Patent Publication Number: US-2011049091-A1

Title: Method of removing photoresist and etch-residues from vias

Description:
COPENDING APPLICATION  
     The following application has been filed by the applicant simultaneously with the present application:
         U.S. Pat. No. 11,861,282
 
The disclosure of this copending application is incorporated herein by reference.
       

     CROSS REFERENCE TO RELATED APPLICATIONS  
     Various methods, systems and apparatus relating to the present invention are disclosed in the following US patents/patent applications filed by the applicant or assignee of the present invention: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
             
            
               
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                 7,249,833 
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                 7,255,430 
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                 10/760,180 
                 7,111,935 
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                 7,261,482 
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                 7,002,664 
               
               
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                 11/014,728 
                 11/014,727 
               
               
                 7,237,888 
                 7,168,654 
                 7,201,272 
                 6,991,098 
                 7,217,051 
               
               
                 6,944,970 
                 10/760,215 
                 7,108,434 
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                 7,210,407 
               
               
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                 7,217,049 
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                 7,261,477 
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                 11/282,768 
               
               
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                 7,154,626 
                 7,079,292 
               
               
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     FIELD OF THE INVENTION  
     The present invention relates to the field of printers and particularly MEMS inkjet printheads. It has been developed primarily to improve fabrication of MEMS inkjet printheads, although the invention is equally applicable to any MEMS fabrication process. 
     BACKGROUND OF THE INVENTION  
     Many different types of printing have been invented, a large number of which are presently in use. The known forms of print have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc. 
     In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature. 
     Many different techniques on ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore, “Non-Impact Printing: Introduction and Historical Perspective”, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207 -220 (1988). 
     Ink Jet printers themselves come in many different types. The utilization of a continuous stream of ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing. 
     U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electro-static field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al) Piezoelectric ink jet printers are also one form of commonly utilized inkjet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element. 
     Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The inkjet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclosed ink jet printing techniques that rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard. 
     As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables. 
     The present Applicant has developed a plethora of inkjet printheads fabricated by MEMS techniques. Typically, MEMS fabrication employs a plurality of photoresist deposition and removal steps. Removal of relatively thin layers of photoresist (c.a. 1 micron or less), used as photolithographic masks, is usually facile. Standard conditions employ an oxygen plasma, which oxidatively removes any photoresist in a process colloquially known in the art as “ashing”. 
     In the fabrication of inkjet nozzle assemblies, the present Applicant has employed photoresist as a sacrificial scaffold onto which other materials (e.g. heater material, roof structures) may be deposited. This technique enables relatively complex nozzle assemblies to be constructed. However, it requires deposition of relatively thick layers of viscous, heat-resistant photoresist. As will be explained in more detail below, photoresist layers or plugs of up to 30 microns may be required. Furthermore, this photoresist must be thoroughly hardbaked and UV cured so that it does not reflow during subsequent high-temperature deposition steps e.g. deposition of metals or ceramic material onto the photoresist. 
     In a typical MEMS printhead fabrication process, a final ashing step removes all remaining photoresist in the nozzle assemblies, including photoresist scaffolds and photoresist plugs employed during the fabrication process. Hitherto, traditional O 2  plasma ashing techniques have been employed for final or late-stage removal of photoresist. 
     However, thick layers of photoresist, which have been hardbaked and UV cured have increased resistance to ashing and are removed relatively slowly by traditional O 2  ashing techniques. This means that prolonged ashing times are required and/or higher ashing temperatures. Prolonged ashing times and/or higher ashing temperatures are undesirable, because there is an increased risk of damage to other MEMS structures (e.g. nozzle chambers, actuators) during the ashing process. Moreover, there is, in general, a need to increase the efficiency of each MEMS processing step so as to reduce processing time and, ultimately, reduce the cost of each printhead. 
     Combinations of O 2  with fluorinated gases (e.g. CF 4 ) are known to improve ashing rates. However, the Applicant has found that O 2 /CF 4  gas chemistries require significant amounts of CF 4  (&gt;10%) to provide improved ashing rates. At high concentrations of CF 4 , the ashing conditions have a deleterious effect on silicon nitride nozzle structures in the Applicant&#39;s printheads. Hence O 2 /CF 4  has proven to be unsatisfactory for removing hardbaked photoresist from the Applicant&#39;s printheads. 
