US20070007593A1 - Metal-oxide-semiconductor device with enhanced source electrode - Google Patents

Abstract

An MOS device is formed including a semiconductor layer of a first conductivity type, a first source/drain region of a second conductivity type formed in the semiconductor layer, and a second source/drain region of the second conductivity type formed in the semiconductor layer and spaced apart from the first source/drain region. A gate is formed proximate an upper surface of the semiconductor layer and at least partially between the first and second source/drain regions. The MOS device further includes at least one contact, the at least one contact including a silicide layer formed on and in electrical connection with at least a portion of the first source/drain region, the silicide layer extending laterally away from the gate. The contact further includes at least one insulating layer formed directly on the silicide layer.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• The present application is a divisional application of and claims priority to U.S. patent application Ser. No. 10/673,539 filed Sep. 29, 2003, the entirety of which is hereby incorporated by reference herein.
FIELD OF THE INVENTION
• The present invention relates generally to semiconductor devices, and more particularly relates to techniques for improving high-frequency performance in a metal-oxide-semiconductor (MOS) device.
BACKGROUND OF THE INVENTION
• Power MOS devices, including laterally diffused metal-oxide-semiconductor (LDMOS) devices, are employed in a variety of applications, such as, for example, power amplifiers in wireless communications systems. A conventional LDMOS device utilizes a multiple-level metal fabrication process for forming an interconnection between regions of differing conductivity types (e.g., n-type and p-type) in a source region of the device. The source current is then generally routed via a low resistance p-type region to the back of the wafer, where a source contact is formed.
• In applications where high-frequency operation is desired, such as in a radio frequency (RF) range (e.g., above 1 gigahertz (GHz)), the conventional methodology for forming the LDMOS device results in a relatively high input capacitance Cgs (e.g., about 80 picofarad (pF) for a 50 micron device). The high input capacitance can cause a variety of undesirable effects, including device mismatching, narrow bandwidth, and low power gain. In order to minimize the input capacitance in the LDMOS device, a conventional approach has been to scale back the source contact area, thereby increasing a distance between the gate and the source interconnection of the device. While this approach may reduce the input capacitance of the LDMOS device, a source resistance Rs of the device is substantially increased due, at least in part, to the reduction in source contact area. In some cases, the increase in source resistance significantly undermines any beneficial reduction in the input capacitance provided by scaling back the source contact area.
• Previous attempts to improve the high-frequency performance of the LDMOS device have primarily focused on optimizing the trade-off between input capacitance and source resistance. These prior attempts, however, have been unsuccessful at providing a CMOS process compatible LDMOS device capable of high-frequency operation. Accordingly, there exists a need for an LDMOS device capable of improved high-frequency performance which does not suffer from one or more of the above-noted deficiencies of the prior art. Furthermore, it would be desirable if such an LDMOS device was fully compatible with a CMOS process technology.
SUMMARY OF THE INVENTION
• The present invention provides techniques for reducing an input capacitance of an MOS device without significantly impacting the source resistance of the device, thereby improving a high-frequency performance of the device. In an illustrative embodiment, when fabricating an LDMOS device, the techniques of the present invention advantageously eliminate the difficulties associated with using a dedicated metal layer to electrically connect an n+ region to a p+ region in a source region of the device, thereby significantly simplifying the process technology and improving device topology. Moreover, the techniques of the present invention can be used to fabricate an integrated circuit (IC) device, for example, an LDMOS device, using conventional CMOS-compatible process technology. Consequently, the cost of manufacturing the IC device is not significantly increased.
• In accordance with one aspect of the invention, an MOS device is formed including a semiconductor layer of a first conductivity type, a source region of a second conductivity type formed in the semiconductor layer, and a drain region of the second conductivity type formed in the semiconductor layer and spaced apart from the source region. A gate is formed proximate an upper surface of the semiconductor layer and at least partially between the source and drain regions. The MOS device further includes at least one contact, the at least one contact including a silicide layer formed on and in electrical connection with at least a portion of the first source/drain region, the silicide layer extending laterally away from the gate. The contact further includes at least one insulating layer formed directly on the silicide layer. In this manner, the LDMOS device exhibits improved high-frequency performance, and is also substantially compatible with a CMOS process technology.
