Abstract:
A method of fabricating an integrated circuit having active components, conductors and isolation regions on a substrate is disclosed, including patterning and etching a portion of at least one of said isolation regions to expose a first area of said substrate, depositing a mask layer over said integrated circuit including said first area, patterning an a itching said mask layer to expose a second area of said substrate within said first area, converting a portion of said substrate to a selectively etchable material, wherein said selectively etchable material lies in an area subjacent to said second area and extends only partially to the bottom surface of said substrate, selectively etching said selectively etchable material to form a void, removing said mask layer to expose said isolation region, depositing a dielectric layer over said void wherein said dielectric layer extends at least to the height of said isolation region and covers the top surface of said wafer, polishing the surface of said dielectric layer until the surface is planar and the top surface of said isolation region is exposed, and forming at least one patterned conductive layer over the surface of said dielectric layer that is coplanar with the surface of said isolation region.

Description:
This application claims priority under 35 USC § 119(e)(1) of provisional application No. 60/174,066 filed Dec. 30, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates, in general, to integrated circuit structures and fabrication methods and, in particular, to isolation of circuit components using the etching of porous silicon areas followed by a dielectric backfill; including decoupling of circuit structure radio frequency transmission lines from a semiconductor substrate to minimize parasitics. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits have been designed to ever-smaller geometries, and required to carry signals of ever-increasing frequencies. As integrated circuit components and signal lines are placed more closely together, and as the frequencies at which the components and signal lines operate are increased to radio frequencies (RF), the components and signal lines strongly couple electromagnetically to the substrate. This results in low power efficiency and restricts the maximum frequency at which the integrated circuit can function. 
     Previous methods have attempted to overcome the problem of coupling by increasing space between radiant components and receptive components, which results in larger die area and increased design costs. Other previous techniques have boosted the voltage levels of low voltage signals requiring a high degree of isolation; resulting in lower power efficiency and relatively high power emissions that may couple undesirably with other signals. 
     Previous designs. have used trenches to isolate components from a substrate; but such attempts typically fail to yield the 100 decibels (dB) of isolation necessary to integrate systems comprising mixed signal devices (such as base band, phase-locked-loop, or voltage controlled oscillators) or functionally distinct circuits requiring different power levels (such as transmitter or receiver) on the same substrate. 
     Even by changing the substrate, or isolating the component from the substrate, the degrees of isolation necessary to integrate many RF systems on a single chip have not been achieved. Thus, commercially viable isolation of RF components from the substrate is now needed. 
     SUMMARY OF THE INVENTION 
     Therefore, a method for fabricating an integrated circuit having active components, high frequency conductors and isolation regions on a substrate is now needed; providing enhanced design performance while overcoming the aforementioned limitations of conventional methods. 
     The present invention provides integrated circuit structures and fabrication methods, including techniques using the etching of porous silicon areas and dielectric backfill to provide isolation for circuit components. The present invention provides for isolation of circuit components, including decoupling of RF transmission lines on a circuit structure from a semiconductor substrate, to minimize parasitics. The present invention provides a method of fabricating an integrated circuit having active components, conductors and isolation regions on a substrate. 
     An embodiment of the present invention comprises patterning and etching at a portion of at least one of the isolation regions to expose a first area of the substrate; depositing a layer of silicon carbide (or other material resistant to a porous silicon formation process) over the substrate including the first area, patterning and etching the silicon carbide layer to expose a second area of substrate within the first area, forming a porous silicon region in at least the second area using HF (Hydrogen Fluoride), wherein the silicon carbide layer protects the active areas from the HF, forming at least one dielectric layer over the substrate, forming at least one patterned metallization layer over the dielectric layer, removing the porous silicon from the backside to form a void, and backfilling the void left,by the removal with a dielectric. 
     One embodiment of the present invention may form angled sidewalls having a slope between 30° and 60° degrees via the step of patterning and etching the isolation regions. This formation can comprise an isotropic plasma etch using CF 4 /O 2 , as well as an HF etch. 
     The present invention may comprise forming an oxide layer over the substrate patterned metallization comprising RF transmission lines; where the removal of porous silicon and backfilling with dielectric decouples RF transmission lines on the circuit structure from the semiconductor substrate to minimize parasitics. 
