This invention relates generally to semiconductor devices, and more particularly to devices with reduced parasitics.
While semiconductor devices have been used in electronic devices such as computers and cellular phones, semiconductor devices are also increasingly used for high power and high frequency applications such as subscriber cable service transmission and cellular base station transmission. Therefore, one of the goals of the semiconductor industry is to develop semiconductor devices that operate at high frequencies as well as provide adequate power for data transmission applications.
One type of semiconductor device used for high power and high frequency applications is the power metal oxide field effect transistor (MOSFET). Of the various forms of power MOSFETs, the Lateral Double-Diffused Metal Oxide Semiconductor (LDMOS) device is commonly used because of its output power capability and high efficiency. Because of its high performance and efficiency, the LDMOS device has found its way into demanding applications such as cellular phone base stations and other transmission equipment.
As performance requirements have become more demanding, however, semiconductor device manufacturers must continually improve power transistor device performance. One limitation to power transistor device performance is parasitic capacitance. In power transistors, parasitic capacitance is particularly problematic because of the large device structures necessary to source and sink large currents, while avoiding breakdown at high voltages. As device size increases, parasitic capacitance contributors, such as source-substrate capacitance, drain-substrate capacitance, gate-drain capacitance, gate-source capacitance, and interconnect capacitance also increase.
Besides limiting device performance through the mere presence of capacitive loading, practical considerations in dealing with parasitic capacitance may also lead to higher resistive parasitics. For example, in order to maintain a minimum parasitic gate-source capacitance due to the presence of gate routing over the source, the number of gate contacts may have to be reduced. Such a reduction in the number of gate contacts results in a correspondingly higher series gate resistance.
In the field of highly efficient power transistors, device structures that minimize parasitic capacitance are needed.
In one embodiment of the present invention, a semiconductor device is provided. The device includes a layer of insulating material disposed over a semiconductor body, and a gate electrode disposed over the layer of insulating material. The gate electrode comprises a first region of gate electrode material having a length, width, a first end at an end of the length and a second end at end of the length opposite the first end. The device also includes a source region disposed within the semiconductor body adjacent to the first region of gate electrode material along the width of the first gate region of gate electrode material. Similarly, the device includes a drain region disposed within the semiconductor body adjacent to the first region of gate electrode material along the width of the first region of gate electrode material opposite the source region. A gate connection comprising a second region of gate electrode material that extends away from the width of the first gate region is provided. The gate connection is electrically coupled to a contact region and to a region of gate electrode material along the width of the first region of gate electrode material between the first and the second end. A first insulating region disposed beneath the gate connection.
The foregoing has outlined rather broadly features of the present invention. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
a-1c illustrate a layout view and corresponding cross-sections of an embodiment of the present invention;
a-2b illustrate a layout view and corresponding cross-sections of an alternate embodiment of the present invention;
a-3b illustrate a layout view and corresponding cross-sections of another embodiment of the present invention; and
a-4b illustrate a layout view and corresponding cross-sections of an additional embodiment of the present invention.
Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. To more clearly illustrate certain embodiments, a letter indicating variations of the same structure, material, or process step may follow a figure number.
The making and using of preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that may be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The invention will now be described with respect to preferred embodiments in a specific context, namely a semiconductor device with reduced parasitics. Concepts of the invention can also be applied, however, to other electronic devices.
Referring first to
Gate contacts 115 provide electrical coupling from a metallization layer (not shown) to the gate regions 103 of the devices. Gate regions 103 are coupled to gate contacts 115 by a region of gate electrode material 102 that is made from the same material as gate region 103. In a preferred embodiment of the present invention, field plate 116 is provided to shield gate region 103 from drain region 108, which reduces the amount of gate-drain capacitance. Excess gate-drain capacitance is undesirable in most common source LDMOS applications because of the Miller multiplication effect.
Turning to
Source regions 106 are preferably p-type regions disposed within p-well regions 124, which are disposed within a p-type epitaxial layer 128 at the surface of substrate 130. Source regions 106 are electrically coupled to metal layer 132 on the back side of the substrate 130 through an ion implanted p+ sinker 126 in series with substrate 130. In a preferred embodiment of the present invention, source regions 106 do not have contacts on the top surface of the top surface. Rather, the source connection is made though the back of the substrate. In a preferred embodiments of the present invention, substrate 130 is a low resistivity substrate, typically about 10 mΩcm. Sinker 126 and epitaxial layer 128 can be fabricated using conventional methods. In alternative embodiments, however, contact to source region 106 can be made from the topside of substrate 130 rather than from the bottom of substrate 130. In yet other embodiments of the present invention, source regions 106, well regions 124, and possibly epitaxial layer 128 can be made from n-type material in order to create a p-type LDMOS device.
