Patent Publication Number: US-2006000493-A1

Title: Chemical-mechanical post-etch removal of photoresist in polymer memory fabrication

Description:
FIELD  
      Embodiments of the invention relate to semiconductor processing techniques, and specifically to photoresist removal techniques.  
     BACKGROUND  
      Memory manufacturers are currently researching and developing the next generation of memory devices. One such development includes technology designed to replace current Flash non-volatile memory technology. Important elements of a Flash successor include compactness, low price, low voltage operation, non-volatility, high density, fast read and write cycles, and long life.  
      Current Flash technology is predicted to survive into 90 nanometer and 65 nanometer process generations. This survival is in part based on, for example, exotic storage dielectric material, cobalt and nickel source and drain regions, copper and low dielectric constant materials for the interconnect levels, and high dielectric constant materials for transistor gate dielectrics. However, there will thereafter exist a need for new memory materials and technology, particularly for non-volatile memory.  
      Ferroelectric memory is one such technology aimed to replace Flash memory. A ferroelectric memory device combines the non-volatility of Flash memory with improved read and write speeds. Simply stated, ferroelectric memory devices rely on the use of ferroelectric materials that can be spontaneously polarized by an applied voltage or electric field and that maintain the polarization after the voltage or field has been removed. As such, a ferroelectric memory device can be programmed with a binary “1” or “0” depending on the orientation of the polarization. The state of the memory device can then be detected during a read cycle.  
      Two crystalline materials have emerged as promising films utilized in a ferroelectric memory scheme, namely lead zirconium titanate (“PZT”) and strontium bismuth tantalite (“SBT”). However, while the materials exhibit appropriate ferromagnetic properties, each is nevertheless expensive to integrate into an existing CMOS process.  
      More recent developments include the use of polymers that exhibit ferroelectric properties. The creation of polymer ferroelectric memory utilizes polymer chains with net dipole moments. Data is stored by changing the polarization of the polymer chain between metal lines that sandwich the layer comprised of the ferroelectric polymer chain. Further, the layers can be stacked (e.g., metal word line, ferroelectric polymer, metal bit line, ferroelectric polymer, metal word line, etc.) to improve memory element density. The polymer ferroelectric memory devices exhibit microsecond initial read speeds coupled with write speeds comparable to Flash.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1 : illustration of a ferroelectric beta phase polyvinylidene fluoride (PVDF) molecule chain  
       FIG. 2 : illustration of a top view of a polymer ferroelectric memory device  
       FIG. 3 : illustration of a substrate cross section of a polymer ferroelectric memory device after the top electrode metal has been blanket deposited  
       FIG. 4 : illustration of a substrate cross section of a polymer ferroelectric memory device after the photoresist has been deposited and patterned  
       FIG. 5 : illustration of a substrate cross section of a polymer ferroelectric memory device after the top electrode metal has been etched  
       FIG. 6 : illustration of a substrate cross section of a polymer ferroelectric memory device after the photoresist removal process of an embodiment  
       FIG. 7 : illustration of a photoresist removal tool and the nitrogen purge of an embodiment  
       FIG. 8 : illustration of a photoresist removal tool an the low pressure chemical spray of an embodiment  
       FIG. 9 : illustration of a photoresist removal tool and the high pressure chemical spray of an embodiment  
       FIG. 10 : illustration of a high pressure chemical spray arm motion of an embodiment  
       FIG. 11 : illustration of a high pressure chemical spray arm motion of an embodiment with a cone-shaped spray  
       FIG. 12 : illustration of a high pressure chemical spray arm motion of an embodiment with a fan-shaped spray  
       FIG. 13 : illustration of a photoresist removal tool and another low pressure spray of an embodiment  
    
    
     DETAILED DESCRIPTION  
      Embodiments of a method of removing photoresist are described. Reference will now be made in detail to a description of these embodiments as illustrated in the drawings. While the embodiments will be described in connection with these drawings, there is no intent to limit them to drawings disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents within the spirit and scope of the described embodiments as defined by the accompanying claims.  
      Simply stated, an embodiment of the invention is a method of removing photoresist. More specifically, an embodiment is a method of removing photoresist utilized to pattern the top electrode metal layer in a polymer memory device substantially without damaging the underlying polymer by utilizing a low pressure photoresist solvent spray, a high pressure photoresist solvent spray, and/or a combination thereof.  