     In one embodiment of the present invention, the step of depositing a layer of silicon carbide is accomplished by using a silicon carbide layer with a thickness in the range of 500-5000 Å. 
     In another embodiment of the present invention, the fabrication of an RF integrated circuit having active components, high frequency conductors and isolation regions on a substrate, comprises the forming of isolation regions in a substrate; forming active components in said substrate; patterning at least one of the isolation regions to expose a first area of said substrate; etching away some of the field oxide; forming a patterned masking layer of silicon carbide over said substrate, preferably by Plasma Enhanced Chemical Vapor Deposition (PECVD); patterning and etching the silicon carbide layer to expose a second area of the substrate within the first area; anodizing the porous silicon region; exposing the porous silicon from the backside, e.g., by back grinding; removing the porous silicon from backside; and spin-coating on glass to fill voids left by the removal. 
     In some embodiments, especially those for high frequency applications, a low-dielectric constant material, such as porous silicon dioxide, e.g., as formed by an aerogel or HSQ process, is used as the dielectric, filling the voids left by the removal of the porous silicon. 
     A stabilizing material, such as a photoresist, can be used to level and strengthen the topside of the wafer, prior to the back-grinding step. The steps of back-grinding, removal of the porous silicon, and back-filling with dielectric may all be done in the same machine to avoid handling of the wafer while it is in a relatively fragile condition. 
     In one embodiment, there is a step of drying out porous silicon at 100-200° C. in vacuum for 6 to 24 hours, then heating in oven at 300-400° C. in oxygen for 1 hour; shifting from oxygen to nitrogen, to reduce the oxygen concentration in the porous silicon before depositing a capping layer. 
     In another embodiment, the anodization to form porous silicon is continued until the porous silicon extends entirely through the substrate and the need for back-grinding is eliminated. 
     The present invention may thus be utilized for isolation in general, not just to decouple conductors from the substrate to minimize RF parasitics. Areas in a substrate may be isolated from one another, even down to an individual transistor level, if desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention taken in conjunction with the accompanying drawings in which like numerals identify like parts and in which: 
     FIG. 1 is a schematic illustration of a transistor and field oxide structure; 
     FIG. 2 is a schematic illustration of the structure of FIG. 1, with an additional layer of silicon dioxide added, e.g., by a Plasma Enhanced Chemical Vapor Deposition (PECVD); 
     FIG. 3 is a schematic illustration of the structure of FIG. 2, with a layer of photoresist spun-on; 
     FIG. 4 is a schematic illustration of patterned photoresist leaving an exposed area of silicon dioxide over the site where Porous Silicon (PS) is to be formed; 
     FIG. 5 is a schematic illustration of silicon dioxide etched with tapered slope down to the Si substrate; 
     FIG. 6 is a schematic illustration of process after photoresist has been removed and after a layer of silicon carbide has been deposited; 
     FIG. 7 is a schematic illustration of the structure after a second layer of photoresist has been deposited and patterned; 
     FIG. 8 is a schematic illustration of Si area exposed after a carbon tetrafluoride RIE etch and photoresist strip; 
     FIG. 9 is a schematic illustration of the structure after a porous silicon region has been anodized in the substrate; 
     FIG. 10 is a schematic illustration of vias cut through Intra-Layer Dielectric (ILD), SiC and TEOS to expose silicide areas on the transistor, using a three (3) step etch; 
     FIG. 11 is a schematic illustration after deposition of a first dielectric layer, and after inductor metallization has been deposited and patterned; 
     FIG. 12 is a schematic illustration of the structure, after deposition of a second dielectric layer and after a support layer of, e.g., photoresist has been added on the wafer topside and after grinding of the wafer backside; 
     FIG. 13 is a schematic illustration of the structure after the porous silicon has been removed, e.g., by backside wet-etching; 
     FIG. 14 is a schematic illustration of the structure after a dielectric has been used in a backside filling of the void left by the porous silicon removal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     A process for forming selective porous silicon (PS) areas according to a first embodiment of the invention will now be discussed in further detail. In this embodiment, the silicon carbide (SiC) masking layer is deposited after the transistor silicide process. Referring to FIG. 1, a semiconductor wafer during fabrication process is depicted as  20  showing substrate  34  having a transistor  21  having poly gate  24 , side walls  26 , gate oxide  30 , and source/drain implants  32  formed therein. The device has been processed through isolation regions  28  and transistor  21  formation. Transistor  21  has been fabricated through silicide  22  formation and anneal. Isolation regions  28  are shown as field oxide regions. 
