SHIELDING OF ANALOG CIRCUITS ON SEMICONDUCTOR SUBSTRATES

Abstract: A semiconductor device, in accordance with the present invention, includes a doped semiconductor substrate (102) wherein the doping of the substrate has a first conductivity and a device region (110) formed near a surface of the substrate. The device region includes at least one device well. A buried well (104) is formed in the substrate below the device region. The buried well is doped with dopants having a second conductivity. A trench region (124) surrounds the device region and extends below the surface of the substrate to at least the buried well such that the device region is isolated from other portions of the substrate by the buried well and the trench region.

Description: SHIELDING OF ANALOG CIRCUITS ON SEMICONDUCTOR SUBSTRATES BACKGROUND 1. Technical Field This disclosure relates to semiconductor devices and more particularly, to a system for isolating analog circuits from a semiconductor substrate by employing a buried well and isolation regions. 2. Description of the Related Art Semiconductor chips are employed in many different devices across many different industries. Semiconductor chip may also include both analog and digital circuits. Such devices have evolved for use in, for example, the telecommunications industry. In telecommunications, cellular technology includes internal processing with digital logic while transmission and receiving operations are performed by analog circuits . To save costs and reduce space occupied by internal circuitry, digital and analog circuits may be placed on a same chip . Analog circuits tend to be more susceptible to noise, and analog circuits formed on a semiconductor substrate tend to pick up noise and transmit noise to and from the substrate. This can have a detrimental effect on circuit performance and may result in significant noise levels introduced to the analog circuits on the chip. Therefore, a need exists for isolating analog circuits from a semiconductor substrate to reduce noise and improve performance of the circuits . SUMMARY OF THE INVENTION A semiconductor device, in accordance with the present invention, includes a doped semiconductor substrate wherein the doping of the substrate has a first conductivity and a device region formed near a surface of the substrate. The device region includes at least one device well. A buried well is formed in the substrate below the device region. The buried well is doped with dopants having a second conductivity. A trench region surrounds the device region and extends below the surface of the substrate to at least the buried well such that the device region is isolated from other portions of the substrate by the buried well and the trench region. Another semiconductor device, in accordance with the present invention, includes a doped semiconductor substrate having a first region surrounded by a trench region. The first region includes circuits, which generate noise or are susceptible to noise which are formed on or near a surface of the substrate wherein the doping of the substrate has a first conductivity. A plurality of regions surrounds the first region. The plurality of regions is separated from the first region by the trench region. The plurality of regions includes other circuits and components. A buried well is formed in the substrate below the first region in the substrate. The buried well is doped with dopants having a second conductivity. The trench region surrounds the first region and extends below the surface of the substrate to at least the buried well such that the first region is isolated from the other circuits and components by the buried well and the trench region. In alternate embodiments, the trench region may be filled with a dielectric material . The device region may include at least one of a P-well and an N-well between the buried well and the surface of the semiconductor substrate. The buried well may be located between about 1400 nm to about 1600 nm below the surface of the semiconductor substrate. The device region preferably includes analog circuits. The device region may include digital circuits. The semiconductor device may include other circuits, and the buried well and the trench region preferably decouple noise and cross-talk between the device region and the other circuits. The device region may include a system on a chip (SOC) . The semiconductor device may include a telecommunications chip. The buried well may be between about 400 nm and about 600 nm in thickness. These and other objects, 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 DRAWINGS This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: FIG. 1 is a cross-sectional view of a semiconductor device having a buried well formed in accordance with the present invention; FIG. 2 is a cross-sectional view showing a well region being formed above the buried well, the well region being doped with a single dopant conductivity in accordance with the present invention; FIG. 3 is a cross-sectional view showing two well regions formed above the buried well, each well region having a different conductivity type in accordance with the present invention; FIG. 4 is a cross-sectional view showing a trench region formed about a circuit region in accordance with the present invention; FIG. 5 is a top plan view of the semiconductor device showing the trench region filled with a dielectric material in accordance with the present invention; and FIG. 6 is a cross-sectional view of the semiconductor device showing circuits and components formed on a surface of the device in accordance with the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention relates to semiconductor devices and more particularly, to a system for isolating analog circuits from a semiconductor substrate by employing a buried well and isolation regions. The present invention provides a substrate with a buried well that extends below an analog circuit or system on a chip (SOC) . In this way, electromagnetic leakage or current leakage to or from the analog circuit, which can result in noise and performance problems, are reduced. In one embodiment, a deep trench isolation region is formed to surround the analog circuit and the buried well to further prevent the propagation of electromagnetic waves or the flow of current parallel to a surface of the semiconductor wafer. The present invention will now be described in greater detail with reference to the FIGS, in terms of illustrative examples which should not be construed as limiting the present invention. Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 1, a cross-sectional view of a semiconductor device 100 is shown in accordance with one embodiment of the present invention. Semiconductor device 100 includes a substrate 102, which preferably includes monocrystalline silicon, although other substrate materials may be employed. Substrate 102 may include a P-doped substrate or an N-doped substrate. For simplicity, the present invention will be described in terms of a P-doped substrate. It is to be understood that the dopant conductivities as well be described herein may be changed. For example, P-type dopants may be switched to N- type dopants and vise versa. One skilled in the art understands that voltage levels and circuit designs may be adjusted accordingly. Substrate 102 is doped in accordance with methods known in the art. A buried well 104 is formed in accordance with the present invention. Buried well 104 is formed by patterning a mask 106 over a surface of substrate 102, for example, a resist mask. A mask open process is employed to open mask 106 over a circuit region 108. Circuit region 108 will be employed to form a circuit, for example an analog circuit and/or components, as will be described hereinafter. Buried well 104 has a dopant type which is opposite the dopant type provided with substrate 102. In one embodiment, buried well 104 includes N-type dopants such as arsenic, antimony or phosphorous. If a P-doped buried well 104 is employed, dopants such as, boron, gallium or indium may be employed. Other dopants or combinations may be employed as well. Buried well 104 is formed by employing an ion implantation process. In one embodiment phosphorous is employed, and ion energies of between, for example, about 0.5 MeV and about 2.0 MeV are employed to set a distance for buried well 104 at about 1400 nm and about 1600 nm below a surface of substrate 102. Buried well 104 may include a dose or density of, for example, between about lxl0 17 to about lxl0 18 atoms/cc. Buried well 104 may include a thickness of, for example, between about 400 nm and about 600 nm. Although these processing parameters are desirable, one skilled in the art may adjust these parameters to achieve desired results for a given application and dopant type. After buried well 104 is formed, mask 106 can be removed or employed for additional implantation processes (see e.g., FIG. 2) . Referring to FIG. 2, in one embodiment, mask 106 is optionally employed for implanting a region 110 of substrate 102. Region 110 may be employed as a transistor well or a capacitor plate, or other component. The ion implantation process described above may be altered to provide ions, which penetrate to a lower depth (i.e., less deep) . In this way, N- and/or P-type regions may be formed in region 110. In one embodiment, as shown in FIG. 3, region 110 may include both N and P type wells, 112 and 114 respectively. Alternately, region 110 may also include a single dopant type well (FIG. 2) , for example a single N type dopant or a single P type dopant region. The structure of FIG. 3, may be formed using a mask 107 to prevent P type dopants from entering N-well 112. In addition, another mask (not shown) may be employed to prevent N type dopants from entering P-well 114. Mask 107 is opened over a portion or opened over all of circuit region 108. Deposition of wells in region 110 may not be needed depending on the types of components or circuits needed in region 108. Referring to FIG. 4, a mask 122 (e.g., resist) is patterned over substrate 102 and opened over regions just outside of circuit region 108. An anisotropic etch process, such as, reactive ion etching, is employed to form deep trenches 124. Deep trenches 124 preferably extend to a depth below buried well 104. In one embodiment, deep trenches 124 extend a depth of about 3 to 6 microns and have a width at a top surface of substrate 102 of between about 0.3 microns and about 1.0 micron. Mask 122 is removed after trenches 124 are formed. Referring to FIG. 5, a top view of semiconductor device 100 is shown in accordance with the present invention. A dielectric material 126 is employed to fill trenches 124. In the embodiment shown, trenches 124 surround buried well 104 (shown in parenthesis to indicate that buried well is below the surface) . In this way, region 110 is electrically isolated from other portions of substrate 102. Dielectric material 126 may include silicon dioxide or other material capable of filling trenches 124 while providing insulation. In one embodiment, trenches 124 are left unfilled. Filling trenches 124 may include employing chemical vapor deposition of physical vapor deposition processes known in the art. An etch or polishing step may be employed to remove deposited dielectric form surfaces of substrate 102. Other circuits and/or components are externally disposed relative to region 110. it is to be understood that electrical connections may be made between the other circuits and/or components exterior to region 110 and circuit region 108 by providing interconnections above the surface of substrate 102. Referring to FIG. 6, circuit components 120 are formed in region 108. Circuit components 120 may include analog circuits, such as receivers, amplifiers, active or passive filters, resistors, inductors, transistors, diodes, inductors or other electronic components. Circuit components 120 may include diffusion regions, metal lines, insulation layers, etc. In one embodiment, circuit components 120 include a plurality of different components to form a system on a chip (SOC) . Circuit components 120 include analog devices which may be susceptible to noise (or generate noise) or cross-talk transmitted to or from substrate 102. Circuit components 120 may include both analog and digital circuits which may be susceptible to current leakage. By providing isolation trenches 128 and buried well 104, electrical leakage, crosstalk and/or transmitted/received noises are significantly reduced between substrate 102 and circuit components 120. Isolation trenches 128 isolate circuit components 120 in region 108 from regions 130. Regions 130 may include other systems on a chip (SOC), analog components, digital components, logic circuitry or memory devices. Buried well 104 prevents cross-talk or noise induction between substrate 102 and circuit components 120. In this way, system performance is improved by eliminating or reducing cross-talk, noise induction and/or current leakage to substrate and between circuit components and regions 130 by isolation/shielding circuit components 120 in region 108. In one embodiment, buried well 104 may be grounded to provide further shielding. Further, a center portion of trenches 124 may include a conductive material, which is grounded to further prevent cross-talk or noise from passing therethrough. It is to be understood that connection between components in region 108 and regions 130 may be connected by upper metal layers formed in later processing. Interconnects may also be formed to connect components within region 108. Interconnects and contact formation are known in the art . The present invention may be employed with a plurality of different device types, for example, chips for mobile phones, telecommunications equipment or other analog chips including but not limited to radio frequency applications. Having described preferred embodiments for shielding of analog circuits on semiconductor substrates (which are intended to be illustrative and not limiting) , it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims .

Claims: WHAT IS CLAIMED IS: 1. A semiconductor device comprising: a doped semiconductor substrate wherein the doping of the substrate has a first conductivity; a device region formed near a surface of the substrate, the device region including at least one device well ; a buried well formed in the substrate below the device region, the buried well being doped with dopants having a second conductivity; and a trench region surrounding the device region and extending below the surface of the substrate to at least the buried well such that the device region is isolated from other portions of the substrate by the buried well and the trench region. 2. The semiconductor device as recited in claim 1, wherein the trench region is filled with a dielectric material . 3. The semiconductor device as recited in claim 1, wherein the device region includes at least one of a P-well and an N-well between the buried well and the surface of the semiconductor substrate. 4. The semiconductor device as recited in claim 1, wherein the buried well is located between about 1400 nm to about 1600 nm below the surface of the semiconductor substrate . 5. The semiconductor device as recited in claim 1, wherein the device region includes analog circuits. 6. The semiconductor device as recited in claim 5, wherein the device region includes digital circuits. 7. The semiconductor device as recited in claim 1, wherein the semiconductor device includes other circuits and the buried well and the trench region decouple noise and cross-talk between the device region and the other circuits. 8. The semiconductor device as recited in claim 1, wherein the device region includes a system on a chip. 9. The semiconductor device as recited in claim 1, wherein the semiconductor device includes a telecommunications chip . 10. The semiconductor device as recited in claim 10, wherein the buried well is between about 400 nm and about 600 nm in thickness. 11. A semiconductor device comprising: a doped semiconductor substrate having a first region surrounded by a trench region, the first region including circuits which generate noise or are susceptible to noise which are formed on or near a surface of the substrate wherein the doping of the substrate has a first conductivity; a plurality of regions surrounding the first region, the plurality of regions being separated from the first region by the trench region, the plurality of regions including other circuits and components; a buried well formed in the substrate below the first region in the substrate, the buried well being doped with dopants having a second conductivity; and the trench region surrounding the first region and extending below the surface of the substrate to at least the buried well such that the first region is isolated from the other circuits and components by the buried well and the trench region. 12. The semiconductor device as recited in claim 11, wherein the trench region is filled with a dielectric material . 13. The semiconductor device as recited in claim 11, wherein the first region includes at least one of a P-well and an N-well between the buried well and the surface of the semiconductor substrate. 14. The semiconductor device as recited in claim 11, wherein the buried well is located between about 1400 nm to about 1600 nm below the surface of the semiconductor substrate . 15. The semiconductor device as recited in claim 11, wherein the first region includes analog circuits. 16. The semiconductor device as recited in claim 15, wherein the first region includes digital circuits. 17. The semiconductor device as recited in claim 11, wherein the first region is decoupled from noise and cross-talk, which is transmitted through the substrate and generated by the other circuits and components, by the buried well and the trench region. 18. The semiconductor device as recited in claim 11, wherein the first region includes a system on a chip. 19. The semiconductor device as recited in claim 11, wherein the semiconductor device includes a telecommunications chip. 20. The semiconductor device as recited in claim 11, wherein the buried well is between about 400 nm and about 600 nm in thickness.

Inventors: PRIGGE ODIN, PRIGGE, ODIN
Applicants: INFINEON TECHNOLOGIES CORP, INFINEON TECHNOLOGIES NORTH AMERICA CORP.
Application year: 2001
Main classification: H01L21
IPC codes: H01L21