# Patent

## Title
Through wafer vias and method of making same

## Abstract

A method of forming and structure for through wafer vias and signal transmission lines formed of through wafer vias. The method of forming through wafer vias includes forming an array of through wafer vias comprising at least one electrically conductive through wafer via and at least one electrically non-conductive through wafer via through a semiconductor substrate having a top surface and an opposite bottom surface, each through wafer via of the array of through wafer vias extending from the top surface of the substrate to the bottom surface of the substrate.

## Background
RELATED APPLICATIONS
This application is a division of U.S. patent application Ser. No. 12/188,236 filed on Aug. 8, 2008.


FIELD OF THE INVENTION
The present invention relates to the field of integrated circuit chips; more specifically, it relates to through wafer vias for use in integrated circuit chips and the method of fabricating the through wafer vias
BACKGROUND OF THE INVENTION
To increase the density of devices using integrated circuit chips it is desirable to allow interconnections to be made to both the top and bottom surfaces of the integrated circuit chip. This requires formation of through wafer vias from the top to the bottom surface of the integrated chip that are compatible with carrying both high frequency and DC signals. Many existing through via schemes are either difficult to integrate into existing integrated circuit fabrication processes or result in unacceptable degradation of signals propagating from/to the front surface of the integrated circuit chip to/from the bottom surface of the integrated circuit chip. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.
SUMMARY OF THE INVENTION
A first aspect of the present invention is a structure, comprising: a semiconductor substrate having a top surface and an opposite bottom surface; and an array of through wafer vias comprising at least one electrically conductive through wafer via and at least one electrically non-conductive through wafer via, each through wafer via of the array of through wafer vias extending from the top surface of to the bottom surface of the substrate.
A second aspect of the present invention is a method, comprising: forming an array of through wafer vias comprising at least one electrically conductive through wafer via and at least one electrically non-conductive through wafer via through a semiconductor substrate having a top surface and an opposite bottom surface, each through wafer via of the array of through wafer vias extending from the top surface of the substrate to the bottom surface of the substrate.
A third aspect of the present invention is a method, comprising: ((a) forming a first trench and a second trench in a semiconductor substrate, the first and second trenches extending in a first direction from a top surface of the substrate toward an opposite bottom surface of the substrate a distance less than a thickness of the substrate in the first direction; after (a), (b) simultaneously completely filling the first trench with a dielectric material and forming a liner of the dielectric material on sidewalls of the second trench; after (b), (c) filling remaining space in the second trench with a polysilicon core, recessing the polysilicon core below the top surface of the substrate, and forming, in the second trench, a dielectric plug on the polysilicon core; after (c), (d) removing the dielectric material from the first trench and filling the first trench with and electrically conductive material; and after (d), (e) thinning the substrate from the bottom surface of the substrate to form a new bottom surface of the substrate, the electrically conductive material of the first trench and the liner and polysilicon core of the second trench exposed in the new bottom surface of substrate.
A fourth aspect of the present invention is a signal transmission line through a semiconductor substrate, the substrate having a top surface and an opposite bottom surface, comprising: a conductive through wafer via extending from the top surface of the substrate to the bottom surface of the substrate, sidewalls of the conductive through via in physical and electrical contact with the substrate; and a non-conductive through via extending from the top surface of the substrate to the bottom surface of the substrate, the nonconductive through via proximate to and separated from the conductive through wafer by a region of the substrate.



BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
FIGS. 1A through 1M are cross-sectional drawings illustrating initial steps in the fabrication of an array of through wafer vias according to embodiments of the present invention;
FIGS. 2A through 2J are cross-sectional drawings illustrating completion of the fabrication of the array of through wafer vias according to embodiments of the present invention and fabrication of a three-dimensional device using arrays of through wafer vias according to embodiments of the present inventions;
FIGS. 3A through 3D are plan views of through wafer vias according to embodiments of the present inventions;
FIGS. 4A through 4D are schematic plan views of waveguide models using through wafer vias according to embodiments of the present invention;
FIG. 5 is cross-sectional drawing illustrating an alternative through wafer via structure according embodiments of the present invention;
FIG. 6A illustrates the phenomena of wide trenches etching deeper than narrow trenches and FIG. 6B and FIG. 6B illustrates a method of mitigating the effect of wide trenches etching deeper than narrow trenches; and
FIGS. 7A through 7D illustrate an alternative method of fabricating through vias according to embodiments of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The term through wafer via defines a structure that extends from a top surface of a substrate, through the substrate, to an opposite bottom surface of the substrate of the packaged integrated circuit or chip. A through wafer via according to the embodiments of the present invention may be electrically conductive or electrically non-conductive. Although in the description infra, both conductive and non-conductive (i.e., insulating) through vias are described and illustrated as extending from the top to the bottom surface of the chip, the present invention may be practiced where the conductive through via extends entirely through the chip, but the non-conductive only extends partially through the chip because one purpose of the non-conductive through vias is for isolation and not passing electrical signals between the top and bottom surfaces of the chip. An electrically conductive through via includes at least one electrically conductive element and may include non-electrically conductive elements. An electrically non-conductive through via includes at least one electrically non-conductive element and may include electrically conductive elements that are surrounded completely by electrically non-conductive elements. The “wafer” of through wafer via derives from the fact that the via is formed before integrated circuits have been singulated from a semiconductor substrate called a wafer. The term “three dimensional device” defines a device comprising two or more individual substrates electrically connected and in physical contact by being stacked one upon another.
FIGS. 1A through 1M are cross-sectional drawings illustrating fabrication of a through wafer via is according a first embodiment of the present invention. In FIG. 1A, a semiconductor substrate 100 has a top surface 105. Formed on top surface 105 is a first dielectric layer 110. Formed on a top surface 115 of first dielectric layer 110 is a second dielectric layer 120. First and second dielectric layers 110 and 120 are exemplary and there may be a few as one dielectric layer or more than two dielectric layers formed over top surface 105 of substrate 100. In one example, substrate 100 comprises silicon. In one example first dielectric layer 110 is silicon dioxide and second dielectric layer 120 is silicon nitride.
In FIG. 1B, trenches 125 and 130 are etched through first and second dielectric layers 110 and 120 and into substrate 100. Trenches 125 and 130 may be formed using a photolithographic/etching process. An exemplary photolithographic/etching process comprises (1) forming a photoresist layer on second dielectric layer 120, (2) forming openings in the photoresist layer by exposing the photoresist layer to actinic radiation through a patterned photomask and developing away either the exposed or unexposed regions of the photoresist, (4) etching through, the first and second dielectric layers using, for example, a reactive ion etch (RIE) process, (5) removing the patterned photoresist layer, and (6) etching substrate 100 using, for example, an RIE process using the pattern in the first and second dielectric layers as a patterned hardmask. Although trenches 125 and 130 are shown etched to the same depth in FIG. 1B, trenches 125 and 130 may be etched to different depths. For example, an etch process commonly referred to as the “Bosch” silicon etch process will etch trenches having wide (W2) openings deeper then trenches having narrow openings (W1). This phenomena illustrated in FIG. 6A. If the etch depth differences are small, this effect presents no problem as the thinning processes described infra can be adjusted to over-thin to compensate. Otherwise the alternative processes illustrated in FIGS. 6A and\7A-7D and described infra can be performed.
Trenches 125 and 130 extend into substrate 100 a distance D1 from top surface 105 of the substrate. Trenches 125 have a width W1 and trench 130 has a width W2. W2 is greater than W1. In one example, W1 is about 1 micron to about 3 microns. In one example, W2 is about 3 microns to about 10 microns. In one example D1 is about 50 microns to about 200 microns. In one example W1 is about 2 microns, W2 is about 5 microns and D1 is about 150 microns. Since trenches 125 and 130 may be elongated in and out of the plane of the drawings (i.e., are rectangles when viewed from above), W1 and W2 measure minimum widths of trenches 125 and 130 (i.e, the short side of the rectangle).
In FIG. 1C, a polysilicon layer 135 is deposited on top surfaces 140 of second dielectric layer 120, sidewalls 145 and bottoms 150 of trenches 125 and 130. In one example, polysilicon layer 135 comprises an N or a P-type doped polysilicon. In one example, polysilicon layer 135 comprises boron doped polysilicon. Polysilicon layer 135 has a thickness T1. In one example, T1 is about 0.8 microns to about 2.4 microns.
In FIG. 1D an oxidation is performed to convert polysilicon layer 135 (see FIG. 1C) to a silicon dioxide layer 155. Silicon dioxide layer 155 completely fills trenches 125, but does not completely fill trench 130 because the width W2 (see FIG. 1B) of trench 130 is greater than twice the thickness T2 of silicon dioxide layer 155. Silicon dioxide layer conformally covers the sidewalls and bottom of trench 130. In one example, T2 is about equal to half of W1 (see FIG. 1B). In one example, the oxidation of polysilicon layer 135 (see FIG. 1C) to form silicon dioxide layer 155 is performed using a high pressure oxidation (HIPOX) process.
HIPOX of boron doped polysilicon (i.e., layer 135 of FIG. 1C) is preferred because of the high oxidation rate of born doped polysilicon and the ability of HIPOX to form uniform oxide thicknesses in deep trenches.
Alternatively, the structure illustrated in FIG. 1D may be formed using a high pressure oxidation process of the sidewalls and bottoms of trenches 125 and 130 of FIG. 1B. In one example, a high pressure oxidation is performed at a pressure greater than about 5 atmospheres. In one example, a high pressure oxidation is performed at a pressure between about 5 atmospheres and about 25 atmospheres. Alternatively, the structure illustrated in FIG. 1D may be formed by deposition (e.g., by chemical vapor deposition (CVD) or atomic layer deposition (ALD)) of oxide on the sidewalls and bottom of trenches 125 and 130.
In FIG. 1E, a polysilicon layer 170 is formed top surface 175 of silicon dioxide layer 155 and completely fills the remaining space in trench 130. In one example, polysilicon layer 170 comprises intrinsic (i.e., undoped) polysilicon.
In FIG. 1F, a polysilicon recess process is performed which removes all of polysilicon layer 170 from top surface 175 of dielectric layer 155 and from an upper region of trench 130. A top surface of polysilicon layer 170 remaining in trench 130 after the recess process is below top surface 105 of substrate 100. The polysilicon recess process may be performed using an RIE, a wet etch, or an RIE followed by a wet etch.
In FIG. 1G, a dielectric layer 185 is formed top surface 175 of silicon dioxide layer 155 and in trench 165. Dielectric layer 185 completely fills the space created in trench 130 by the polysilicon recess etch of FIG. 1F. In one example dielectric layer 185 comprises TEOS oxide (oxide formed by CVD using tetraethoxysilane precursor).
In FIG. 1H, using a chemical-mechanical-polish (CMP) process, dielectric layer 185, silicon dioxide layer 155, second dielectric layer 120 and first dielectric layer 110 (see FIG. 1G) are removed and a new first dielectric layer 190 and a new second dielectric layer 195 are formed on new top surface 105A of substrate 100. Silicon dioxide layer 155 remains in trenches 125 and silicon dioxide layer 155, polysilicon layer 170 and dielectric layer 185 remain in trench 130 protected by dielectric layers 190 and 195.
In FIG. 1I, first, shallow trench isolation (STI) 200 has been formed in substrate 100 by a photolithographic/etching process in combination new first and second dielectric layers 190 and 195 (see FIG. 1H) similar to that described supra followed by a TEOS CVD followed by a CMP. Second, additional photolithographic/etching process/deposition process have been performed to form a FET 205 and a trench capacitor 210. FET 205 and trench capacitor are examples of integrated circuit devices that may be formed at this point in fabrication. Other devices that may be formed at this point include bipolar transistors, BiCMOS SiGe transistors, diodes and resistors. FET 205 includes source/drains 215, a gate dielectric 220, a gate electrode 225, and silicide contacts 230. Trench capacitor 210 includes an inner plate 235 and a dielectric layer 238. Third, new first and second dielectric layers 190 and 195 are removed and a first interlevel dielectric layer 240 is formed over a new top surface 105B of substrate 100. Interlevel dielectric layer 240, by way of example, comprises a lower dielectric layer 245 and an upper dielectric layer 250. Interlevel dielectric layer 240 may be a single layer or may include more than two layers. In one example, lower dielectric layer 245 comprises silicon nitride and upper dielectric layer 250 boro-phosphosilicate glass (BPSG).
In FIG. 1I and subsequent FIGS. 1J through 1M, lower dielectric layer 245 is shown as not covering gate 225 of FET 205 for clarity. In actuality, lower dielectric layer 245 may cover gate 225 of FET 205.
In FIG. 1J, openings 252 over silicon dioxide layer 155 filled trenches 125 and opening 253 over silicide layer 230 of FET 205 are formed using a photolithographic/etching process as described supra.
In FIG. 1K, a patterned photoresist layer 255 is formed having openings 254 over trenches 125, but not over opening 253 and silicon dioxide layer 155 (see FIG. 1J) is removed from trenches 125, for example, by wet etching. Photoresist layer 255 protects all exposed surfaces of upper and lower dielectric layers 245 and 250 during the removal of silicon dioxide layer 155 (see FIG. 1J) from trenches 125.
In FIG. 1L, photoresist layer 255 (see FIG. 1J) is removed and an electrically conductive layer 260 is formed (for example, by evaporation, sputtering or deposition) over interlevel dielectric layer 240. Electrically conductive layer 260 completely fills trenches 125 and opening 252 over trenches 125 and opening 253 in interlevel dielectric layer 240 over silicide layer 230. In one example, electrically conductive layer 260 comprises a metal. In one example, electrically conductive layer 260 comprises tungsten (W) or tungsten and titanium nitride (TiN). In one example, electrically conductive layer 260 comprises a first deposited conformal layer of titanium nitride and a second deposited layer of tungsten. In one example, t electrically conductive layer 260 comprises a first deposited conformal layer of titanium nitride, a second deposited layer of conformal titanium (Ti) and a third deposited layer of tungsten. Titanium, titanium nitride and tungsten may be deposited using CVD.
In one example, electrically conductive layer 260 comprises tungsten or tungsten and tantalum nitride (TaN). In one example, electrically conductive layer 260 comprises a first deposited conformal layer of tantalum nitride and a second deposited layer of tungsten. In one example, electrically conductive layer 260 comprises a first deposited conformal layer of tantalum nitride, a second deposited layer of conformal tantalum (Ta) and a third deposited layer of tungsten. Tantalum and tantalum nitride and may be deposited using CVD.
Other metallurgical combinations that may be used for electrically conductive layer 260 include combinations of copper (Cu), ruthenium (Ru), Ta and TaN. Those combinations include the following combinations, each of which is in the order of formation: Ta/Cu, TaN/Cu, Ru/Cu, TaN/Ta/Cu/, TaN/Ru/Cu, Ta/Ru/Cu, Ru/Ta/Cu, Ru/TaN/Cu, TaN/Ta/Ru/Cu.
In FIG. 1M, a CMP process is performed to remove excess layer 260 (see FIG. 260) to expose a top surface of upper dielectric layer 250 and simultaneously form a stud contact 265 to silicide layer 230 of FET 205 and a through wafer via 275 having an integral contact 280. Top surfaces of contacts 280 and stud contact 265 are coplanar with the top surface of upper dielectric layer 250.
It should be understood, that many other stud contacts 265 are formed at this time to other devices in substrate 100. It should be also understood that by thinning substrate 100 from the bottom a through wafer via will be formed as illustrated in FIGS. 2A through 4J and described infra. Core 275 is in physical and electrical contact with substrate 100. Polysilicon region 170 in trench 130 is electrically isolated from substrate 100 by silicon dioxide layer 155.
FIGS. 2A through 2J are cross-sectional drawings illustrating completion of the fabrication of the array of through wafer vias according to embodiments of the present invention and fabrication of a three-dimensional device using arrays of through wafer vias according to embodiments of the present inventions. In FIGS. 2A through 2J, the depth of the narrow trenches are illustrated as being the same. As mentioned supra, there are processes that may be applied if the widths if the wide and narrow trenches are such as to result in significant differences in etch depth, These alternative processes illustrated in FIGS. 6A and\7A-7D and described infra.
In FIG. 2A, (not drawn to scale) formed in a set of interlevel dielectric layers 300 are corresponding wires and vias 305. An optional terminal pad 310 is formed on a top surface 315 of the set of interlevel dielectric layers 300 and is in electrical contact with an uppermost wire 305 in an uppermost interlevel dielectric layer of the set of interlevel dielectric layers 300. Wires in a lowermost interlevel dielectric layer of the set of interlevel dielectric layers are in physical and electrical contact with stud contacts 265 and integral contact regions 280. Individual interlevel dielectric layers of the set of interlevel dielectric layers 300 are not illustrated in FIG. 2A. A handle substrate 325 is attached to top surface 315 of the set of interlevel dielectric layers 300. Handle wafer 325 is attached using a layer of adhesive (not shown). In one example, handle substrate 325 is a quartz wafer.
In FIG. 2B, substrate 100 is thinned from the bottom (for example, by grinding) to form a new bottom surface 320 that is a distance D2 from trenches 125 and 130. In one example, D2 is about 5 microns to about 50 microns. In one example, D2 is about 20 microns. After thinning, the thickness of substrate 100 is D3. In one example, D3 is about 50 microns to about 200 microns. In one example, D3 is about 170 microns.
In FIG. 2C, a RIE or wet etch selective to silicon is preformed to recess bottom surface 320 (see FIG. 1B) of substrate 100 so the fill material of trenches 125 and 130 protrudes above a new top surface 320A.
In FIG. 2D, a CMP is performed to remove the fill material protruding above top surface 320A (see FIG. 2C) to form through wafer via arrays 330. In the example of FIG. 2D, each through wafer via array 330 includes two conductive through wafer vias 125A and one non-conductive conductive through via 130A. While the core formed of polysilicon layer 170 of through wafer via 130A is conductive, because of the plug formed of dielectric layer 185 (see FIG. 1I) through wafer via 130A is defined as a non-conductive through wafer via relative to conducting electricity from front surface 105 to bottom surface 335. After the CMP, the electrically conductive through wafer region 275 and silicon dioxide layers 155 (see FIG. 2C) are exposed at bottom surface 320A of substrate 100.
If trenches 125 and 130 (see FIG. 1B) had been etched to different depths, then the etch back of substrate 100 performed in relation to FIG. 2C is performed to expose the fill material in both trenches 125 and 130 and the CMP performed in relation to FIG. 2D, is performed so as to result in the structure illustrated in FIG. 2D.
In FIG. 2E, a RIE or wet etch selective to preferentially etch silicon over silicon dioxide is preformed to recess bottom surface 320A (see FIG. 2D) below the bottoms of through wafer via arrays 330 and to form a new bottom surface 335 of substrate 100.
In FIG. 2F, a dielectric layer 340 is formed over bottom surface 335 of substrate and over through wafer via arrays 330. In one example, dielectric layer 340 is a plasma enhanced chemical vapor deposition (PECVD) silicon oxide.
In FIG. 2G, a CMP is performed to remove dielectric layer 340 from over the bottom surfaces of through wafer via arrays 330. Dielectric layer 340 remains on bottom surface 335 of dielectric layer 340 and dielectric layer 340 fills any spaces between through wafer via arrays 330 between individual through wafer vias 125A and 130A of each of the through wafer via arrays. Bottom surfaces of through wafer vias 125A and 130A are coplanar or substantially coplanar with a top surface 350 of dielectric layer 340.
Alternatively, the backside grind process illustrated in FIG. 2B and described supra, can be continued until conductive through wafer vias 125A and non-conductive through wafer vias 125A of FIG. 2D are formed directly (skipping the processes of FIG. 2C) or after grinding and a “clean-up” CMP to remove any grinding damage to the through wafer vias and surface 320A (see FIG. 2D).
It should be noted that through wafer via 125A comprises a first trench filled only with an electrical conductor and through wafer via 130A comprises a second trench having a filling only consisting of a dielectric liner surrounding an electrically conductive core with a dielectric plug between the dielectric liner and over the conductive core at one end.
In FIG. 2H, electrically conductive pads 345 are formed on top surface 350 of dielectric layer 340 on through wafer via arrays 330 and electrically conductive solder bumps 355 are formed on pads 345. In one example, pads 345 and solder bumps 355 are formed by electroplating through a patterned photoresist layer or by evaporation through a metal mask. If pads 345 are formed by plating, a thin electrically seed layer is first deposited, which is removed after the photoresist layer is removed, by RIE or wet etching.
In FIG. 2I, handle wafer 325 (see FIG. 2G) is removed, either prior to or after chip dicing. An exemplary method of removing handle wafer 325 is to expose the adhesive to ultraviolet radiation as is well known in the art. In a preferred embodiment, handle wafer 325 is removed after dicing to minimize the potential for breaking the thinned wafer.
FIG. 2J is an exploded view prior to a solder reflow step. In FIG. 2J, an upper substrate 360 containing electrical components is aligned to terminal pad 310 by electrically conductive solder bumps 365, and substrate 100 is aligned to a lower substrate 370 having electrically conductive pads 375 and containing electrical components (not shown) by solder bumps 355. This arrangement allows for self alignment of the three components prior to the anneal which melts the solder bumps, electrically wires substrates 100, 360 and 370 together and completes the fabrication process. Examples of electrical components include but are not limited to, transistors, diodes, resistors, capacitors, inductors and wires.
While pad to solder bump connections are illustrated in FIG. 2J, other connection types such as pad to pad may be used (i) between substrate 360 and substrate 100, (ii) between substrate 100 and substrate 370 or (iii) between substrate 360 and substrate 100 and between substrate 100 and substrate 370 substrate. While solder bumps are shown on substrate 360 and pads on substrate 100, pads may be formed on substrate 360 and solder bumps on substrate 370. While the solder bumps are shown on substrate 100 and pads on substrate 370, pads may be formed on substrate 100 and solder bumps on substrate 370. Substrate 360 may be replaced with wire or tab bonds. If the solder bumps and pads of substrate 100 are swapped, then substrate 370 may be replaced with wire or tab bonds.
FIGS. 3A through 3D are exemplary plan views of through wafer vias according to embodiments of the present inventions. In FIG. 3A, a through wafer via array 330A consists of a single electrically conductive through wafer via 125A consisting of electrical conductor 275 and a single electrically non-conductive through wafer via 130A consisting of silicon dioxide layer 155 surrounding a core of silicon layer 170.
In FIG. 3B, a through wafer via array 330B consists of two electrically non-conductive through wafer vias 130A on opposite sides of a single electrically conductive through wafer via 125A.
In FIG. 3C, a through wafer via array 330C comprises four electrically non conductive through wafer vias 130A opposite each of the four sides of a single electrically conductive through wafer via 125A.
In FIG. 3D, through wafer via array 330C consists of seven electrically non-conductive through wafer vias 130A and two electrically conductive through wafer via 125A. Three of the electrically non-conductive through wafer vias 130A are positioned between the two electrically conductive through wafer vias 125A. Four of the electrically non-conductive through wafer vias 130A are positioned opposite each of the four sides formed by the combination the first three electrically non-conductive through wafer vias 130A and the two electrically conductive through wafer vias 125A.
In each of FIGS. 3A, 3B, 3C and 3D every through wafer via 125A and 130A is surrounded by a region of substrate 100. Through wafer vias 330B, 330C and 330D function as coplanar waveguides.
Through wafers vias of the present embodiment of the present invention includes at least one electrically conductive element extending from the top surface of the substrate, through the substrate, to the bottom surface of the substrate and at least one non-electrically conductive (i.e., dielectric or insulator) element also extending from the top surface of the substrate, through the substrate, to the bottom surface of the substrate.
It should be understood that very many other through wafer via arrays having different numbers and configurations of through wafers vias 125A and 130A are possible and are not limited to those illustrated in FIGS. 3A, 3B, 3C and 3D.
FIGS. 4A through 4D are schematic plan views of waveguide models using through wafer vias according to embodiments of the present invention. In FIGS. 4A, 4B, 4C and 4D, G indicates an electrical conductor filled trench that is not insulated from the substrate and that is connected to ground, S indicates an electrical conductor filled trench that is not insulated from the substrate and is connected to a signal source, I indicates an electrical insulator filled trench, IG indicates an electrical conductor filled trench connected to ground and that is insulated from the substrate and IS indicates an electrical conductor filled trench connected to a signal source and that is insulated from the substrate. The space between the G, S, I, IG and IS structures is substrate.
The structures of FIGS. 4A, 4B, 4C and 4D were modeled as signal waveguides for characteristic impedance, propagation loss and effective dielectric constant (Er). Low propagation loss and small effective dielectric constant are preferred. The model was based on a silicon substrate with a relative dielectric constant of 11.9 and a conductivity of 7.41 Siemens/meter; tungsten with a conductivity of 1.82E7 Siemens/meter for the electrical conductor for the G, IG, S and IS structures; and silicon dioxide with a relative dielectric constant of 4.1 for the insulator of the I, IG and IS structures.
The dimensions of the G and S structures in top view was 50 by 3 microns. The dimensions of the IG and IS structures in top view was 52 by 5 microns (the G and S structures with a surrounding insulator 1 micron thick). The dimensions of the I structures in top view was 52 by 5 microns. Simulation was performed on an Ansoft HFSS-3D full wave EM simulator. Table I gives the result of the simulation for each of the structures of FIGS. 4A, 4B 4C and 4D.











TABLE I







Propagation





Characteristic
Loss
Propagation Loss
Effective


CASE
Impedance
(dB/mm)
% of Case 1
Er



















FIG. 4A
22.61 + j0.96
1.329
100
12.136


FIG. 4B
24.08 + j071 
1.062
79.9
10.722


FIG. 4C
27.07 + j0.37
0.777
58.5
8.4657


FIG. 4D
28.42 + j0.23
0.635
47.8
7.7056









The following conclusions can be reached as a result of the simulation. For coplanar waveguides, insulated through wafer vias have a higher characteristic impedance, less propagation loss, lower effective dielectric constant and is better for signal propagation with less potential un-wanted coupling. The reasons are, silicon is lossy, but silicon dioxide is not. The higher dielectric constant of silicon causes higher parasitic capacitance relative to parasitic capacitance of the lower dielectric constant silicon dioxide.
Thus, a through wafer via that contacts the substrate directly (G structure) as in the embodiments of the present invention, may be used for the ground structure as long as the voltage across the through wafer via is low enough for little or no current is conducted through the substrate. For the signal structure, an insulated conductor (IS structure) such as in the second and third embodiments of the present invention is preferred to reduce signal conduction through the substrate.
FIG. 5 is cross-sectional drawing illustrating an alternative through wafer via structure according embodiments of the present invention. When polysilicon is deposited into a deep trench, there is a tendency for a void to be formed where the polysilicon deposited on opposite sidewalls meet in what is called the seam. The void forms because of pinch-off at the top of the trench. This can result in a slightly different structure than that shown in FIG. 1M. FIG. 5 is similar to FIG. 1M except dielectric layer 185 extends into a void 180 in polysilicon layer 170.
FIG. 6A illustrates the phenomena of wide trenches (130B) etching deeper than narrow trenches (125). FIG. 6A is a cross-sectional drawing illustrating an alternative structure to that illustrated in FIG. 1B. In FIG. 5, trench 130B extend a distance D3 into substrate 100 from top surface 105, while trenches 125 extend into substrate 100 from top surface 105 the distance D2 as described supra with respect to FIG. 1B. D3 is greater than D2, while D2 is equal to D1 (see FIG. 1B).
FIG. 6B illustrates a method of mitigating the effect of wide trenches etching deeper than narrow trenches. In FIG. 6B, a buried oxide layer 102 has been formed a distance D2 into substrate 100. Trenches 125 and 130B will stop etching vertically when buried oxide layer 102 is reached. In one example substrate 100 is a silicon-on-insulator substrate and buried oxide layer 102 is silicon oxide. Dielectric layer 102 needs to be thick enough to act as a RIE stop layer for etching trenches 125 and 130B but not too thick to avoid adding process complexity related to it's subsequent exposure and/or removal. In one example, dielectric layer 102 is 1 micron thick. In another example, dielectric layer 102 is 0.1 to 10 microns thick. The through wafer via 125 and 130B depth is determined by the integration requirements and, for example, can vary from 5 to 500 microns and typically. In one example, through wafer via 125 and 130B depth is 100 microns and width is 3 microns.
FIGS. 7A through 7D illustrate an alternative method of fabricating through vias according to embodiments of the present invention. FIG. 7A is similar to 1H except trenches 125 and 130 were the same width so after the processing described supra with respect to FIGS. 1A through 1G are preformed, the fill of trenches 125 is the same as for trench 130. FIG. 7B is similar to FIG. 1J and FIG. 7C is similar to FIG. 1K except plug 1285, polysilicon 170 and liner 155 are removed in trenches 125. FIG. 7D is similar to FIG. 1M after the steps described in reference to FIGS. 1L and 1M have been performed.
Thus, the embodiments of the present invention provide a structure and method to integrate through wafer vias into existing integrated circuit fabrication processes having in good propagation of signals from/to the front surface of the integrated circuit chip to/from the bottom surface of the integrated circuit chip.
The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.

## Claims

1. A method, comprising:
(a) forming a first trench and a second trench in a semiconductor substrate, said first and second trenches extending in a first direction from a top surface of said substrate toward an opposite bottom surface of said substrate a distance less than a thickness of said substrate in said first direction;
after (a), (b) simultaneously completely filling said first trench with a dielectric material and forming a liner of said dielectric material on sidewalls of said second trench;
after (b), (c) filling remaining space in said second trench with a polysilicon core, recessing said polysilicon core below said top surface of said substrate, and forming, in said second trench, a dielectric plug on said polysilicon core;
after (c), (d) removing said dielectric material from said first trench and filling said first trench with and electrically conductive material; and
after (d), (e) thinning said substrate from said bottom surface of said substrate to form a new bottom surface of said substrate, said electrically conductive material of said first trench and said liner and polysilicon core of said second trench exposed in said new bottom surface of substrate.

2. The method of claim 1, wherein (b) includes:
forming a boron doped polysilicon layer on sidewalls of said first trench and on said sidewalls of said second trench; and
oxidizing said polysilicon layer to form said dielectric material in said first trench and said liner of said dielectric material on said sidewalls of said second trench.

3. The method of claim 2, wherein said substrate includes a buried oxide layer and in (a) said first and second trenches extend to said buried oxide layer, a surface of said buried oxide layer exposed in bottoms of said first and second trenches.

4. The method of claim 1, wherein (e) includes:
grinding said bottom surface of said substrate to form a ground surface of said substrate;
chemically etching said ground surface of said substrate to expose said electrically conductive material of said first trench;
chemical-mechanical-polishing said conductive material of said first trench and said liner
and polysilicon core of said second trench.

5. The method of claim 1, wherein (b) includes:
performing a high pressure oxidation to form said dielectric material in said first trench and said liner of said dielectric material on said sidewalls of said second trench.

6. The method of claim 5, further including:
forming an insulating layer on said new bottom surface of said substrate, said electrically conductive material of said first trench extending through said insulating layer and said dielectric liner and polysilicon core of said second trench extending through said insulating layer; and
forming an electrically conductive backside pad in physical and electrical contact with said electrically conductive material of said first trench, said dielectric liner of said second trench and said polysilicon core of said second trench, said backside pad formed on said insulating layer.

7. The method of claim 1, wherein said electrically conductive material comprises tungsten.

8. The method of claim 1, wherein said electrically conductive material comprises copper.

9. The method of claim 1, wherein said electrically conductive material comprises (i) tungsten in combination with titanium or titanium nitride or (ii) tungsten in combination with titanium and titanium nitride.

10. The method of claim 1, wherein said electrically conductive material comprises copper in combination with one or more of titanium, titanium nitride and ruthenium.

11. The method of claim 1, wherein said electrically conductive material comprises copper in combination with one or more of tantalum, tantalum nitride and ruthenium.

12. The method of claim 1, further including:
before (a), forming a dielectric layer on a top surface of said substrate; and
wherein said first trench and said second trench extend in said first direction from a top surface of said dielectric layer through said dielectric layer into said substrate said distance less than said thickness of said substrate in said first direction.

13. The method of claim 12, wherein (c) includes: after forming said dielectric plug, removing said dielectric layer and forming a replacement dielectric layer on said top surface of said substrate and on top surfaces of said dielectric material in said first and second trenches and said dielectric plug.

14. The method of claim 1, wherein after (d) said top surface of said substrate and top surfaces of said dielectric material in said first and second trenches and said dielectric plug are coplanar.

15. The method of claim 1, further including:
between (d) and (e), forming shallow trench isolation in said substrate.

16. The method of claim 1, further including:
between (d) and (e), forming a source, a drain and a channel of a field effect transistor in said substrate and a gate dielectric and gate electrode on said channel.

17. The method of claim 1, further including:
between (d) and (e) forming a trench capacitor in said substrate.

18. The method of claim 1, wherein after (d), (i) a top surface of said electrically conductive material extends above a top surface of said substrate and (ii) said top surface of said substrate and top surfaces of said dielectric material and said dielectric plug in said second trench are coplanar.

19. The method of claim of claim 1, wherein said dielectric plug comprises a first region in physical contact with said dielectric material in said second trench and a second region separated from said dielectric material by regions of said polysilicon core.

20. The method of claim 1, wherein:
(a) includes forming a third trench on an opposite side of said second trench from said first trench;
(b) includes completely filling said third trench with said dielectric material; and
(d) includes removing said dielectric material from said third trench and filling said third trench with said electrically conductive material.