• These and other features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a cross-sectional view illustrating at least a portion of an LDMOS device in which the techniques of present invention can be implemented.
• FIG. 2 is a cross-sectional view depicting at least a portion of an exemplary LDMOS device, formed in accordance with an illustrative embodiment of the invention.
• FIG. 3 is a schematic diagram illustrating a circuit representing a simplified electrical model of the source region of the exemplary LDMOS device depicted inFIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The present invention will be described herein in the context of an illustrative CMOS integrated circuit fabrication technology suitable for forming discrete RF LDMOS transistors, as well as other devices and/or circuits. It should be appreciated, however, that the present invention is not limited to the fabrication of this or any particular device or circuit. Rather, the invention is more generally applicable to an MOS device comprising a novel source electrode which advantageously enables the MOS device to provide improved high-frequency performance. Moreover, the device is fully compatible with a CMOS process technology. Although implementations of the present invention are described herein with specific reference to an LDMOS device, it is to be appreciated that the techniques of the present invention are similarly applicable to other devices, such as, but not limited to, a vertical diffused MOS (DMOS) device, an extended drain MOS device, etc., with or without modifications thereto, as will be understood by those skilled in the art.
• It is to be understood that the various layers and/or regions shown in the accompanying figures may not be drawn to scale. Furthermore, certain semiconductor layers may have been omitted for ease of explanation.
• FIG. 1 illustrates a cross-sectional view of at least a portion of asemiconductor wafer100. Thewafer100 includes an LDMOS device formed on ap+ substrate102. The LDMOS device includes ann+ source region116 and adrain region110 formed in anepitaxial region104 of thewafer100. The LDMOS device further includes agate122 formed above achannel region114 of the device. Thechannel region114 is at least partially formed between the source and drain regions. A drift region is generally formed in theepitaxial layer104 of the LDMOS device which may comprise a first lightly-doped drain (LDD) region (ldd1)106 and a second LDD region (ldd2)108 formed between thechannel region114 anddrain region110. The LDMOS device also includes ap+ region118 formed in theepitaxial layer104 which connects the p+ substrate to an upper surface of thewafer100 via one ormore trench sinkers124 formed through theepitaxial layer104. Thetrench sinkers124 provide a low resistance (e.g., less than about 1 ohm per square) path between the substrate and the upper surface of the wafer. Thisp+ region118 is connected to then+ source region116 by adedicated metal layer120.
• The LDMOS device also includes adrain contact112 electrically connected to thedrain region110, and agate contact128 electrically connected to thegate122. Electrical contact to thesource region116 maybe made from a bottom of thesubstrate102 by way of thetrench sinkers124. Themetal layer120 which is formed over at least a portion of thesource region116 and thep+ region118, for electrically connecting the n+ source region and p+ region, may also be formed over at least a portion of thegate122 for shielding purposes. Anoxide layer126 is generally formed on an upper surface of the wafer to electrically isolate the source, drain and gate contacts of the device as well as to protect the device.
• Due to the relatively close proximity of themetal layer120, which is electrically connected to thesource region116, to thegate contact128, a gate-to-source capacitance Cgs1 of the LDMOS device is formed which undesirably affects the high-frequency performance of the device. The gate shielding provided by themetal layer120 helps reduce a capacitance between the gate and drain region which would otherwise improve the high-frequency performance of the device. However, a capacitance Cgs2 is formed as a result of the close proximity of themetal layer120 to thegate122, which further undesirably affects the high-frequency performance of the device. In order to reduce the capacitance Cgs2, themetal layer120 is often scaled back from thesource region116 which increases the distance between themetal layer120 and thegate122. However, this reduces the source contact area resulting in a substantial increase in the source resistance Rs of the LDMOS device. As previously stated, in some cases, the increase in source resistance significantly undermines any beneficial reduction in the input capacitance Cgs2 provided by scaling back the source contact area.
• FIG. 2 illustrates a cross-sectional view of at least a portion of asemiconductor wafer200 in which the techniques of the present invention are implemented. As previously stated, the various layers and/or regions shown in the figure may not be drawn to scale and certain semiconductor layers may have been omitted for ease of explanation. Thewafer200 includes an exemplary LDMOS device formed on asemiconductor substrate202. Thesubstrate202 is commonly formed of single-crystal silicon, although alternative materials may be used, such as, but not limited to, germanium (Ge), gallium arsenide (GaAs), etc. Additionally, the substrate may have been modified by adding an impurity or dopant, such as by a diffusion or implant step, to change the conductivity of the material (e.g., n-type or p-type). In a preferred embodiment of the invention, thesubstrate202 is of p-type conductivity, and hence may be referred to as a p-substrate.
• The term “semiconductor layer” as may be used herein refers to any semiconductor material upon which and/or in which other materials may be formed. The semiconductor layer may comprise a single layer, such as, for example, thesubstrate202, or it may comprise multiple layers, such as, for example, thesubstrate202 and anepitaxial layer204. Thesemiconductor wafer200 comprises thesubstrate202, with or without theepitaxial layer204, and preferably includes one or more other semiconductor layers formed on the substrate. The term “wafer” is often used interchangeably with the term “silicon body,” since silicon is typically employed as the semiconductor material comprising the wafer. It should be appreciated that although the present invention is illustrated herein using a portion of a semiconductor wafer, the term “wafer” may include a multiple-die wafer, a single-die wafer, or any other arrangement of semiconductor material on or in which a circuit element may be formed.
• The exemplary LDMOS device includes asource region216 and a drain region210 formed in theepitaxial layer204 of thewafer200, such as by a conventional implant and diffusion process. The source and drain regions are preferably doped, such as by a conventional implant step, with an impurity of a known concentration level to selectively change the conductivity of the material as desired. Preferably, the source and drainregions216,210 have a conductivity type associated therewith which is opposite a conductivity type of thesubstrate202, so that active regions can be formed in the device. In a preferred embodiment of the invention, the source and drainregions216,210 are of n-type conductivity. Electrical connection between thesource region216 and thesubstrate202 may be provided by forming one ormore trench sinkers224 through theepitaxial layer204 of thewafer200. The trench sinkers may be formed in a conventional manner, such as, for example, by opening windows in the epitaxial layer204 (e.g., by photolithographic patterning and etching) to expose thesubstrate202, and filling thetrenches224 with a conductive material, as will be understood by those skilled in the art. In a preferred embodiment of the invention, thetrench sinkers224 are of p-type conductivity.
• It is to be appreciated that, in the case of a simple MOS device, because the MOS device is symmetrical in nature, and thus bidirectional, the assignment of source and drain designations in the MOS device is essentially arbitrary. Therefore, the source and drain regions may be referred to generally as first and second source/drain regions, respectively, where “source/drain” in this context denotes a source region or a drain region. In an LDMOS device, which is generally not bidirectional, such source and drain designations may not be arbitrarily assigned.
• The exemplary LDMOS device may include ap+ region218 formed in theepitaxial layer204, such as by a conventional implant and diffusion process. Thep+ region218 is preferably formed adjacent to thesource region216 and extends laterally in a direction opposite the drain region210. Thep+ region218 is preferably doped, such as by a conventional implant step, with an impurity of a known concentration level to selectively change the conductivity of the material as desired. Preferably, thep+ region218 has a conductivity type associated therewith which is opposite a conductivity type of thesource region216.
• Achannel region214 and a drift region, which may comprise a first LDD region (ldd1)206 and a second LDD region (ldd2)208, are formed in the exemplary LDMOS device. Thechannel region214 is formed at least partially below and adjacent to thesource region216 while the drift region extends laterally from thechannel region214 to the drain region210. Thechannel region214 may be formed of a material having the same conductivity type as the substrate, preferably p-type in the exemplary LDMOS device, and may therefore be referred to as a p-channel. The drift region may be formed of material having the same conductivity type as the source and drain regions, preferably n-type, although the relative doping concentration of the drift region compared to the source and drain regions is typically lower.
• The exemplary LDMOS device further includes agate222 formed above at least a portion of thechannel region214 and proximate an upper surface of thewafer200. The gate maybe formed of, for example, polysilicon material, although alternative suitable materials (e.g., metal) may be similarly employed. A shieldingelectrode226, which may be referred to herein as a dummy gate, may be formed in the exemplary LDMOS device at least partially between thegate222 and the drain region210. Thedummy gate226 is spaced laterally from thegate222 and preferably non-overlapping relative to the gate. It is to be appreciated that thedummy gate226 may be formed in virtually any configuration and/or shape that is substantially non-overlapping with respect to thegate222, as will be understood by those skilled in the art. Thedummy gate226 is formed in close relative proximity (e.g., 200 nanometers (nm)) to an upper surface of thewafer200. Although not shown, thedummy gate226 in the exemplary LDMOS device, if used, is preferably electrically connected (i.e., strapped) to thesource region216.
• Thedummy gate226 beneficially reduces a Miller capacitance Cgd between the gate and drain region of the LDMOS device, thereby improving the high-frequency performance of the device, and reduces hot-carrier induced (HCI) degradation in the device. A dummy gate suitable for use in conjunction with the present invention can be found in a related U.S. application Ser. No. 10/623,983 entitled “Shielding Structure for Use in a Metal-Oxide-Semiconductor Device” filed on Jul. 15, 2003 and assigned attorney docket number Xie 3-4, which is incorporated by reference herein.
• Traditionally, adedicated metal layer120, which may comprise aluminum, gold, etc., is used to electrically connect the source region116 (n-type) with thep+ region118 of the device, as illustrated inFIG. 1. An important aspect of the present invention is that the exemplary LDMOS device is configured so as to eliminate this metal connection layer. Instead, electrical connection between thesource region216 andp+ region218 is achieved via asilicide layer220 formed over at least a portion of the source and p+ regions, such as by using a conventional deposition process. Suitable materials used to form thesilicide layer220 may include, for example, titanium, cobalt and tungsten, although essentially any material which is capable of forming a low-resistance (e.g., less than about one ohm per square) connection with the silicon may be used. As stated above, thesilicide layer220 is used in place of the metal layer120 (seeFIG. 1) for conducting current between thesource region216 andp+ region218 of the LDMOS device.
• FIG. 3 is a schematic diagram illustrating acircuit300 representing a simplified electrical model of the source region of the exemplary LDMOS device depicted inFIG. 2. Thecircuit300 includes aresistor302 representing thesilicide layer220 in the exemplary LDMOS device and adiode304 representing an active p-n junction formed by thep+ region218 and then+ source region216 in the device. An anode ofdiode304 is formed by thep+ region218 and a cathode of the diode is formed by then+ source region216. Afirst terminal306 of thecircuit300 represents the upper surface of the p-substrate and asecond terminal308 represents thechannel region214 of the device. The importance of thesilicide layer220 thus becomes apparent, since the silicide layer serves as a low-resistance path for current in the LDMOS device which by-passes the p-n junction formed by thep+ region218 andsource region216. It is to be appreciated that substantially all of the source current passes through thesilicide layer220, and therefore the silicide layer is preferably designed to handle this current without exhibiting an appreciable resistance.
• With continued reference toFIG. 2, because thesilicide layer220 is preferably formed as a relatively thin layer (e.g., about 0.05 to about 0.1 micron), at least in comparison to themetal layer120 associated with the LDMOS device illustrated inFIG. 1, the distance between the silicide layer, which is electrically coupled to thesource region216, and agate contact230 formed proximate an upper surface of thewafer200 above at least a portion of thep+ region218, is substantially increased, thereby advantageously reducing a first component Cgs1 of the overall gate-to-source capacitance. Furthermore, a second gate-to-source capacitance component Cgs2 associated with a side edge of thesilicide layer220 is significantly smaller compared to the side edge capacitance component associated with the metal layer120 (seeFIG. 1). As such, thesilicide layer220 may be placed in closer relative proximity to thegate222 without significantly increasing the gate capacitance Cgs2. In this manner, the source contact area can be increased, thereby beneficially reducing the source resistance, without any significant increase in the gate capacitance.
• The exemplary LDMOS device further includes an insulatinglayer228 formed on at least a portion of an upper surface of thewafer200. The insulatinglayer228 functions, at least in part, to protect thewafer200 and to electrically isolate two or more conductive regions of the LDMOS device. The insulatinglayer228 may comprise an oxide, for example, silicon dioxide (SiO₂), although alternative materials may be used for forming the insulating layer. It is to be appreciated that the insulatinglayer228 may comprise a multiple-layer structure. Furthermore, the insulatinglayer228 may include one or more conductive layers, as long as each of the conductive layers is electrically isolated from thesilicide layer220. For example, the insulatinglayer228 may include a first oxide layer (e.g., SiO₂) formed directly on thesilicide layer220, a conductive layer (e.g., aluminum) formed on the first oxide layer, and a second oxide layer (e.g., SiO₂) formed on conductive layer, thus forming multiple-layer sandwich structure (not shown).
• In addition to the improvements in high-frequency performance achieved by the LDMOS device formed in accordance with the techniques of the present invention, the elimination of themetal layer120 electrically connecting the p+ and source regions of a traditional LDMOS device simplifies the semiconductor fabrication process, thereby reducing the cost of manufacturing the LDMOS device. Moreover, a topology of an upper surface of the LDMOS formed in accordance with the present invention is significantly more planar in comparison to the topology of the upper surface of a conventional LDMOS device employingmetal layer120. Consequently, the LDMOS formed in accordance with the present invention will advantageously exhibit an improved reliability over conventional LDMOS devices.
• Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made therein by one skilled in the art without departing from the scope of the appended claims.