      As noted, a large portion of the historical research in ferroelectric memory device technology has centered on select crystalline materials such as PZT and SBT. More current trends, however, include utilizing polymer chains that exhibit ferroelectric properties. Polyvinylidene Fluoride (“PVDF”) is a fluoropolymer with alternating CH 2  and CF 2  groups for which the relative electron densities between the hydrogen and fluorine atoms create a net ionic dipole moment.  FIG. 1  illustrates the ferroelectric beta phase PVDF  100 , including a chain of carbon  110  and alternating and opposing hydrogen  120  and fluorine  130  pairs. A particular PVDF copolymer is polyvinylidene fluoride trifluoroethylene (“PVDF-TrFE”). The addition of the trifluoroethylene C 2 HF 3  (essentially substituting a hydrogen with a fluorine) in the chain reduces the overall theoretical ionic dipole moment of a ferroelectric PVDF beta phase chain, but increases the likelihood of forming the ferroelectric PVDF beta phase versus the paraelectric PVDF alpha phase during crystallization. The crystalline PVDF-TrFE polymer is ferroelectric in that it can be given a remanent polarization that can be switched in a sufficiently high electric field (i.e., a coercive field). The polarization can be used to store a binary “0” state and a binary “1” state of a memory device fabricated therewith based on the orientation of the polarization.  
      Memory elements utilizing polymer ferroelectric materials can be passive in the sense that there is no need for active components (e.g., a transistor coupled to a MOS capacitor in DRAM). Data is stored by changing the polarization of the polymer chain between metal lines that sandwich the layer comprised of the ferroelectric polymer. The elements are driven externally by applying a voltage to the appropriate word and bit lines to read or write to a polymer ferroelectric memory cell. Configured as such, the read cycle is destructive and the memory cell must be rewritten akin to a DRAM refresh cycle.  
       FIG. 2  illustrates a top view of a single layer polymer ferromagnetic memory device. Bit lines  250 - 280  and word lines  210 - 240  sandwich a layer of polymer ferroelectric material  200 . When a voltage is applied across overlapping bit and word lines (e.g., bit line  250  and word line  240 ) a number of operational processes are possible. A relatively high voltage (e.g., ranging approximately between 8 and 10 volts), can create a coercive electric field sufficient to program a binary “1” state or a binary “0” state based on altering the orientation of the remanent polarization of the polymer ferroelectric material  200  sandwiched between the bit and word lines  250  and  240  respectively. A separate voltage can be applied, in conjunction with external detection circuitry not illustrated, to read the binary state of the memory cell.  
      There are a variety of processing challenges associated with fabricating polymer ferroelectric memory devices. One challenge is to deposit and pattern materials adjacent to the ferroelectric polymer layer as the ferroelectric polymer is susceptible to damage by certain processing steps common to, for example, photoresist removal. Further, the photoresist removal that is compatible with the ferroelectric polymer must simultaneously not damage (e.g., by etching) exposed metal.  
      As is well known in the art, photoresist is a photosensitive organic polymer utilized in the photolithographic process. Once the photoresist has been used to pattern for example an etch, deposition, or implant process step as is well known in the art, it is removed and the exposed substrate is cleaned in preparation for subsequent process steps. Photoresist removal (also called photoresist strip, or PR strip) can occur by a variety of different mechanisms and combinations thereof. For example, the photoresist may be removed with a solvent, and may further be subject to sonic energy while being exposed to the solvent. The photoresist may also be removed by ashing whereby the substrate is exposed to an oxygen-containing plasma that thermally decomposes the photoresist. The ashing may be followed by a solvent or rinse process step to remove any remaining photoresist or photoresist ash.  
       FIGS. 3 through 6  depict substrate cross sections to illustrate metal layer patterning processing steps associated with a metal electrode of a polymer memory device. A substrate  300  onto which the polymer ferroelectric memory is fabricated can be any substrate onto which it would be useful to fabricate a memory device, ranging from, for example, a bulk silicon wafer to the top interconnect, dielectric, or passivation layer of a dual damascene process architecture. Metal  310  is the bottom electrode of the polymer memory and forms, for example, one of word lines  210 - 240  word line illustrated by  FIG. 2 . Metal  310  can be any metal suitable as electrode material in a polymer memory device. For example, metal  310  may be titanium, titanium oxide, titanium nitride, aluminum, tantalum, gold, silver, tungsten, ruthenium, rhodium, palladium, platinum, cobalt, nickel, iron, copper, or alloys thereof. A polymer layer  320  is deposited atop the metal  310  layer. In an embodiment, the polymer layer  320  is polyvinylidene fluoride. In another embodiment, the polymer layer  320  is a copolymer of polyvinylidene fluoride and trifluoroethylene. The addition of the trifluoroethylene reduces the overall theoretical electrical dipole of the PVDF molecule chain, but increases the likelihood that the PVDF molecule will orient in its ferroelectric beta phase. Metal  330  is the basis for the top electrode of the polymer memory that will form, for example and following the processes of  FIGS. 4 through 6 , bit lines  250 - 280 . Metal  330  can be any metal suitable as electrode material in a polymer memory device. For example, metal  330  may be titanium, titanium oxide, titanium nitride, aluminum, tantalum, gold, silver, tungsten, ruthenium, rhodium, palladium, platinum, cobalt, nickel, iron, copper, or alloys thereof.  
       FIG. 4  illustrates the substrate  300  of  FIG. 3  following the deposition and patterning of photoresist layer  400 . Though not illustrated, photoresist layer  400  is, for example, spin-coat deposited as a blanket layer on top of the metal  330  blanket layer to be patterned. Using well known photolithographic techniques, the photoresist is patterned to expose select areas of the metal  330  layer.  
       FIG. 5  illustrates the substrate  300  of  FIG. 4  following the removal of select portions of the metal  330  layer to fabricate, for example, bit lines  250 - 280 . As introduced, metal  330  may be titanium, titanium oxide, titanium nitride, aluminum, tantalum, gold, silver, tungsten, ruthenium, rhodium, palladium, platinum, cobalt, nickel, iron, copper, or alloys thereof. In an embodiment, the metal is removed with a reactive ion etch with BCl 3 , Cl 2 , argon, helium, or combinations thereof. After portions of the metal  330  layer have been removed, portions of the polymer layer  320  are exposed.  
       FIG. 6  illustrates the substrate  300  of  FIG. 5  following the removal of photoresist layer  400 . As noted, there are a variety of methods common to photoresist removal including ashing and/or solvent strip as introduced above. However, traditional methods of photoresist removal are not fully compatible with the polymer layer  320 . For example, given that the photoresist and polymer layer are organic polymers, solvents useful to remove photoresist may also damage the polymer layer. Similarly, ashing the photoresist with an oxygen-containing plasma may also cause damage to the polymer layer. A method of an embodiment removes photoresist  400  in the presence of exposed ferroelectric polymer  320  substantially without damaging the ferroelectric polymer  320  and substantially without damaging metal  330  by adding mechanical energy to a wet photoresist removal chemistry. During the photoresist  400  removal, the polymer  320  is not exposed to oxygen-containing plasma that may damage the polymer  320 .  
       FIG. 7  illustrates a cross section of a photoresist removal tool  700 . Inside a chamber  770 , the photoresist removal tool includes a fixture  720  to hold a wafer  710  in place during the photoresist removal process. As used herein, wafer  710  includes or is the substrate on which the polymer memory is fabricated. In an embodiment, the wafer  710  is oriented such that the face of the wafer  710  (i.e., the side of the wafer including the fabricated circuit elements) is facing down and toward the source of a solvent spray that is sprayed up toward the wafer  710  surface. Further, in an embodiment, the fixture  720  is configured to spin the wafer  710 .  
      Once the wafer  710  is secure in the fixture  720 , the ambient within the chamber  770  is purged with, for example, nitrogen to evacuate substantially all of the oxygen in the chamber. In an embodiment, and as will be discussed more fully below, the wet photoresist removal chemistry may include, for example, metal corrosion inhibitors that degrade if oxidized. The nitrogen purge reduces that oxidizing exposure.  
      Once the chamber  770  is purged with, for example, nitrogen, the low pressure chemical spray manifold  750 , including a plurality of low pressure nozzles  780 , sprays the surface of the wafer  710  with a wet photoresist removal chemistry as illustrated by  FIG. 8 . In an embodiment the wafer  710  is spinning in the fixture  720  to aid uniformity in wet photoresist removal chemistry coverage. The pressure of the wet photoresist removal chemistry is approximately between 10 and 100 pounds per square inch. In an embodiment, the wet photoresist removal chemistry pressure is approximately 90 pounds per square inch. The wet photoresist removal chemistry has a temperature of approximately between 20° C. and 90° C. In an embodiment, the wet photoresist removal chemistry temperature is approximately 70° C. The low pressure chemical spray manifold  750  sprays the wafer  710  with the aforementioned parameters for approximately between 10 and 200 seconds. In an embodiment the low pressure chemical spray manifold  750  sprays the wafer  710  for approximately 90 seconds. During the spray, the wafer  710  may be spun in the fixture  720  at approximately between 25 and 1500 revolutions per minute. In an embodiment, the wafer is spun in the fixture  720  at approximately 50 revolutions per minute. During the low pressure spray of an embodiment, a high pressure chemical spray arm  760  is withdrawn, swung aside, or otherwise moved so as to not interfere with the spray from the low pressure chemical spray manifold  750 .  