Gate regions 103 disposed within oxide layer 134 are preferably made from polysilicon using conventional methods. In some preferred embodiments this gate can be a two-layer gate including a polysilicon layer on the bottom and a titanium silicide layer on top, although in other embodiments of the present invention, other materials may be used. For example, cobalt silicide, nickel silicide, and tungsten silicide can be used. In other embodiments of the present invention, a polysilicon/tungsten nitride/tungsten gate stack can be used which utilizes non-silicided gate materials. Titanium silicide, however, is a preferred material for the fabrication of power devices. Field plate 116 is preferably formed adjacent to gate region 103 in order to shield gate region 103 from drain region 108 and via 138. Field plate 116 is preferably made from metal and is formed using conventional methods. In alternative embodiments of the present invention, however, field plate 116 may be omitted.
Turning to
In the conventional LDMOS devices, gate connection region 102 would overlie source regions 106 creating excess gate-source capacitance. In the preferred embodiment of the present invention, however, STI regions 104 are formed underneath gate connection region 102. The presence of STI region 104 (region 104a in
In a preferred embodiment of the present invention, STI regions 104 are trenches filled with high-density plasma oxide at a depth of about between about 200 nm and about 1000 nm, preferably 300 nm to 700 nm, using conventional methods. Because STI regions 104 contain insulating material, they present a much higher impedance to any adjoining semiconductor region or metal layer than epitaxial layer 128 or substrate 130. As a result of its higher impedance, STI regions 104 present a lower capacitive load than epitaxial layer 128 or substrate 130. In alternate embodiments of the present invention, other materials and other depths may be used.
In order to further reduce parasitic capacitance, such as drain-substrate, source-substrate, and gate-substrate capacitance, STI region 104b of
Turning to
b shows a cross-sectional view of a preferred embodiment of device pair 154 taken at cross-sectional line 2b-2b of
Turning to
b shows a cross-sectional view of this preferred embodiment of device pair 156 taken at cross-sectional line 3b-3b of
Turning to
b shows a cross-sectional view of this preferred embodiment of device pair 158 taken at cross-sectional line 4b-4b of
Returning to
Resist is deposited over the hard mask and exposed lithographically according to a mask pattern. In a preferred embodiment of the present invention, the mask pattern defines the location of STI trenches 104 located according to the embodiments of the invention described hereinabove. Once the resist is exposed, the BPSG hard mask is etched and the resist is stripped using conventional techniques.
Using the BPSG hard mask, trenches in STI regions 104 are etched to a depth of between about 300 nm and about 700 nm. In other embodiments, the trench depth is preferably between about 200 nm and about 1000 nm. The opened silicon then undergoes a thermal oxidation step. Because STI regions 104 underlie regions of gate material, for example, under gate connections 102 the trenches 104 are filled with oxide using an HDP process. The HDP oxide deposition process is preferable compared to a conventional embodiment of LOCOS isolation for power devices to avoid the problem of SiN becoming embedded into the oxide, as is the case with thermally grown oxide used in conventional embodiments. The existence of such SiN particles underneath gate 103 is typically undesirable because it can lead to gate oxide failure by a mechanism known in the art as the Kooi effect. In alternative embodiments of the present invention, however, other processes besides HDP can be used which can create an oxide that promotes gate oxide reliability. For example, a low pressure chemical vapor deposition (LPCVD) of tetraethyl orthosilicate (TEOS) followed by a densification step can be used in place of HDP in other embodiments.
After the trenches in STI regions 104 are filled with oxide, a reactive ion etch (RIE) is used to remove oxide in non-STI regions in order to facilitate subsequent planarization. A CMP is then performed to remove most of the oxide (approximately 70%) from the surface of the stopping layer. Next, an oxide recess step, such as a deglaze, is performed to adjust the step height of STI regions 104 followed by a hot phosphoric acid etch of the Si3N4 pad nitride. A sacrificial oxide step is performed followed by the deposition of the gate oxide. The fabrication process then continues with the creation of the LDMOS devices, as hereinbefore described according to conventional techniques.
It will also be readily understood by those skilled in the art that materials and methods may be varied while remaining within the scope of the present invention. It is also appreciated that the present invention provides many applicable inventive concepts other than the specific contexts used to illustrate preferred embodiments. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
Number | Name | Date | Kind |
---|---|---|---|
7084462 | Warnock et al. | Aug 2006 | B1 |
7119399 | Ma et al. | Oct 2006 | B2 |
20020145172 | Fujishima et al. | Oct 2002 | A1 |
20070034986 | Hokomoto et al. | Feb 2007 | A1 |
20070228497 | Shimizu | Oct 2007 | A1 |
Number | Date | Country | |
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20090026539 A1 | Jan 2009 | US |