Claims (8)

1. A method for forming a metal-oxide-semiconductor (MOS) device, comprising the steps of:

forming a gate proximate an upper surface of a semiconductor layer, the semiconductor layer being of a first conductivity type;

forming first and second source/drain regions of a second conductivity type in the semiconductor layer proximate the gate, the gate being between the first and second source/drain regions;

forming a silicide layer on and in electrical connection with at least a portion of the first source/drain region, the silicide layer extending laterally away from the gate; and

forming at least one insulating layer directly on the silicide layer,

wherein the first source/drain region comprises an n-type region and a p-type region formed adjacent to said n-type region and extending laterally away from said gate, and wherein the silicide layer is formed substantially proximate the n-type and p-type regions such that the silicide layer forms a substantially low-resistance electrical path in parallel with an electrical path formed between the n-type and p-type regions.

2. The method ofclaim 1, wherein substantially all current associated with the first source/drain region passes through the silicide layer in a direction from the n-type region to a p-type region that is proximate the upper surface of the semiconductor layer.

3. The method ofclaim 1, wherein the at least one insulating layer comprises at least one conductive layer, the at least one conductive layer being electrically isolated from the silicide layer.

4. The method ofclaim 1, wherein the silicide layer forms a substantially low-resistance electrical path for conducting current between two or more regions in the semiconductor layer that are electrically isolated from the device.

5. The method ofclaim 1, further comprising the step of forming a shielding structure proximate the upper surface of the semiconductor layer and between the gate and the second source/drain region, the shielding structure being electrically connected to the first source/drain region, the shielding structure being spaced laterally from the gate and being substantially non-overlapping relative to the gate.

6. The method ofclaim 1, wherein the first source/drain region comprises a source region and the second source/drain region comprises a drain region.

7. The method ofclaim 1, wherein the MOS device comprises a diffused MOS (DMOS) device, the first source/drain region comprises a source region and the second source/drain region comprises a drain region.

8. The method ofclaim 7, wherein the MOS device comprises a lateral DMOS (LDMOS) device.

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