     Referring to FIG. 2, an oxide layer  36  may be deposited at this point. This oxide layer may be deposited by, for example, PETEOS, to a thickness on the order of 500-10,000 Å. 
     Next, a resist layer  38  is formed over the surface as shown in FIG.  3 . The resist layer  38  is patterned as shown in FIG. 4 to expose an area  42  of isolation region  28 . Area  42  is located over the desired PS site. Then, the oxide of isolation regions  28  is etched, preferably leaving sidewalls  44  having a slope between 30° and 60° degrees, as shown in FIG.  5 . This may, for example, be accomplished in one of the following two ways: (1) an isotropic plasma etch using CF 4 /O 2  or its equivalent or (2) an HF deglaze. Resist layer  38  is then removed using standard ash/cleanup processes. The above oxide pattern and etch steps may alternatively be performed prior to transistor formation. 
     A p+ diffusion of boron or similar dopant may be performed prior to the transistor formation to convert the p− epi to p+ with resistivity on the order of 0.01-cm. If necessary, a deglaze to remove the boron or similar dopant contaminated oxide is then performed. 
     Referring to FIG. 6, a masking layer  46  of SiC is deposited over the structure. Layer  46  may typically have a thickness in the range of 500 to 5000 Å. The following process may be used: PECVD (plasma-enhanced chemical vapor deposition) using silane/methane, trimethylsilane, tetramethylsilane or other organosilicon precursor gas and Ar or He as carrier gas. The pressure may be on the order of 5 Torr. The gas flow may be in the range of 500-5000 sccm. RF power density may be on the order of 2 W/cm 2  (13.56 MHZ). The substrate temperature may be in the range of 200-500° C. If desired, a double deposition step (using the same conditions) may be used to reduce defects. 
     Next, a second resist mask  48  is formed over the SiC masking layer  46 . The second resist mask  48  is patterned to expose a portion  50  of SiC masking layer  46  approximately in the center of where the PS region is desired, as shown in FIG.  7 . The exposed portion  50  is generally significantly smaller than the width of the desired PS region. The relationship between the size of portion  50  and the desired PS region is optimized based on the PS formation process parameters. 
     Referring to FIG. 8, the exposed portion  50  of SiC masking layer  46  is removed. The following are some exemplary methods for removing portion  50  of SiC masking layer  46 . (1) Cl 2 /O 2  based etch using a chamber pressure: 300 mTorr (gas flow 10-50 sccm), O 2  fraction 0-90%, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); (2) CF 4 /O 2 /H 2  based etch using an O 2  fraction 0-90%, H 2  flow 0-20 sccm, (fluorinated-O 2  gas flow 10-50 sccm),chamber pressure; 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); (3) SF 6 /O 2 /H 2  based etch using O 2  fraction 0-90%, H 2  flow 0-20 sccm, (fluorinated-O 2  gas flow 10-50 sccm), chamber pressure: 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); and (4) CHF 3 /CF 4 /Ar/O 2 /H 2  based etch using an O 2  fraction 0-50%, H 2  flow 0-100 sccm, (fluorinated-Ar gas flow 50-200 sccm), chamber pressure: 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ). Second resist layer  52  is then removed to give the structure shown in FIG.  8 . 
     Next, the PS regions  54  are formed by anodization, as shown in FIG.  9 . The thickness of PS region  54  may vary between 10 μm and the wafer thickness, depending on the application. Typically, PS region  54  may be in the range of 10-200 μm. Any suitable anodization method may be used. For example, the porosity may be in the range of 45-70% and the current density may be in the range of 50-200 mA/cm2. Since the wafers used will typically be either p−, or have a p+ substrate with a p− epi layer on top, the electrolyte may have a larger than usual HF concentration, such as 50-70% (49%) HF. The electrolyte generally contains a surfactant such as isopropyl alcohol, and optionally additional water. Higher HF concentrations may be necessary to prevent the porosity of the PS in the p− from becoming too large and then cracking, while at the same time maintaining reasonable etch rates during anodization. 