     The use of O 2 /N 2  is also known to improve ashing rates, although the addition of N 2  shows only moderate improvement over pure O 2  for the removal of hardbaked photoresist. 
     Accordingly, from the foregoing, it will be appreciated that there is a need to improve the efficiency of photoresist removal in MEMS fabrication techniques. 
     It would be further desirable to remove ‘veils’ from etched vias concomitantly with photoresist removal. Post-etch residues or ‘veils’ form along via sidewalls as a byproduct of anisotropic etch processes (e.g. Bosch process). Veils are a well-recognized problem in the art and are notoriously difficult to remove. Veils typically contain entrapped species of the materials etched, which are generally silicon-oxy-carbon compounds. Polymer-forming anisotropic etch chemistries (e.g. Bosch process) create veils that can usually only be removed using aggressive, wet chemical solvents. Furthermore, conventional ashing using O 2  at elevated temperature typically compounds the problem of veils, making them even more difficult to remove. Accordingly, there is a need for a dry de-veiling process, which is reliable and which does not require aggressive wet chemicals that may damage the wafer. 
     Whilst the above-mentioned needs have been presented in the context of printhead fabrication, it will be appreciated that any MEMS fabrication process would benefit from improved techniques for photoresist removal and/or de-veiling, especially those MEMS fabrication processes which use a relatively thick layer of sacrificial photoresist that has been hardbaked and/or UV cured. 
     SUMMARY OF THE INVENTION  
     In a first aspect, there is provided a method of removing photoresist from a substrate, the method employing a plasma formed from a gas chemistry comprising: O 2 , NH 3  and a fluorine-containing gas. The method according to the present invention surprisingly and advantageously improves ashing rates by at least 20%, at least 50% or at least 100%, compared with ashing rates using a conventional O 2  plasma or an O 2 /N 2  plasma. 
     The method according to the present invention concomitantly de-veils etched vias in the substrate in contrast with conventional O 2  or O 2 /N 2  ashing plasmas. 
     Optionally, fluorine-containing gas is CF 4 . 
     Optionally, the fluorine-containing gas is present in said gas chemistry in a concentration of less than 5% by volume. The amount of fluorine-containing gas is usually kept low so as to avoid damaging any silicon nitride printhead structures in the substrate. 
     Optionally, the fluorine-containing gas is present in the gas chemistry in a concentration of less than 3% by volume. 
     Optionally, a ratio of O 2 :NH 3  is in the range of 20:1 to 5:1. 
     Optionally, a ratio of O 2 :CF 4  is in the range of 40:1 to 20:1. 
     Optionally, the gas chemistry consists only of O 2 , NH 3  and CF 4 . However, inert gases such as He and Ar may be present in the gas chemistry, if required. 
     Optionally, the photoresist is hardbaked photoresist and/or UV-cured photoresist, which is particularly difficult to remove using conventional O 2  or O 2 /N 2  ashing plasmas. Moreover, the use of conventional ashing plasma usually leaves residues (‘veils’) which are problematic in themselves. 
     Optionally, the photoresist has a thickness of at least 5 microns, such as the photoresist used as a sacrificial scaffold in the formation MEMS structures (e.g. inkjet nozzle assemblies). 
     Optionally, the substrate is attached to a chuck, and the chuck is cooled to a temperature in the range of −5 to −30° C. 
     Optionally, the method is a step of a MEMS fabrication process, such as a printhead fabrication process. 
     Optionally, the photoresist is contained in inkjet nozzle chambers and/or ink supply channels. 
     Optionally, the photoresist is a protective coating for inkjet nozzle assemblies and/or a mask for an anisotropic deep reactive ion etching (DRIE) process. 
     In a second aspect, there is provided a method of fabricating an inkjet printhead, the method comprising the steps of: 
     forming inkjet nozzle chambers on a frontside of a wafer substrate, each nozzle chamber having a corresponding ink inlet plugged with photoresist; 
     etching ink supply channels from a backside of the wafer substrate to meet with the ink inlets plugged with photoresist; and 
     removing at least some of the photoresist and concomitantly de-veiling the ink supply channels by subjecting the backside to a first plasma formed from a first gas chemistry comprising: O 2 , NH 3  and a fluorine-containing gas. 
     Optionally, the method comprises the further step of: 
     removing further photoresist by subjecting the frontside to a second plasma formed from a second gas chemistry comprising: O 2  and NH 3 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Optional embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a partial perspective view of an array of nozzle assemblies of a thermal inkjet printhead; 
         FIG. 2  is a side view of a nozzle assembly unit cell shown in  FIG. 1 ; 
         FIG. 3  is a perspective of the nozzle assembly shown in  FIG. 2 ; 
         FIG. 4  shows a partially-formed nozzle assembly after deposition of side walls and roof material onto a sacrificial photoresist layer; 
         FIG. 5  is a perspective of the nozzle assembly shown in  FIG. 4 ; 
         FIG. 6  is the mask associated with the nozzle rim etch shown in  FIG. 7 ; 
         FIG. 7  shows the etch of the roof layer to form the nozzle opening rim; 
         FIG. 8  is a perspective of the nozzle assembly shown in  FIG. 7 ; 
         FIG. 9  is the mask associated with the nozzle opening etch shown in  FIG. 10 ; 
         FIG. 10  shows the etch of the roof material to form the elliptical nozzle openings; 
         FIG. 11  is a perspective of the nozzle assembly shown in  FIG. 10 ; 
         FIG. 12  shows the nozzle assembly after backside wafer thinning; 
         FIG. 13  is a perspective of the nozzle assembly shown in  FIG. 12 ; 
         FIG. 14  is the mask associated with the backside etch shown in  FIG. 15 ; 
         FIG. 15  shows the backside etch of the ink supply channel into the wafer; 
         FIG. 16  is a perspective of the nozzle assembly shown in  FIG. 15 ; 
         FIG. 17  shows the nozzle assembly after backside ashing; and 
         FIG. 18  is a perspective of the nozzle assembly shown in  FIG. 17 ; 
     
    
    
     DESCRIPTION OF OPTIONAL EMBODIMENTS  
     As foreshadowed above, the present invention may be used in connection with any process requiring removal of photoresist. However, it will now be exemplified using the example of MEMS inkjet printhead fabrication. The present Applicant has previously described a fabrication of a plethora of inkjet printheads for which the present invention is suitable. It is not necessary to describe all such printheads here for an understanding of the present invention. However, the present invention will now be described in connection with a thermal bubble-forming inkjet printhead and a mechanical thermal bend actuated inkjet printhead. Advantages of the present invention will be readily apparent from the discussion that follows. 
     Referring to  FIG. 1 , there is shown a part of printhead comprising a plurality of nozzle assemblies.  FIGS. 2 and 3  show one of these nozzle assemblies in side-section and cutaway perspective views. 
     Each nozzle assembly comprises a nozzle chamber  24  formed by MEMS fabrication techniques on a silicon wafer substrate  2 . The nozzle chamber  24  is defined by a roof  21  and sidewalls  22  which extend from the roof  21  to the silicon substrate  2 . As shown in  FIG. 1 , each roof is defined by part of a nozzle plate  56 , which spans across an ejection face of the printhead. The nozzle plate  56  and sidewalls  22  are formed of the same material, which is deposited by PECVD over a sacrificial scaffold of photoresist during MEMS fabrication. Typically, the nozzle plate  56  and sidewalls  21  are formed of a ceramic material, such as silicon dioxide or silicon nitride. These hard materials have excellent properties for printhead robustness, and their inherently hydrophilic nature is advantageous for supplying ink to the nozzle chambers  24  by capillary action. 
     Returning to the details of the nozzle chamber  24 , it will be seen that a nozzle opening  26  is defined in a roof of each nozzle chamber  24 . Each nozzle opening  26  is generally elliptical and has an associated nozzle rim  25 . The nozzle rim  25  assists with drop directionality during printing as well as reducing, at least to some extent, ink flooding from the nozzle opening  26 . The actuator for ejecting ink from the nozzle chamber  24  is a heater element  29  positioned beneath the nozzle opening  26  and suspended across a pit  8 . Current is supplied to the heater element  29  via electrodes  9  connected to drive circuitry in underlying CMOS layers of the substrate  2 . When a current is passed through the heater element  29 , it rapidly superheats surrounding ink to form a gas bubble, which forces ink through the nozzle opening. By suspending the heater element  29 , it is completely immersed in ink when the nozzle chamber  24  is primed. This improves printhead efficiency, because less heat dissipates into the underlying substrate  2  and more input energy is used to generate a bubble. 
     As seen most clearly in  FIG. 1 , the nozzles are arranged in rows and an ink supply channel  27  extending longitudinally along the row supplies ink to each nozzle in the row. The ink supply channel  27  delivers ink to an ink inlet passage  15  for each nozzle, which supplies ink from the side of the nozzle opening  26  via an ink conduit  23  in the nozzle chamber  24 . 
     The complete MEMS fabrication process for manufacturing such printheads was described in detail in our previously filed U.S. application Ser. No. 11/246,684 filed on Oct. 11, 2005, the contents of which is herein incorporated by reference. The latter stages of this fabrication process are briefly revisited here so as to illustrate one example of the present invention. 
       FIGS. 4 and 5  show a partially-fabricated printhead comprising a nozzle chamber  24  encapsulating sacrificial photoresist  16 . During nozzle fabrication, the photoresist  16  was used firstly to plug the ink inlet  15  (shown in  FIG. 2 ), secondly as a scaffold for deposition of heater material to form the suspended heater element  29 , and thirdly as a scaffold for deposition of the sidewalls  22  and roof  21  (which defines part of the nozzle plate  56 ). The photoresist plugging the ink inlet  15  has a depth of about  20  microns, while the photoresist used as a scaffold in the nozzle chambers has a thickness of at least 5 microns. Furthermore, all the photoresist  16  was hardbaked and UV cured and must be removed later on in the fabrication process. 
     Referring to  FIGS. 6 to 8 , the next stage of MEMS fabrication defines the elliptical nozzle rim  25  in the roof  21  by etching away 2 microns of roof material  20 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone rim mask shown in  FIG. 6 . The elliptical rim  25  comprises two coaxial rim lips  25   a  and  25   b,  positioned over their respective thermal actuator  29 . 
     Referring to  FIGS. 9 to 11 , the next stage defines an elliptical nozzle aperture  26  in the roof  21  by etching all the way through the remaining roof material  20 , which is bounded by the rim  25 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone roof mask shown in  FIG. 9 . The elliptical nozzle aperture  26  is positioned over the thermal actuator  29 , as shown in  FIG. 11 . 
     Once frontside MEMS processing of the wafer is completed, the wafer is then thinned by backside grinding and etching to a thickness of about 150 microns ( FIGS. 12 and 13 ). After wafer thinning, ink supply channels  27  are etched from the backside of the wafer to meet with the ink inlets  15  using a standard anisotropic DRIE ( FIGS. 14 to 16 ). This backside etch is defined using a layer of hardbaked photoresist  50  exposed by the dark tone mask shown in  FIG. 14 . The ink supply channel  27  will make a fluidic connection between the backside of the wafer and the ink inlets  15  after removal of all the sacrifical photoresist  16  used in the fabrication of frontside MEMS nozzles assemblies. 
     Removal of the photoresist proceeds firstly with backside ashing to remove the backside hardbaked photoresist layer  50  and a portion of the plug of photoresist  16  plugging the frontside ink inlets  15  ( FIGS. 17 and 18 ). Backside ashing utilizes the ashing conditions described in the Example below with a sequential three-stage ashing process. 
     In a conventional ashing processes, an O 2  plasma is employed for ashing the photoresist  16 . However, in accordance with the present invention, the ashing plasma is formed using a gas chemistry comprising O 2 , NH 3  and CF 4 . When the plasma is formed from a gas chemistry comprising this gas chemistry, superior ashing is achieved in terms of increased ashing rate and reduced damage to nozzle structures. Moreover, veils resulting from backside anisotropic etching of the ink supply channels  27  are also removed using this gas chemistry, obviating the need for aggressive wet-chemical removal of veils. Experimental details of ashing conditions are described in more detail in the Example section below. 
     Finally, frontside ashing removes the remainder of the photoresist  16  to provide the completed printhead shown in  FIG. 1 to 3 . Frontside ashing may utilize the O 2 /NH 3 /CF 4  gas chemistry in accordance with the present invention. Alternatively, frontside ashing may utilize an O 2 /NH 3  gas chemistry as described the Applicant&#39;s US Publication No. US 2009/0078675, the contents of which are herein incorporated by reference. 
       FIG. 1  shows three adjacent rows of nozzles in a cutaway perspective view of a completed printhead integrated circuit. Each row of nozzles has a respective ink supply channel  27  extending along its length and supplying ink to a plurality of ink inlets  15  in each row. The ink inlets, in turn, supply ink to the ink conduit  23  for each row, with each nozzle chamber receiving ink from a common ink conduit for that row. 
     It will be appreciated by the person skilled in the art that the exact ordering of late-stage MEMS fabrication steps may be varied. For example, the wafer may be subjected to backside ashing only or frontside ashing only. Regardless, it will be appreciated that the wafer must be subjected to ashing, either frontside ashing and/or backside ashing, in order to remove the photoresist  16  and furnish the printhead. 
     EXAMPLES  
     Backside ashing of the wafer shown in  FIGS. 17 and 18  was performed in an ashing oven, using the optimized ashing sequence shown in Table 1. Recipe 1 was used for 15 minutes, followed by Recipe 2 for 5 minutes and then Recipe 3 for 10 minutes. The temperature in Table 1 refers to the chuck temperature, which is cooled using helium. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Recipe 1 
                 Recipe 2 
                 Recipe 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pressure (mTorr) 
                 80 
                 20 
                 20 
               
               
                   
                 ICP Power (W) 
                 2200 
                 2200 
                 2200 
               
               
                   
                 NH 3  (sccm) 
                 10 
                 10 
                 10 
               
               
                   
                 O 2  (sccm) 
                 100 
                 100 
                 100 
               
               
                   
                 CF 4  (sccm) 
                 3 
                 3 
                 0 
               
               
                   
                 Temperature (° C.) 
                 −20 
                 −20 
                 −20 
               
               
                   
                 Time (mins) 
                 15 
                 5 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Under the sequential ashing conditions shown in Table 1, an excellent rate of photoresist removal was observed. Moreover the ink supply channel  27  and the ink inlet had been completely de-veiled, as confirmed by SEM. By way of comparison, conventional O 2  ashing or O 2 /N 2  ashing required about 70-90 minutes of ashing time to remove the same photoresist, and left significant veils which had to be removed by subsequent wet-chemical treatment. 
     As expected, the excellent ashing rates and de-veiling were also observed in frontside ashing experiments using the O 2 /NH 3 /CF 4  gas chemistry. 
     From these experiments, it can be concluded that gas chemistries comprising O 2 /NH 3 /CF 4  provide superior ashing rates and surprising efficacy in de-veiling compared to conventional ashing conditions. 
     It will be appreciated by ordinary workers in this field that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.