Abstract:
A micromachined structure, comprising: a substarte; a first wet etched pit disposed in the substrate; a second wet etched pit disposed in the substrate, the second pit extending into the substrate a greater depth than the first pit; and a dry pit disposed between, and adjacent to, the first and second pits. Also disclosed is a micromachined substrate comprising: a wet etched pit; and a dry-etched hole disposed in the wet etched pit, wherein the dry hole extends through the substrate.

Description:
RELATED APPLICATIONS 
     The present application claims the benefit of priority from copending provisional parent applications 60/269,011 filed on Feb. 14, 2001 and 60/269,010 filed on Feb. 14, 2001 and which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to micromachining. More particularly, the present invention relates to a new method for combining directional ion etching and anisotropic wet etching. The present invention is particularly applicable to silicon micromachining. 
     BACKGROUND OF THE INVENTION 
     Silicon optical bench chips often have anisotropically etched grooves for holding optical fibers or other components. Also, SiOB chips can have dicing saw cuts that function as fiber stops, thereby providing passive longitudinal alignment for an optical fiber. Such optical bench chips are well known in the art. 
     In some cases, it is not desirable or practical to have dicing saw cuts. Particularly, dicing saw cuts can be undesirable because they typically must extend across an entire wafer. It would be an advance in the art to provide fiber stops in optical bench chips without requiring dicing saw cuts. Also, it would be an advance in the art of micromachining to provide a wider array of precision-made structures. Particularly, it would be advance to combine multiple micromachining techniques to provide unusual, useful structures. 
     SUMMARY OF THE INVENTION 
     The present invention provides novel micromachined structure, comprising: a substarte; a first wet etched pit disposed in the substrate; a second wet etched pit disposed in the substrate, the second pit extending into the substrate a greater depth than the first pit; and a dry pit disposed between, and adjacent to, the first and second pits. The present invention also provides a micromachined substrate comprising: a wet etched pit; and a dry-etched hole disposed in the wet etched pit, wherein the dry hole extends through the substrate. 
     In a further aspect of the present invention, a method for micromachining a substrate is provided, comprising: providing a hard mask on a selected portion of the substrate to provide a region of the substrate without a hard mask; removing a portion of the substrate adjacent the region without the hard mask using one or more of ultrasonic drilling, laser etching, sawing and electrochemical etching to provide a micromachined feature in the substrate; and wet etching the substrate to provide a wet etched feature adjacent to the micromachined feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of the preferred embodiments of the present invention will be understood when read in conjunction with the appended drawings, in which: 
         FIG. 1  illustrates a top view of an exemplary structure in accordance with the present invention having two flat, planar surfaces that are both parallel with the substrate surface and a dry pit disposed between the planar surfaces; 
         FIG. 2  illustrates a cross-sectional view of the structure of  FIG. 1 ; 
         FIG. 3  illustrates the structure of  FIG. 2  with a VCSEL disposed on the deeper flat surface; 
         FIG. 4  illustrates a top view of another exemplary structure in accordance with the present invention having two flat, planar surfaces and a linear dry pit disposed between the planar surfaces; 
         FIGS. 5 and 6  illustrate top views of further exemplary structures in accordance with the present invention having two flat, planar surfaces and a dry pit having curved boundaries disposed between the planar surfaces; 
         FIG. 7  illustrates a top view of an exemplary structure in accordance with the present invention having a V-groove with a fiber stop and a wet-etched pit for retaining a ball lens; 
         FIG. 8  illustrates a top view of the structure of  FIG. 7  with an optical fiber in the V-groove and a ball lens in the pit; 
         FIGS. 9 and 10  illustrate a top view and a cross-sectional view taken along the line A—A in  FIG. 9 , respectively, of an exemplary structure in accordance with the present invention having a wet etched pit disposed within another wet etched pit with a ring-shaped dry pit disposed therebetween; 
         FIGS. 11 and 12  illustrate a top view and a cross-sectional view taken along the line A—A in  FIG. 11 , respectively, of an exemplary structure in accordance with the present invention having a U-shaped dry pit disposed within a wet etched pit to provide a wedge in the wet edged pit; 
         FIGS. 13 and 14  illustrate a top view and a cross-sectional view taken along the line A—A in  FIG. 13 , respectively, of an exemplary structure in accordance with the present invention having a U-shaped dry pit disposed within a wet etched pit to provide a wedge in the wet edged pit that lies below the surface of the substrate; 
         FIGS. 15 and 16  illustrate a top view and a cross-sectional view taken along the line A—A in  FIG. 15 , respectively, of an exemplary structure in accordance with the present invention having two U-shaped dry pits disposed within a wet etched pit to provide two wedges in the wet edged pit; 
         FIG. 17  illustrates a top view of an exemplary structure in accordance with the present invention having a plurality of wet pits each having a micromachined hole disposed therethrough; 
         FIG. 18  illustrates a cross-sectional view of one of the wet pits and through-holes of  FIG. 17  taken along the line of A—A in  FIG. 17 ; 
         FIG. 19  illustrates a top view of an exemplary structure in accordance with the present invention having a plurality of wet pits each having an off-center micromachined hole disposed therethrough; 
         FIG. 20  illustrates a top view of an exemplary structure in accordance with the present invention having V-grooves with a plurality of micromachined holes disposed therethrough; 
         FIGS. 21A-21H  illustrate a method for making structures in accordance with the present invention; 
         FIGS. 22A-21E  illustrate additional, optional steps for use with the method illustrated in  FIGS. 21A-21H ; 
         FIGS. 23A-23G  illustrate another method for making structures in accordance with the present invention; 
         FIGS. 24A-24G  illustrate a further method for making structures in accordance with the present invention; and 
         FIGS. 25A-25E  illustrate yet a further method for making structures in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides novel micromachined structures made by combined wet/dry etching. The structures can be used in a variety of micromachined devices, including micro-optical submounts and the like. For example, referring now to the figures, wherein like elements are numbered alike throughout,  FIGS. 1 and 2  illustrate an exemplary structure  100  in accordance with the present invention comprising two wet pits  10 ,  20  disposed in a substrate  40  with a dry pit  30  disposed between the two wet pits  10 ,  20 . Each wet pit  10 ,  20 , includes a corresponding flat, planar surface  12 ,  22  that is parallel with an upper substrate surface  15 . As provided in the exemplary structure of  FIGS. 1 and 2 , the flat surfaces  12 ,  22  may be located at different depths within the substrate  40 . For example, the flat surface  22  of the second wet pit  20  may be deeper than flat surface  12  of the first wet pit  10 . As provided by the methods described below, the dry pit  30  is desirably etched before the wet pits  10 ,  20 , with the dry pit  30  being coated with a mask layer before the wet pits  10 ,  20  are etched. 
     The structure  100  is particularly useful in an application where a VCSEL or photodetector is disposed on the flat surface  22  of the second wet pit  20 , because planar electrical connections  227  (e.g., wire bonds) can be made between the flat surface  12  of the first wet pit  10  and the VCSEL  25 , as illustrated in FIG.  3 . Planar electrical connections are desirable because they have better characterized impedance characteristics than wire bonds that extend between two different levels. 
       FIGS. 1-3  illustrate a first exemplary structure  100  in accordance with the present invention. However, there are various configurations of dry and wet pits that can provide wet pits that have flat surfaces at different depths. For example, as illustrated in  FIG. 4 , the dry pit  430  may comprise a straight line trench that is disposed between two wet pits  410 ,  420  to provide flat surfaces  412 ,  422  disposed at different depths. The dry pit  430  maybe created by dry-etching or a dicing saw cut, for example. Alternatively, as shown in  FIG. 5 , the dry pit  530  may comprise curved sidewall portions  538  disposed between two wet pits  510 ,  520 . However, if the curved sidewall portions  638  are too tightly curved, wedges  639  may be formed between the wet pits  610 ,  620 , as illustrated in FIG.  6 . 
     In another aspect of the present invention, a structure  700  for coupling an optical fiber  750  and a ball lens  760  is provided, as illustrated in  FIGS. 7 and 8 . The structure  700  comprises a V-groove  710  with a fiber stop  711  and a wet-etched pit  720  for a ball lens  760 . 
     In yet another aspect of the present invention, a structure  900  for use as a silicon optical bench can be provided, as illustrated in  FIGS. 9 and 10 . An interior wet etched pit  910  can be formed within an exterior wet etched pit  920  with a ring-shaped dry pit  930  disposed therebetween. The exterior wet etched pit  920  can have a flat bottom surface  932 . The interior wet pit  910  can hold a ball lens, and the flat bottom surface  932  of the exterior pit  930  can be used to hold optical devices such as switches or fibers. 
