherein a substantial portion of the first overlapping gate region occurs over the active region; and
wherein a width of the overlapping gate portion is at least equal to or greater than a width of the active region.

13. The method of claim 12 wherein the first portion of the first gate is formed over the active region; and
wherein at least a portion of the second gate is formed over only a portion of the first portion of the first gate.

14. The method of claim 1 wherein a substantial portion of the first overlapping gate region occurs over the active region;
wherein the first portion of the first gate is formed over the active region; and wherein at least a portion of the second gate is formed over the entirely of the first portion of the first gate.

15. The method of claim 1 wherein the first portion of the first gate is interposed between the second portion of the second gate region and active region of the circuit portion.

16. The method of claim 1 wherein the logic element corresponds to an element selected from a group of: NAND gates, AND gates, NOR gates, OR gates, XOR gates, and latches.

17. The method of claim 1 wherein said logic element is devoid of a floating gate.

18. The method of claim 17 wherein the logic element corresponds to an element selected from a group of: NAND gates, AND gates, NOR gates, OR gates, XOR gates, and latches.

19. The method of claim 1 further comprising forming the second dielectric layer between the first gate and the second gate.

20. The method of claim 1 wherein the first transistor gate has associated therewith a first threshold voltage value; and
wherein the second transistor gate has associated therewith a second threshold voltage value which is different than the first threshold voltage value.

21. The method of claim 20 wherein the first gate is formed to not function as a floating gate; and
wherein the second gate is formed to not function as a floating gate.

22. The method of claim 1 further comprising forming the logic element in a manner which does not enable the logic element to perform a memory storage operation.

23. A method of forming a logic element of an integrated circuit, the integrated circuit being fabricated on a substrate, the integrated circuit including a circuit portion, the method comprising
forming a first gate over the substrate, the first gate being comprised of a first transistor gate layer;
forming a first dielectric layer between the first gate and the substrate such that the first gate is conductively isolated from the substrate;
forming a second gate over the substrate, the second gate being comprised of a second transistor gate layer;
forming a second dielectric layer between the second gate and the substrate such that the second gate is conductively isolated from the substrate;
wherein a portion of the second gate is formed over a first portion of the first gate, thereby forming a first overlapping gate region;
forming a first electrical contact region on the first gate;
forming a second electrical contact region on the second gate;
wherein the first gate corresponds to a first finalized transistor gate and wherein the second gate correspond to a second finalized transistor gate of the circuit portion;
wherein the logic element corresponds to a transistor cell which includes the first finalized transistor gate and second finalized transistor gate;

forming an active region in the substrate, said active region being designed to enable current flow between source and drain regions of the transistor cell;
wherein the first finalized transistor gate includes an active region portion and a contact region portion, the active region portion corresponding to that portion of the first transistor gate which is over the active region; and
removing portions of the second transistor gate layer to thereby form the second finalized transistor gate such that at least the portion of the second finalized transistor gate is formed over the entirety of the active region portion of the first finalized transistor gate.

24. The method of claim 23 further comprising forming the logic element in a manner which enables the logic element to perform at least one logic operation.

25. The method of claim 23 wherein the logic element includes a circuit portion which is designed to exhibit performance characteristics substantially similar to performance characteristics of two serially connected transistors.

26. The method of claim 25 wherein method further comprises:
forming a first doped region in the substrate which functions as a source region of the circuit portion;
forming a second doped region in the substrate which functions as a drain region of the circuit portion; and
forming an active region in the substrate between said source and drain regions, said active region being designed to enable current flow between said source and drain regions.

27. The method of claim 26 wherein said circuit portion is devoid of third doped region having doping characteristics substantially similar to doping characteristics of the first doped region.

28. The method of claim 25 wherein said circuit portion is devoid of a floating gate.

29. The method of claim 26 wherein a substantial portion of the first overlapping gate region occurs over the active region; and
wherein a width of the overlapping gate portion is at least equal to or greater than a width of the active region.

30. The method of claim 29 wherein the first portion of the first gate is formed over the active region; and
wherein at least a portion of the second gate is formed over only a portion of the first portion of the first gate.

31. The method of claim 26 wherein a substantial portion of the first overlapping gate region occurs over the active region;
wherein the first portion of the first gate is formed over the active region;
and wherein at least a portion of the second gate is formed over the entirety of the first portion of the first gate.

32. The method of claim 26 wherein the portion of the first gate is interposed between the second portion of the second gate region and active region of the circuit portion.

33. The method of claim 23 wherein the logic element corresponds to an element selected from a group of: NAND gates, AND gates, NOR gates, OR gates, XOR gates, SRAM cells, and latches.

34. The method of claim 23 further comprising forming the second dielectric layer between the first gate and the second gate.

35. The method of claim 23 wherein the first gate is formed in a manner to function as a first transistor gate;
wherein the second gate is formed in a manner to function as a second transistor gate;
wherein the first transistor has associated therewith a first threshold voltage value; and
wherein to second transistor has associated therewith a second threshold voltage value which is different than the first threshold voltage value.

36. The method of claim 35 wherein the gate is formed to not function as a floating gate; and
wherein the first gate is formed to not function as a floating gate.

37. The method of claim 23 further comprising forming the logic element in a manner which does not enable the logic element to perform memory storage operation.

38. The method of claim 23 wherein the first dielectric layer is formed having a first thickness, and
wherein the second dielectric layer is formed having a second thickness; and
wherein the first thickness is different than the second thickness.

39. The method of claim 23 wherein the first gate is formed having a first thickness, and
wherein the second gate is formed having a second thickness; and
wherein the first thickness is different than the second thickness.