Abstract:
A potassium hydroxide (KOH) etch process can produce deep high aspect ratio trenches in (110) oriented silicon substrates. The trenches, however, are perpendicular to the (111) direction of the silicon substrate&#39;s crystal lattice. The trenches are used to produce thermally isolating areas and through the wafer electrical connections. These structures can be produced in a cost effective manner because of the nearly ideal capabilities of the KOH etch process when it is applied to appropriate materials at appropriate orientations.

Description:
TECHNICAL FIELD 
     Embodiments relate to the field of semiconductor processing. Embodiments also relate to using potassium hydroxide (KOH) etch processing on (110) oriented silicon to create deep high aspect ratio trenches. The deep high aspect ratio trenches can then be used in the production of other structures that are otherwise difficult to create. 
     BACKGROUND 
     Many devices can be produced via semiconductor processing. Semiconductor processing, like any other type of manufacturing, involves moving material through a series of transformative steps until a finished product is produced. Different steps have different complexity and incur different costs. A manufacturer that can produce a product with fewer steps, simpler steps, or less costly steps can often produce the product more inexpensively. 
     Most semiconductor processing is performed using silicon wafers as a substrate. Some small wafers have a 50 millimeter (mm) diameter and are usually thicker than 200 micrometers (microns). The most common wafers have diameters of 150 mm, 200 mm or 300 mm and are usually thicker than 400 microns. 
     Silicon wafers are produced from a single crystal. A crystal is a solid material in which the atoms occur in a repeating pattern. The repeating pattern can be used to define a crystal&#39;s main crystallographic axes. A flat surface on a crystal is called a face. The relationship between a crystal face and the main crystallographic axes can be expressed in terms of a Miller Index. An example of a Miller Index is (110). One skilled in crystallography can, given knowledge of a crystal and a face&#39;s Miller Index, determine exactly how the face intersects the repeating pattern of atoms that forms the crystal. Similarly, a crystal direction refers to a specific direction along the repeating atomic pattern. For example, a (111) direction refers to a specific direction along the main crystallographic axes. 
     Some semiconductor devices, such as through the wafer (TTW) flow sensors on silicon substrates, have structures requiring costly manufacturing steps. One such structure is a thermal isolator. A thermal isolator can be produced on a silicon substrate by etching a wide deep hole in the substrate and then filling the hole with a thermal insulator. A thermal insulator is a material that has low thermal conductivity. A wide deep hole can be produced fairly inexpensively, but requires an expensive process to fill. For example, achieving good thermal isolation using silicon dioxide, also known as oxide, as the insulating material requires an oxide thickness exceeding 30 microns and a width exceeding 100 microns. Depositing over 30 microns of oxide, however, is an expensive process step. 
     Another costly structure is a TTW electrical connection. The connection must go completely through the wafer which can be 750 microns thick. Furthermore, the TTW electrical connection must be large enough to carry the amount of electrical current that is required of it. A thick wire can carry more electrical current than a thin wire of the same material. Similarly, a large TTW electrical connection can carry more electrical current than a small one. 
     Deep reactive ion etching is capable of producing a hole from one side of a wafer to the other large enough for a TTW electrical connection, but is expensive. After the hole is produced, its sidewalls can be oxidized and then it can be filled with an electrically conductive material to produce the TTW electrical connection. Filling the hole with electrically conductive material is also expensive. Current technology requires two expensive steps for producing a TTW electrical connection. 
     A KOH etch process is capable of producing deep high aspect ratio trenches in (110) oriented silicon. Deep high aspect ratio means that the trench is much deeper than it is wide. The process, however, only produces high sect ratio trenches perpendicular to the (111) direction of the silicon substrate. As such, the KOH etch process has experienced limited use. 
     Aspects of the embodiments directly address the shortcoming of current technology by exploiting a property of KOH etching of (110) oriented silicon substrates to inexpensively produce deep high aspect ratio trenches. These trenches are then used for inexpensively producing TTW electrical connections and thermal isolators. 
     BRIEF SUMMARY 
     It is therefore an aspect of the embodiments to select a silicon substrate with a (110) orientation. As discussed above, (110) orientation refers to a specific relationship between the pattern of atoms in the silicon crystal and the face of the substrate. 