     In still another aspect of the present invention, a wedge  1190  is provided on the flat bottom  1115  of a wet etched pit  1110 , as illustrated in FIG.  11 . In order to form the wedge  1190 , a U-shaped dry pit  1130  is dry etched or machined and coated with a hard mask. As illustrated in  FIGS. 13 and 14 , the U-shaped dry pit  1330  may be shaped so that the size of the wedge  1390  may be limited by the size of the U-shaped pit  1330 . In this case, the top corner  1392  of the wedge  1390  will be located below the substrate surface  1315 . In yet a further aspect of the invention, multiple U-shaped areas  1590 ,  1591  can be combined to form a V-groove  1595  or similar shapes in a flat-bottom pit  1510 , as shown in FIG.  15 . 
     The present invention also provides a 2-D fiber array  1700 , as shown in  FIGS. 17 and 18 . The fiber array  1700  holes  1730  that are drilled or etched or machined (e.g., by reactive ion etching, ultrasonic drilling, laser drilling, laser machining, or electrodischarge machining) into a substrate  1740 . The holes  1730  are then coated with a hard ask material (e.g., SiO 2  or Si nitride). Then, the substrate  1740  is wet-etched. The wet etching step can form a wet pit  1710  around each hole  1730 . Alternatively, the wet pit can have the shape of a V-groove  2010  and can include a row of holes  2030  disposed therein, as shown in FIG.  20 . The V-groove  2010  provides passive alignment in one direction when aligning the optical fibers with the holes  2030 . 
     The wet pit  1710  associated with each hole  1730  acts as a funnel for guiding an optical fiber into the hole  1730 . In this way, the assembly process of a fiber array is simplified. It is noted that the holes  1710  can be located off-center within each wet pit  1730 , as shown in FIG.  19 . Having off-center holes can be useful for locating the fibers in cases where the fibers are guided into the holes from one direction (e.g., from the right side in FIG.  19 ). 
     The present invention also provides several methods for creating the above-described structures. For example, with to reference  FIGS. 21A-21H , a method in accordance with the present invention is illustrated. First, a hard mask layer  2142  (e.g., nitride) is deposited and patterned on substrate  2140 , such as a silicon wafer, FIG.  21 A. The hard mask layer  2142  does not cover areas of the substrate  2140  to be wet etched. The hard mask layer  2142  may partially cover areas to be machined (i.e., ultrasonically drilled, electrochemically etched, sawed, etc.), provided that the machining process can remove the hard mask material. Subsequently, a coating  2144  of CVD oxide, PSG or BPSG is provided as shown in FIG.  21 B. Then, the machining process is preformed to provide a pit  2110 , FIG.  21 C. The machining process can be any process that removes substrate material. The machining process should be able to cut through the hard mask layer  2142  and oxide layer  2144 . After the machining, a coating of CVD nitride  2146  is provided, as shown in FIG.  21 D. Next, nitride  2146  is removed from the top surface  2145  by planarization, FIG.  21 E. Then the oxide  2144  is removed by a dilute HF etch, for example,  FIG. 21F , and the substrate  2140  is wet etched, FIG.  21 G. Finally, hard mask layer  2142  and CVD nitride  2146  is removed, FIG.  21 H. 
     Optionally, after the step of  FIG. 21D , the pit  2110  can be filled with a fugitive mask material  2148  that resists nitride etches (e.g., wax, polymer or photoresist), FIG.  22 A. After filling the pit  2110 , the nitride layer  2146  on th upper surface  2145  is removed by etching, FIG.  22 B. Subsequently, the oxide layer  2144  is etched away, FIG.  2 C. Then the substrate  2140  is wet etched, FIG.  22 D. Finally, the fugitive mask material  2148 , nitride layer  2146 , and hard mask  2142  are removed, FIG.  22 E. 
     Combined wet and dry etching can be performed according to a number of different methods. The dry pit can be coated with CVD nitride or oxide, or can be thermally oxidized. The present invention can be used with silicon or other materials (e.g., GaAs) that can be dry etched and wet etched (isotropic or anisotropic) and can be conformally coated with a mask material. 