     Under certain anodization conditions, the SiC mask may also crack as the etch proceeds beneath it. To avoid this, the anodization current density may initially be started at low levels and gradually increased to the desired level during the first 30-50% of the etch. This way the latter part of the etch may proceed at a high rate, again keeping the overall etch time reasonable. After the formation of the PS region  54 , a capping layer of silicon carbide or other dielectric  47  may be deposited over the exposed porous silicon region as shown in FIG.  9 . 
     Referring to FIG. 10, an interlayer dielectric (ILD)  58  is deposited. ILD  58  typically comprises a PECVD TEOS. However, other ILD materials may alternatively be used such as fluorosilicate glass, high density, plasma TEOS, silicon nitride or spin-on glass. 
     The ILD  58  is then patterned and a contact etch is performed to etch through the ILD  58 , SiC masking layer  46  and the oxide layer down to the silicide  22 , as shown in FIG. 10. A multi-step etch is used, because an etch that will remove both SiC and SiO 2  will probably have poor selectivity between the dielectrics and the silicide, and most likely cut into the silicide. Without the multi-step etch, poor process control may result. 
     Referring to FIG. 11, a schematic illustration of only the right portion of the preceding figures is shown after deposition of a first dielectric layer after inductor  64  metallization has been deposited and patterned. 
     Referring to FIG. 12, a second dielectric layer  65  is added followed by a strengthening layer of photoresist  66  and then followed by a back-grinding  62  step which exposes the porous silicon from the backside. 
     Referring to FIG. 13, the exposed porous silicon is then removed from the backside by the process of wet etching to leave a void  70 . Voids left by removal of the porous silicon are back-filled with dielectric fill  72  and planarized as shown in FIG.  14 . The steps of back-grinding, removal of the porous silicon, and back-filling with dielectric may all be done in the same machine, to avoid handling of the wafer while it is in a relatively fragile condition. 
     In a second embodiment, the silicon carbide (SiC) masking layer is deposited after the transistor silicide process. Referring to FIG. 1, a semiconductor wafer during fabrication process is depicted as  20  showing substrate  34  having a transistor  21  having poly gate  24 , side walls  26 , gate oxide  30 , and source/drain implants  32  formed therein. The device has been processed through isolation regions  28  and transistor  21  formation. Transistor  21  has been fabricated through silicide  22  formation and anneal. Isolation regions  28  are shown as field oxide regions. 
     Referring to FIG. 2, an interlayer dielectric (ILD)  36  may be deposited at this point as a Pre-Metal Deposition (PMD) layer. This layer may be deposited by, for example, PETEOS, to a thickness on the order of 3000-10,000 Å. The resulting surface may then be planarized by Chemical Mechanical Polishing (CMP). 
     Next, a resist layer  38  is formed over the surface as shown in FIG.  3 . The resist layer  38  is patterned as shown in FIG. 4 to expose an area  42  over the isolation region  28 . Area  42  is located over the desired PS site. Then, the oxide of isolation regions  28  is wet oxide etched (using HF), preferably leaving sidewalls  44  having a slope between 30° and 60° degrees, as shown in FIG.  5 . This may, for example, be accomplished in one of the following two ways: (1) an isotropic plasma etch using CF 4 /O 2  or its equivalent or (2) an HF deglaze. Resist layer  38  is then removed using standard ash/cleanup processes. 
     A p+ diffusion of boron or similar dopant may be performed prior to the transistor formation to convert the p− epi to p+ with resistivity on the order of 0.01-cm. A shallow implant or diffusion of Boron will also be required on the backside of p− wafers to convert the backside surface to p+ and prevent charge inversion during the PS anodization step. If necessary, a deglaze to remove the boron or similar dopant contaminated oxide is then performed. Referring to FIG. 6, a masking layer  46  of SiC is deposited over the structure. Layer  46  may typically have a thickness in the range of 1000 to 5000 Å. The following process may be used: PECVD (plasma-enhanced chemical vapor deposition) using silane/methane, trimethylsilane, tetramethylsilane or other organosilicon precursor gas and Ar or He as carrier gas. The pressure may be on the order of 5 Torr. The gas flow may be in the range of 500-5000 sccm. RF power density may be on the order of 2 W/cm 2  (13.56 MHZ). The substrate temperature may be in the range of 200-500° C. If desired, a double deposition step (using the same conditions) may be used to reduce defects. 