     It is another aspect of the embodiments to use a potassium hydroxide (KOH) etch process to create an array of trenches in the silicon substrate that are perpendicular to the (111) direction of the silicon substrate. The array of trenches has at least two trenches and the trenches themselves are 5 microns wide or less. The trenches are separated by a rib that is five microns wide or less. 
     It is also another aspect of the embodiments to oxidize the ribs. The ribs are silicon at first because they are the same material as the silicon substrate. Oxidation is the process of exposing a material to oxygen. In semiconductor processing, silicon is often exposed to oxygen under controlled conditions. Controlled conditions means that the temperature, pressure, amount of oxygen, and other environmental conditions are kept within a narrow range. The oxidation process transforms silicon to silicon dioxide, or oxide. 
     It is a further aspect of the embodiments to fill the trenches with a thermally insulating material. A thermally insulating material has low thermal conductance. For example, oxide has low thermal conductance. Therefore, the trenches can be filled with oxide. A thermal isolator is formed by oxidizing the ribs and filling the trenches with a thermal insulator such as oxide. 
     It is an additional aspect of the embodiments to use a KOH etch process to etch a slot in the silicon substrate. A slot is different from a trench because a slot goes completely through the wafer while a trench does not. The slot is ten microns wide or less and is perpendicular to the (111) direction. 
     It is a yet further aspect of an embodiment to oxidize the sides of the slot. At first, the sides of the slot are silicon because the substrate is silicon. As discussed above, oxidation converts the sidewalls to oxide. 
     It is a still yet further aspect of an embodiment to fill the slot with electrically conductive material. For example, doped polysilicon is an electrically conductive material and the slot can be filled with it. Another possibility is to deposit polysilicon on the slot sidewall and then use the polysilicon as a seed layer for depositing nickel or tungsten. In this manner, the bulk of the material filling the slot is a metallic conductor. A TTW electrical connection is formed by filling the slot with conductive material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
         FIG. 1  illustrates a silicon substrate in accordance with an aspect of an embodiment; 
         FIG. 2  illustrates a silicon substrate with patterned photoresist and nitride/oxide in accordance with an aspect of an embodiment; 
         FIG. 3  illustrates a silicon substrate with patterned nitride/oxide and a trench array in accordance with an aspect of an embodiment; 
         FIG. 4  illustrates a top view of a silicon substrate with patterned nitride/oxide and a trench array in accordance with an aspect of an embodiment; 
         FIG. 5  illustrates a silicon substrate with patterned nitride/oxide and a trench array after oxidation in accordance with an aspect of an embodiment; 
         FIG. 6  illustrates a silicon substrate and a trench array after oxidation and filling in accordance with an aspect of an embodiment; 
         FIG. 7  illustrates a silicon substrate and a trench array after oxidation and filling with oxide in accordance with an aspect of an embodiment; 
         FIG. 8  illustrates a silicon. substrate with patterned nitride/oxide in accordance with an aspect of an embodiment; 
         FIG. 9  illustrates a silicon substrate with patterned nitride/oxide and two trenches in accordance with an aspect of an embodiment; 
         FIG. 10  illustrates a silicon substrate with patterned nitride/oxide and a slot in accordance with an aspect of an embodiment; 
         FIG. 11  illustrates a silicon substrate with patterned nitride/oxide and a slot with oxidized sidewalls in accordance with an aspect of an embodiment; 
         FIG. 12  illustrates a filled slot in a silicon substrate having oxidized sidewalls in accordance with an aspect of an embodiment; 
         FIG. 13  illustrates TTW electrical connections in a silicon substrate in accordance with an aspect of an embodiment; 
         FIG. 14  illustrates a group of slots in a silicon substrate having oxidized sidewalls after filling and deposition of a pad in accordance with an aspect of an embodiment; and 
         FIG. 15  illustrates a filled slot in a silicon substrate having oxidized sidewalls coated with one material and then filled in accordance with an aspect of an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
       FIG. 1  illustrates a silicon substrate  101  in accordance with an aspect of an embodiment. As previously discussed, the silicon substrate  101  has a (110) orientation. 
       FIG. 2  illustrates a silicon substrate  101  with patterned photoresist  202  in accordance with an aspect of an embodiment. The deposition and patterning of photoresist layers is a standard part of semiconductor processing. The pattern in the patterned photoresist  202  and nitride/oxide  201  is a series of trenches with a line of photoresist between each pair of trenches. The trench widths are five microns or less. The line widths are five microns or less. Five micron trenches and five micron lines give a trench to trench, or line to line, spacing of ten microns. Similarly, a three micron trench and a four micron line give a trench to trench, or line to line, spacing of seven microns. The trenches and lines are perpendicular to the (111) direction of the silicon substrate. 
       FIG. 3  illustrates a silicon substrate  101  with patterned nitride/oxide  201  and a trench array  302  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 3  can be produced from that of  FIG. 2  in a number of different ways. One of those ways is potassium hydroxide (KOH) etching. As described above, KOH etching is advantageous because of orientation of the trench  301  and the crystal lattice of the silicon substrate  101 . Deep high aspect ration trenches  302  are produced. Here, deep means thirty microns or more. Etching the trenches leaves ribs  303  of material separating the trenches. The aspect ratio is the ratio of trench depth to trench width. Recall that the trench  301  is at most five microns wide and at least thirty microns deep. As such, the aspect ratio is at least six. 
       FIG. 4  illustrates a top view of a silicon substrate (not shown) with patterned nitride/oxide  201  and a trench array  302  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 4  is a top view of the structure illustrated in  FIG. 3 . The structures illustrated in  FIGS. 1-3  and  FIGS. 5-15  are side views. 
       FIG. 5  illustrates a silicon substrate  101  with patterned nitride/oxide  201  and a trench array  502  after oxidation in accordance with an aspect of an embodiment. As discussed above, oxidation is process wherein the structure is exposed to oxygen under controlled conditions. Exposing silicon to oxygen produces oxide  501 . In  FIG. 3 , the ribs  303  are silicon because they are the same material as the substrate  101 . In  FIG. 5 , oxidation has caused the ribs  503  to become oxide. The process of oxidation converts the surface of a silicon structure to an oxide layer. The depth of the layer varies based on the conditions under which oxidation occurs. The initial conversion, to a depth of less than a micron, is fast. As the oxide layer thickens, however, oxidation slows. Once the oxide layer reaches a thickness of 2.5 microns, the oxidation process has slowed considerably. Further oxidation can be too slow to be worthwhile. Recalling that the ribs are at most 5 microns wide and that oxidation proceeds from the surface inwards. Each side of each rib  503  is oxidized until the entire rib  503  is oxidized. 
       FIG. 6  illustrates a silicon substrate  101  and a trench array  602  after oxidation and filling in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 6  can be produce from that illustrated in  FIG. 5  by filling the trenches  602  with a thermal insulator  601  and stripping the photoresist. The area of the trench array  602  has a series of strips of oxide  501  where the trench walls once were and thermal insulator  601  where the trenches once were. Oxide is also a thermal insulator. As such, the trench array  602  has become a wide thermally isolating area. 
     The steps outlined and discussed in relation to  FIGS. 1-6  are not intended to be a recipe for creating a wide thermally isolating area. Rather, some steps in creating a wide thermally isolating area are described. One practiced in the art of semiconductor processing can, on contemplation of the foregoing discussion, incorporate these steps, or similar steps, into new and existing processes and create wide thermally isolating areas thereby. 
       FIG. 7  illustrates a silicon substrate  101  and a trench array after oxidation and filling with oxide in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 7  can be produced from that illustrated in  FIG. 5  by filling the trenches with oxide and stripping the resist. As the ribs were already oxide, a wide oxide area  701  is created. The wide oxide area  701  is also a wide thermally isolating area because oxide is a thermal insulator. 
       FIG. 8  illustrates a silicon substrate  101  with patterned nitride/oxide layers  801  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 8  can be produced from that of  FIG. 1  by coating both sides of the substrate with nitride/oxide and patterning via standard semiconductor processing techniques. The patterned nitride/oxide layers  801  each have a trench  802  that is no more than ten microns wide. Each trench  802  in the patterned nitride/oxide  801  is oriented such that an etch process would produce a trench perpendicular to the (111) direction of the (110) oriented silicon substrate  101 . 
       FIG. 9  illustrates a silicon substrate  101  with patterned nitride/oxide  801  and two trenches  901  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 9  can be produced from that of  FIG. 8  by exposing both side of the wafer to an etch process. A KOH etch process is ideal for this operation because it is a wet etch process and thereby easier than other etch processes to perform on both sides simultaneously. The KOH etch process is also ideal because, given the orientation of the substrate  101  and trenches  901 , it can produce extremely deep high aspect ratio trenches  901 . 
       FIG. 10  illustrates a silicon substrate  101  with patterned nitride/oxide  801  and a slot  1001  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 10  can be produced from that of  FIG. 9  by continuing the etch process until the two trenches  901  meet within the substrate  101 . 
       FIG. 11  illustrates a silicon substrate  101  with patterned nitride/oxide  801  and a slot  1001  with oxidized sidewalls  1101  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 11  can be produced from that illustrated in  FIG. 10  by exposure to an oxidizing environment. Oxidation processes are discussed above. As is normal for oxidation, the oxide layer  1101  is typically 2.5 microns thick or less. 
       FIG. 12  illustrates a filled slot  1201  in a silicon substrate  101  having oxidized sidewalls  1101  in accordance with an aspect of an embodiment. The structure illustrated in  FIG. 12  can be produced from that illustrated in  FIG. 11  by filling the slot  1001  with an electrical conductor. The filled slot  1201  forms a through-the-wafer (TTW) electrical connection  1202  that is electrically isolated from the silicon substrate  101  by the oxidized sidewalls  1101 . The amount of electrical current that can be carried through the connection  1202  is determined by the material properties of the electrical conductor and the area of the slot. The area of the slot is the width times the length. For example, a five micron wide and 100 micron long slot has an area of 500 square microns. The current capacity of the TTW connection  1202  can be doubled by making the slot  1201  twice as long. 
     The steps outlined and discussed in relation to  FIGS. 8-12  are not intended to be a recipe for creating a wide thermally isolating area. Rather, some steps in creating a wide thermally isolating area are described. One practiced in the art of semiconductor processing can, on contemplation of the foregoing discussion, incorporate these steps, or similar steps, into new and existing processes and create wide thermally isolating areas thereby. 
       FIG. 13  illustrates TTW electrical connections  1202  in a silicon substrate  101  in accordance with an aspect of an embodiment. As discussed above, the current capacity of a TTW connection  1202  can be increased by increasing the slot length. Another way to double current capacity is to produce and use two TTW connections  1202 .  FIG. 13  illustrates a grouped TTW connection  1301  wherein many individual TTW connections  1202  are grouped together to obtain higher electrical current capacity. The same process steps as used for producing a single TTW connection  1202  can be used to produce a grouped TTW connection  1301 . 
       FIG. 14  illustrates a grouped TTW connection  1301  in a silicon substrate  101  and a pad  1401  in accordance with an aspect of an embodiment. The structure of  FIG. 14  can be produced from that of  FIG. 13  by producing a pad  1401  that overlays the grouped TTW connection  1301 . The pad  1401  can be produced using any of a variety of common semiconductor processing techniques, such as masking followed by deposition and stripping. One skilled in the art of semiconductor processing can easily create variations of existing process steps for creation of the pad  1401  and all such variations should be considered equivalent. The pad  1401  material is electrically conducting such that an electric current can easily pass into the pad and then through the grouped TTW connection  1301 . The purpose of the pad  1401  is to allow the grouped TTW connection  1301  to be treated as a single large connection. 
       FIG. 15  illustrates a filled slot  1201  in a silicon substrate  101  having oxidized sidewalls  1101  coated with a first material  1501  and then filled in accordance with an aspect of an embodiment. The production of a slot with oxidized sidewalls is discussed above. The structure of  FIG. 15  can be produced from that of  FIG. 11  by coating the oxidized sidewalls  1101  with a first material, such as doped polysilicon, and then filling the slot with a second material. The reason for using the first material as a coating is that it can act as a seed layer for the subsequent deposition of the second material. For example, a polysilicon layer  1501  can act as a seed layer for a metallic conductor  1201  that fills the slot. Tungsten, nickel, aluminum, copper, and gold are examples of metallic conductors. It is also possible for the second material and the first material to be the same material. For example, a polysilicon layer can be deposited on the sidewalls to act as a seed layer for subsequently filling the slot with polysilicon. 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.