     For example, referring to  FIGS. 23A-23G , another method in accordance with the present invention is provided. First, an SiO 2  layer  2342  and a nitride layer  2344  are deposited and patterned over a substrate  2340 , such as a silicon wafer, to provide the structure shown in FIG.  23 A. The SiO 2  layer  2342  should be thick enough to serve as a mask during the dry etch step (e.g., the SiO 2  layer  2342  can be about 2 microns thick for a 100 micron deep dry pit  2310 ). The patterns in the SiO 2  layer  2342  and nitride determine the wet and dry etch areas as shown in FIG.  23 A. Next, the dry pit  2310  is formed. The dry pit  2310  can be formed by reactive ion etching, plasma etching, ion milling or any other directional process, FIG.  23 B. Then, the substrate  2340  is thermally oxidized to yield the structure shown in FIG.  23 C. The sidewalls are necessarily oxidized in this step to provide a sidewall oxide  2312 . The thermal oxidization step causes the SiO 2    2342  to thicken in outside areas outside of the nitride layer  2344 . Afterwards, the nitride layer  2344  is removed,  FIG. 23D , which can be done with a wet etch. A short duration oxide etch (wet or dry) removes the SiO 2  layer  2342  that was under the nitride layer  2344  to yield the structure shown in FIG.  23 E. The other SiO 2  layer areas are not under the nitride layer  2344  remain intact because they are thicker. Next, the substrate  2340  is expose in an anisotropic wet etch to yield the structure shown in FIG.  23 F. KOH should not be used because it will attack the SiO 2  layer  2342 . EDP or TMAH can be used because they will not attack the SiO 2  layer  2342  as strongly. Then, the SiO 2  layer  2342  and sidewall oxide  2312  are optionally removed, which can be done in a dilute HF etch, FIG.  23 G. 
     Referring now to  FIGS. 24A-24G , yet another method in accordance with the present invention is provided. First, a nitride layer  2442  is deposited and patterned on a substrate  2440 , such as a silicon wafer. Then an oxide layer  2444  is deposited and patterned to yield the structure shown in FIG.  24 A. The oxide layer  2444  can be thicker than the nitride layer  2442 . The oxide layer  2444  can comprise PSG or BPSG, for example. The nitride and oxide patterns determine the wet and dry areas as shown in FIG.  24 A. Next, the dry pit  2410  is etched, to yield the structure shown in  FIG. 24B , which can be done with RIE, ion milling or similar processes. Then, the substrate is conformally coated with CVD nitride  2446 , resulting in the structure of  FIG. 24C , where the dry pit  2410  is coated with nitride  2446 . The substrate  2440  is then planarized or polished so that CVD nitride  2446  is removed from the top surface, FIG.  24 D. Then the oxide layer  2444  is removed,  FIG. 24E , which can be done with diluted HF. The exposed areas of the substrate  2440  are then wet etched, to provide the structure shown in FIG.  24 F. This can be done with KOH since the mask is made of nitride. Optionally, the nitride layer  2442  and CVD nitride  2446  are removed with etchant that does not damage the silicon, FIG.  24 G. 
     Referring now to  FIGS. 25A-25G , still another method in accordance with the present invention is provided. A hard mask layer  2504  that blocks oxide formation (e.g., silicon nitride) is deposited and patterned over a substrate  2540 , such as a silicon wafer, followed by depositing and patterning a photoresist layer  2504 , resulting in the structure of  FIG. 25A  with the dry and wet etch areas defined as shown. The photoresist layer  2504  does not need to cover the entire hard mask layer  2504 . The portion of the substrate  2540  exposed by the photoresist layer  2504  and hard mask layer  2504  is dry etched, FIG.  25 B. Then the photoresist layer  2504  is removed and the substrate  2540  oxidized to provide an oxide layer  2506 , as shown in FIG.  25 C. Oxide will not grow under the hard mask layer  2502 . Next, the hard mask layer  2502  is removed, FIG.  25 D. Subsequently, the substrate  2540  is wet-etched with an anisotropic etchant, resulting in the structure of FIG.  25 E. Optionally, the oxide layer  2506  can be removed after the wet etch. 
     These and other advantages of the present invention will be apparent to those skilled in the art form the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.