     Next, a second resist mask  48  is formed over the SiC masking layer  46 . The second resist mask  48  is patterned to expose a portion  50  of SiC masking layer  46  approximately in the center of where the PS region is desired, as shown in FIG.  7 . The exposed portion  50  is generally significantly smaller than the width of the desired PS region. The relationship between the size of portion  50  and the desired PS region is optimized based on the PS formation process parameters. The width of the desired PS region may be smaller than the width of the substrate  34  that is directly covered by the SiC masking layer; the width of substrate  34  in excess of the width of the desired PS region may be used in later process steps to provide a suitable horizontal surface upon which structures for contacting upper-layer metallization may be made. 
     Referring to FIG. 8, the exposed portion  50  of SiC masking layer  46  is removed. The following are some exemplary methods for removing portion  50  of SIC masking layer  46 . (1) Cl 2 /O 2  based etch using a chamber pressure: 300 mTorr (gas flow 10-50 sccm), O 2  fraction 0-90%, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); (2) CF 4 /O 2 /H 2  based etch using an O 2  fraction 0-90%, H 2  flow 0-20 sccm, (fluorinated-O 2  gas flow 10-50 sccm),chamber pressure; 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); (3) SF 6 /O 2 /H 2  based etch using O 2  fraction 0-90%, H 2  flow 0-20 sccm, (fluorinated-O 2  gas flow 10-50 sccm),chamber pressure: 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ); and (4) CHF 3 /CF 4 /Ar/O 2 /H 2  based etch using an O 2  fraction 0-50%, H 2  flow 0-100 sccm, (fluorinated-Ar gas flow 50-200 sccm),chamber pressure: 10-50 mTorr, and RF Power density 0.5-1 W/cm 2  (13.56 MHZ). Second resist layer  52  is then removed to give the structure shown in FIG.  8 . 
     Next the PS regions are formed by anodization. The thickness of the PS region may vary between 10 □m and the wafer thickness, depending on the application. Typically, the PS region may be in the range of 10-300 □m. Any suitable anodization, method may be used. For example, the porosity may be in the range of 45-70% and the current density may be in the range of 50-200 mA/cm2. Since the wafers used will typically be either p−, or have a p+ substrate with a p− epi layer on top, the electrolyte may have a larger than usual HF concentration, such as 50-70% (49%) HF. The electrolyte generally contains a surfactant such as isopropyl alcohol, and optionally additional water. Higher HF concentrations may be necessary to prevent the porosity of the PS in the p− from becoming too large and then cracking, while at the same time maintaining reasonable etch rates during anodization. The anodization chemistry may comprise a ratio by volume of 60:30:10 of an HF solution, an alcohol, and deionized water. 
     Under certain anodization conditions, the SiC mask may also crack as the etch proceeds beneath it. To avoid this, the anodization current density may initially be started at low levels and gradually increased to the desired level during the first 30-50% of the etch. This way the latter part of the etch may proceed at a high rate, again keeping the overall etch time reasonable. 
     The exposed porous silicon Is then removed by the process of wet etching to leave a void. The etch chemistry may comprise a radon by volume of 10:6:50 of a buffered HF solution (−40% NH 4 F and ˜4.5% HF), an alcohol, and a peroxide solution (˜30% H 2 O 2 ). The SiC masking layer may then be removed by a chlorine based Reactive Ion Etch (RIE) 
     A dielectric layer such as an SOG (Spun-On-Glass) Is deposited over the conductive layer so that the dielectric layer extends at least to the height of the isolation region and interlayer dielectric The SOG may be densified by baking it in a furnace at between 200 □ and 330 □ degrees Celsius for one hour. 
     The surface of wafer is then planarized by an oxide CMP process down to the interlayer dielectric of isolation region. 
     The dielectric layer may be patterned and etched to expose silicide regions of the transistor structure so that silicide regions can have metal contacts attached to them. 
     Inductor metallization is then deposited and patterned. 
     The wafer is thinned by a back-grinding step which optionally exposes the dielectric layer depending on extent of the back-grinding. 
     While the making and using of various embodiments are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention.