This invention relates to prismatic silicon and a method of producing the same.
Crystalline anisotropic etching is one method of fabricating a silicon wafer. This is a method of etching a wafer into a specified shape by making use of the property of certain etching solutions, such as KOH, allowing the etching reaction to proceed in a certain crystalline direction but significantly less in certain other directions.
A silicon wafer is obtained by slicing a silicon crystal ingot into a planar form but its single crystals are of a diamond structure. The crystal surfaces of single crystal silicon include (100), (110) and (111) surfaces and various researches have been carried out according to these various crystal surfaces.
Most of the researches on anisotropic etching of silicon have been regarding the (100) substrates. If anisotropic etching is done on a silicon (100) substrate by appropriately using a resist or the like for patterning, for example, pyramid-shaped grooves are formed with (111) surfaces as their side surfaces. Examples of the etching technologies on the (100) surface include DNA nano-tweezers, sockets for micro-connectors and sensor devices, making use of the property that a triangular shape can be thereby formed. Indeed, most of the researches on anisotropic etching of silicon have been on the (100) substrates.
It is known that perpendicular grooves having the (111) surfaces as the side surfaces are formed if anisotropic etching is carried out on the (110) surface, which is different from the (100) surface. Since a shape with perpendicular walls can be formed, there have been reports on researches on the production of half mirrors, etalons and micro-minors by using such perpendicular wall surfaces (such as Japanese Patent Publication Tokkai 8-90431; Y. Uenishi, M. Tsugai and M. Mehegny, “Micro-opto mechanical devices fabricated by anisotropic etching of (100) silicon”, Proc. IEEE Micro Electro Mechanical Systems Workshop, Oiso, Japan 319-324 (1994); and Y. Uenishi, M. Tsugai and M. Mehegny, “Micro-opto mechanical devices fabricated by anisotropic etching of (100) silicon”, J. Micromech. Microeng., Vol. 5, 305-312 (1995)).
As described above, most of the conventional silicon fabrication technologies using crystalline anisotropic etching related to objects making use of a pyramid-like shape or having grooves with perpendicular walls or perpendicular wall surfaces. By contrast, there has been no technology developed for fabricating silicon in a prismatic shape and no such method of fabrication has been known.
In view of the situation as described above and with the idea that silicon in a prismatic shape would be useful in hitherto unconsidered ways in various industrial areas such as fabrication of mother dies for filters and high-density micro-electrodes, the present inventors have accomplished the present invention by diligently conducting researches on the technology of fabricating silicon in a prismatic shape having a high aspect ratio.
On the basis of conventional methods of fabricating the (110) surface of silicon, it may be considered possible to obtain silicon in a prismatic shape by patterning a silicon (110) substrate with squares of a resist and carrying out an anisotropic etching process, but it is in reality not so. This is because when the (111) surfaces cross each other, the bottom point where they cross at valleys can function as an etch stop but the top of hills cannot function as such. Accordingly, a new theory for the fabrication process other than by simple patterning has been considered necessary.
In view of the above, it is an object of this invention to provide a method of producing prismatic silicon based on a completely new technical principle, and it is another object of this invention to provide prismatic silicon with a high aspect ratio.
A method (hereinafter also referred to as the first production method) according to this invention is characterized as using a silicon wafer with (110) surface for producing silicon in a prismatic shape and as comprising sequentially carrying out an alignment configuration forming step for forming alignment configurations having surfaces that are along two (111) surfaces perpendicular to a substrate surface inside the silicon wafer, a primary anisotropic etching step for forming perpendicular walls having wall surfaces aligned to one of these (111) surfaces, and a secondary anisotropic etching step for forming silicon in the prismatic shape having wall surfaces aligned to the other of these (111) surfaces with respect to the perpendicular walls.
By a production method as described above, silicon prisms having (110) surface as the top surface and (111) surfaces as the side surfaces can be obtained because a silicon wafer with (110) surface is employed and the etching processes are carried out such that two internal (111) surfaces perpendicular to the substrate surface will come to be exposed. It is also made possible by such a production method to dig out perpendicular (111) surfaces accurately and hence to obtain silicon prisms with side walls that are accurately perpendicular to the top surface since the etching processes include both the primary anisotropic etching which is carried out after alignment configurations are formed with two perpendicular (111) by aligning according to one of the (111) surfaces and the secondary anisotropic etching according to the other of the (111) surfaces.
Another method (hereinafter also referred to as the second production method) according to this invention is characterized as using a silicon wafer with (110) surface for producing silicon in a prismatic shape and as comprising sequentially carrying out an alignment configuration forming step for forming alignment configurations having surfaces that are along two (111) surfaces perpendicular to a substrate surface inside the silicon wafer, a primary anisotropic etching step for forming perpendicular walls by carrying out an anisotropic etching process on the silicon wafer with resist patterning aligned to one of these (111) surfaces such that this one (111) surface becomes a perpendicular wall surface, a protective film forming step for forming a protective film on surfaces of the silicon wafer inclusive of the wall surface of the perpendicular walls, and a secondary anisotropic etching step for forming silicon in the prismatic shape by carrying out an anisotropic etching process on the silicon wafer with resist patterning aligned to the other of these (111) surfaces such that portions of the perpendicular walls become the other (111) surface and a perpendicular wall surface.
This production method, including the alignment configuration forming step, the primary anisotropic etching step and the secondary anisotropic etching step, has all the advantages of the first production method. By this production method as descried above, furthermore, the protective film can be attached to the side walls of the perpendicular walls formed by the primary isotropic etching and the side walls can be prevented from being abraded at the time of the secondary isotropic etching since the protective film forming step is inserted between the primary isotropic etching and the secondary isotropic etching. As a result, only the surface crossing the side walls having the protective film is subjected to the etching process at the secondary isotropic etching and the prisms are completed when the etch stop is effected at the (111) surface of the side wall. Thus, it becomes possible to obtain silicon prisms having four accurately perpendicular (111) surfaces as side surfaces and theoretically having a high aspect ratio.
A third method according to this invention is characterized as using a silicon wafer with (110) surface for producing silicon in a prismatic shape and as comprising sequentially carrying out an alignment configuration forming step for forming alignment configurations having surfaces that are along two (111) surfaces perpendicular to a substrate surface inside the silicon wafer, a pattern forming step for forming a first resist pattern along one of these (111) surfaces and a second resist pattern on the first resist pattern along the other of said (111) surfaces, a primary anisotropic etching step for forming perpendicular wall surfaces to the first resist pattern by digging on the silicon wafer between mutually adjacent ones of the first resist pattern by anisotropic etching, a protective film forming step for forming a protective film entirely over the silicon wafer inclusive of the wall surfaces of the perpendicular walls, a cutting step for cutting a silicon surface underneath the second resist pattern on the perpendicular walls, and a secondary anisotropic etching step for forming silicon in the prismatic shape having a portion with the first resist pattern as top surface by digging on the silicon wafer by crystalline anisotropic etching.
Silicon in a prismatic shape (or a silicon prism) according to this invention is characterized as having a (110) surface as a top surface and four side surfaces that are (111) surfaces and perpendicular to this top surface. Such a silicon prism may be further characterized as being produced by any of the production methods of this invention as characterized above.
This production method, including the alignment configuration forming step, the primary anisotropic etching step and the secondary anisotropic etching step, has all the advantages of the first production method. By this production method as descried above, furthermore, no particular method is necessary for the coating of resist or for the exposure to light because the protective film forming step can be carried out prior to the formation of the perpendicular walls while the silicon surface is still flat. Moreover, the protective film can be attached to the side walls of the perpendicular walls formed by the primary anisotropic etching step since it is formed between the primary and secondary anisotropic etching steps. The side walls can thus be prevented from being abraded at the time of the secondary anisotropic etching. As a result, only the portions not having the protective film are etched in the secondary anisotropic etching step and the prismatic shapes are formed at the moment of the etch stop at the (111) surface of these side walls. Thus, silicon prisms having four accurately perpendicular (111) surfaces as side walls can be obtained.
Silicon in a prismatic shape (or silicon prism) of this invention is characterized as having a (110) surface as a top surface and four side surfaces that are (111) surfaces and perpendicular to said top surface. It has a prism-shape with no burrs or chipping. Thus, prisms with a high aspect ratio are possible and can be useful in many applications. Such silicon prisms of this invention can be produced by any of the production methods of this invention and can have a high aspect ratio and hence are usable in many applications requiring such a high aspect ratio.
Embodiments of this invention are explained next with reference to the drawings.
This invention is based on the technical principle of using the (111) surfaces inside a (110) silicon wafer perpendicular to the substrate surface for forming a prismatic shape with a high aspect ratio having side walls perpendicular to the top (110) surface. The position of the (111) surface of a (110) silicon wafer is known to be as shown in
The silicon wafer to be used according to this invention is a wafer sliced from an ingot such that its surface will be a (110) surface.
The invention includes the following two methods, which are herein referred to as the first method and the second method.
The first method of this invention is explained next. This method includes the following twelve steps (Steps 1-12), Steps 1, 2 and 3 being alignment configuration forming steps, Steps 4, 5 and 6 being primary anisotropic etching steps, Steps 7 and 8 being protective film forming steps and Steps 11 and 12 being secondary anisotropic etching steps.
Step 1 is for forming a protective film. As shown in
Step 1 is not only for forming alignment configurations which is carried out in Step 2 to be described below but also for preventing the perpendicular walls from melting at the time of crystalline anisotropic etching in Step 6.
For the formation of the film in this step, Si3N4 and SiO2 may be used together with any known CVD method.
Si3N4 is considered optimal for the formation of alignment configurations and perpendicular walls. Representative examples of etching liquid for use in crystalline anisotropic etching include KOH and TMAH (tetramethyl ammonium hydroxide, or (CH3)4NOH). Either of these solutions may be used for anisotropic etching but a choice between Si3N4 and SiO2 must be made as the mask material, depending on which of these solutions is used. Si3N4 is extremely hard to dissolve in KOH and TMAH and hence will not be depleted during an etching process, making it easy to control the film thickness by time management. If KOH is used, on the other hand, SiO2 has a large etching speed and hence the time management is difficult for controlling the film thickness but may be used as a mask material if a film with a sufficient thickness is formed.
Step 2 is for preparing the formation of alignment configurations.
As shown in
The positions and number of the alignment holes 3 are arbitrary, provided that they are located so as to be visible by a microscope at the time of mask alignment. If a mask aligner allowing the entire surface of the wafer 1 to be visible is used, these holes 3 may be located at any places. Their number is totally arbitrary. Although
Step 3 is for forming alignment configurations. As shown in
As shown also in
Step 4 is for a resist patterning according to one of the perpendicular (111) surfaces.
As shown in
A known device called “mask aligner” for matching the positions of a wafer and a mask may be used for aligning the resist patterning to the (111) surface of an alignment configuration 4. Since the resist patterning is very small, the positioning process with respect to the alignment configurations 4 must be carried out with a microscope being used for observation by moving the wafer in the x- and y-directions, as well as in the rotary θ-direction.
Silicon in a prismatic form is formed as will be explained below where a plurality of mutually parallel rows of such belt-like resist pattern are thus formed.
Step 5 is for removing the protective film from the silicon surface. As shown in
Step 6 is for forming perpendicular walls by the primary anisotropic etching of the silicon surface.
As shown in
Step 7 is for forming a protective film on the entire surface.
As shown in
Step 7 makes uses of an oxide film (SiO2), instead of Si3N4. This is because the formation of an oxide film makes it easier to carry out the etching of the mask after the patterning of the resist in Step 8 to be explained below. It is permissive, however, to use Si3N4 or SiO2 as the protective film 7 and to carry out the etching by RIE, and it is also possible to use Si3N4. It is only to be reminded that the film thickness and the kind of solution must be selected appropriately.
Any known method may be employed for the formation of the oxide film (SiO2). For example, an oxidation furnace may be used with the internal temperature of about 1000° C. and with oxygen gas introduced therein to oxidize the wafer. The oxidation method inside the oxidation furnace includes both dry oxidation and wet oxidation. Either method may be used for the purpose of this invention. A film forming method by CVD and the so-called atmospheric pressure chemical vapor deposition (APCVD) method may be appropriately selected.
Step 8 is for resist patterning according to the other of the perpendicular (111) surfaces.
As shown in
For this purpose, the upper surface of the wafer 1 is coated with a resist 8. It is preferable to apply a thick film of resist (with thickness of about 30 μm). It is necessary to have the entire height of the perpendicular walls 11 coated. Accordingly, a thick-film resist (such as PMER and SU-8) capable of forming a thick resist is employed. As a method of coating, the spray coating method, instead of the spin coating method, may be employed because all that is necessary is to coat the entire surface. Moreover, a metallic material may be attached, instead of a resist, to use it as a mask material. Thereafter, a plurality of linear patterns are made according to the other of the perpendicular (111) surfaces. The same alignment method as used in Step 4 may be used in this step. The SEM photograph of
Step 9 is for removing the protective film on the silicon surface.
As shown in
Step 10 is for removing the resist and exposing the silicon surface.
As shown in
The removal of the resist is carried out as in Step 5 by using heated sulfuric acid and hydrogen peroxide water (their mixture at the ratio of 3:1). Other methods are known for the removal of a resist. Any of these known methods may be used.
Step 11 is for the secondary anisotropic etching of the silicon surface.
As shown in
When the silicon surface of the wafer 1 has been dug, there is an etch stop between the perpendicular (111) surface formed earlier in Step 6 and the other of the perpendicular (111) surface (at the edge of the mask part of the top surface), there resulting silicon prisms 8.
Step 12 is for removing the protective film.
The oxide film is finally removed as shown in
The first production method as described above has the following merits.
Firstly, silicon prisms having (110) surface as the top surface and (111) surfaces as the side surfaces can be obtained because a silicon wafer with (110) surface is employed and etching processes are carried out such that two internal (111) surfaces perpendicular to the substrate surface will come to be exposed.
Secondly, it is made possible to dig out perpendicular (111) surfaces accurately and hence to obtain silicon prisms with side walls that are accurately perpendicular to the top surface since the etching processes include both the primary anisotropic etching which is carried out after alignment configurations are formed with two perpendicular (111) surfaces by aligning according to one of the (111) surfaces and the secondary anisotropic etching according to the other of the (111) surfaces.
Thirdly, the protective film 7 can be attached to the side surfaces of the perpendicular walls 6 formed by the primary isotropic etching and the side walls can be prevented from being abraded at the time of the secondary isotropic etching since the protective film forming step is inserted between the primary isotropic etching and the secondary isotropic etching. As a result, only the surfaces crossing the side walls having the protective film are subjected to the etching process at the secondary isotropic etching and the prisms 8 are completed when the etch stop is effected at the (111) surface of this side wall. Thus, it becomes possible to obtain silicon prisms 8 having four accurately perpendicular (111) surfaces as side surfaces and theoretically having a high aspect ratio.
The silicon prisms 8 obtained by the first method according to this invention are of a prismatic shape, being surrounded by four perpendicular (111) surfaces. The height of the silicon prisms produced experimentally according to this method was about 30 μm but it should be possible in principle to obtain shapes with higher aspect ratios.
Each of the four side surfaces comprising a (111) surface means that the atomic combination is very stable and that the rigidity against external forces is very strong. Thus, even if silicon is obtained in a prismatic shape with a large height with respect to its cross-sectional area, or with a high aspect ratio, it can be used in many practical applications. Since it is formed along the crystalline surfaces of four atoms on the side surfaces, it becomes very smooth. For this reason, it can be used in various ways by making use of its prismatic shape.
Next, the second production method according to this invention will be explained.
The second production method is characterized in that incisions are made on the perpendicular walls after they are formed but it is based on the same idea regarding the crystalline orientations as the first production method. This method is advantageous because patterning is possible on wafers without resist patterning, that is, wafers without high steps formed thereon and hence the process is very simple.
Fifteen steps (Steps 1-15) are sequentially carried out according to the second production method, Steps 1, 2 and 3 being alignment configuration forming steps, Steps 4-9 being protective film forming steps, Step 10 being a primary anisotropic etching step, Steps 11 and 12 being protective film forming steps, Step 13 being a cutting step, and Steps 14 and 15 being secondary anisotropic etching steps. Steps 1-5 are the same as explained above for the first production method and hence detailed explanations will be omitted, the explanations being given only in a simple manner with like or equivalent components indicated by the same numerals as used in the description of the first production method.
Step 1 is for forming a protective film. As shown in
Step 2 is for preparing to form alignment configurations. As shown in
Step 3 is for forming alignment configurations. As shown in
Step 4 is for a resist patterning according to one of the perpendicular (111) surfaces. As shown in
Step 5 is for removing the protective film from the silicon surface. As shown in
Step 6 is for forming a protective film on the entire surface. The primary anisotropic etching step of the first production method described above is not carried out according to the second production method. Instead, as shown in
Step 7 is for resist patterning according to the other of the perpendicular (111) surfaces. As shown in
Step 8 is for removing the protective film on the silicon surface. As shown in
Step 9 is for removing the resist and exposing the silicon surface. Next, as shown in
In summary, SiO2 and Si3N4 will appear alternately on the belt-like portions respectively as the protective film 2 and the protective film 4, as a result of Steps 4-9. This is done because the protective (Si3N4) film 2 is to be removed to expose the silicon surface in Step 12 and incisions are to be made in Step 13 on the exposed portions of the silicon according to one of the perpendicular (111) surfaces.
Steps 4-9 described above are advantageous because a mask can be formed with the silicon wafer 1 in a flat condition, aligned to two perpendicular (111) surfaces, that is, the formation of a mask becomes easier. For coating a wall with steps as high as several hundred μm with a resist for patterning, for example, it will be necessary, say, to apply a thick-film resist. Application of a resist alone may be relatively simple because the use of the spray coating method may be possible, but the exposure to light may be a different matter because light may not reach the bottom of a deep groove for exposure. Thus, the present method according to this invention is advantageous, being capable of forming a mask material according to two mutually perpendicular (111) surfaces, while the wafer surface is kept in a flat condition.
Although the second method is described above as forming the protective (Si3N4) film 2 first in Step 1, the protective (SiO2) film 4 may be formed first before the formation of the protective (Si3N4) film 2. In this way, the protective (SiO2) film 4 will appear in Step 5 where the silicon is appearing and hence the patterning of Step 7 may be directly carried out. In other words, the same configurations as by Step 6 can be obtained also by first forming the protective (SiO2) film 4.
Step 10 is for the primary anisotropic etching of the silicon surface. As shown in
Step 11 is for forming a protective film on the perpendicular walls. As shown in
Step 12 is for partially removing the protective film on the upper surface of the perpendicular walls 6. As shown in
Step 13 is for cutting the perpendicular walls 6. As shown in
Step 14 is for the secondary anisotropic etching of the silicon surface. As shown in
As a result of the secondary anisotropic etching, silicon in prismatic shape can be obtained with the protective film 7 remaining on the top surface.
Under the conditions of
Step 15 is for removing the oxide film. As shown in
The second production method of this invention, as described above, has the following advantages.
Firstly, silicon in prismatic shape can be obtained with a (110) surface as the top surface and (111) surfaces as side surfaces because a silicon wafer with a (110) surface is used and etched such that two (111) surfaces perpendicular to the substrate surface inside will appear.
Secondly, perpendicular (111) surfaces can be dug out accurately and silicon in prismatic shape having side surfaces accurately made perpendicular to the top surface can be obtained because the etching is carried out after alignment configurations having two perpendicular (111) surfaces are formed and both the primary anisotropic etching with alignment to one of these two (111) surfaces and the secondary anisotropic etching with alignment to the other of these two (111) surfaces are carried out.
Thirdly, no particular method is necessary for the coating of resist or for the exposure to light because the protective film forming step can be carried out prior to the formation of the perpendicular walls 6 while the silicon surface is still flat.
Fourthly, the protective film 7 can be attached to the side walls of the perpendicular walls 6 formed by the primary anisotropic etching step since it is formed between the primary and secondary anisotropic etching steps. The side walls can thus be prevented from being abraded at the time of the secondary anisotropic etching. As a result, only the portions not having the protective film 7 are etched in the secondary anisotropic etching step and the prismatic shapes are formed at the moment of the etch stop at the (111) surface of these side walls. Thus, silicon prisms 8 having four accurately perpendicular (111) surfaces as side walls can be obtained.
Silicon in a prismatic form 8 obtained by the second production method of this invention is surrounded by four perpendicular (111) surfaces, and it can theoretically be formed in a shape with a high aspect ratio by the method of this invention.
To have four sides surfaces all comprising a (111) surface means that the condition of atomic bonding is stable and that there is no occurrence of burrs or chipping. Thus, even if silicon is made into a prismatic shape with a large height with respect to its cross-sectional area or with a high aspect ratio, it can be used widely in many applications.
All four side wall surfaces can also be made very smooth. This also serves to find many areas of application by utilizing the prismatic shape.
In summary, silicon in a prismatic shape according to this invention is characterized as having four side surfaces that are perpendicular to its top surface, and since the side surfaces are surrounded only by (111) surfaces, they are prism-shapes with no burrs or chipping. Thus, prisms with a high aspect ratio are possible and can be useful in many applications such as electrodes.
Silicon in a prismatic shape according to this invention can be useful in various industrial fields such as fabrication of mother dies for filters and high-density micro-electrodes but the fields of application are not limited by these examples.
Number | Date | Country | Kind |
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2007-067708 | Mar 2007 | JP | national |
This application is a continuation of International Application No. PCT/JP2007/060298, filed May 14, 2007 which claims priority on Japanese Patent Application 2007-067708 filed Mar. 16, 2007.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2007/060298 | 5/14/2007 | WO | 00 | 9/4/2009 |