      Generally speaking, the wet photoresist removal chemistry is a glycol ether based solution that, among other constituents, may contain water and a metal etch inhibitor so as to mitigate damage to metal  330  during the photoresist  400  removal. In an embodiment, photoresist  400  is T.O.K.  601 B. In an embodiment, the wet photoresist removal chemistry is ASHLAND EZSTRIP 100, ARCH MS5010, or SHIPLEY XP-0215. Though an embodiment described herein utilizes the same wet photoresist removal chemistry (i.e., solvent), it is to be understood that each of the first low pressure, high pressure, and second low pressure sprays may utilize different solvents.  
       FIG. 9  illustrates the high pressure chemical spray of an embodiment. After the wafer  710  has been sprayed by the low pressure chemical spray manifold  750 , the high pressure chemical spray arm  760 , including a high pressure nozzle  790 , extends, swings or otherwise positions underneath the wafer  710 . The high pressure nozzle then sprays the face of the wafer  710  with a wet photoresist removal chemistry. The pressure of the wet photoresist removal chemistry is approximately between 100 and 500 pounds per square inch. In an embodiment, the wet photoresist removal chemistry pressure is approximately 400 pounds per square inch. The wet photoresist removal chemistry has a temperature of approximately between 20° C. and 90° C. In an embodiment, the wet photoresist removal chemistry temperature is approximately 70° C. The high pressure nozzle  790  sprays the wafer  710  with the aforementioned parameters for approximately between 10 and 1000 seconds. In an embodiment, the high pressure nozzle  790  sprays the wafer  710  for 300 seconds. During the spray, the wafer  710  may be spun in the fixture  720  at approximately between 25 and 1500 revolutions per minute. In an embodiment, the wafer is spun in the fixture  720  at approximately 50 revolutions per minute.  
       FIG. 10  illustrates a bottom view of the wafer  710  and the high pressure chemical spray arm  760  including the high pressure nozzle  790 . During the high pressure spray, the high pressure chemical spray arm  760  is rotated about, for example, a pivot so that the high pressure nozzle  790  sweeps an arc across the surface of the wafer  710 . In an embodiment, the wafer  710  is spinning in the fixture  720  while the high pressure nozzle  790  is swept back and forth in an arc across the surface of the wafer  710 . The combination of sweeping the high pressure nozzle  790  and spinning the wafer  710  improves the uniformity with which the surface of the wafer  710  is exposed to the wet photoresist removal chemistry.  
      The shape of the wet photoresist removal chemistry spray emitting from the high pressure spray nozzle  790  can be altered to adjust the coverage of the wafer. For example, the high pressure spray nozzle  790  may spray the wet photoresist removal chemistry substantially in a cone shape as illustrated by  FIG. 11  and cone-shaped spray  1100 . The angle of the cone vertex may be altered to control the shape of the cone. Further, the distance between the high pressure spray nozzle  790  and the wafer  710  may be altered to control the surface area covered by the spray for a given cone vertex angle created by the high pressure spray nozzle  790 .  
       FIG. 12  illustrates a fan-shaped spray  1200  of an embodiment. The fan-shaped spray  1200  of an embodiment operates in conjunction with the high pressure chemical spray arm  760  rotated about, for example, a pivot so that the high pressure nozzle  790  sweeps an arc across the surface of the wafer  710  to uniformly expose the surface of the wafer  710  to the wet photoresist removal chemistry. As with the cone-shaped spray  1100 , the vertex angle of the fan-shaped spray  1200  and/or the distance between the wafer  710  and the high pressure spray nozzle  790  may be adjusted to control the surface area covered by the fan-shaped spray  1200  of an embodiment.  
      The effectiveness of the photoresist layer  400  removal depends in significant part on the addition of mechanical energy to the wet photoresist etch chemistry. As noted, adding sonic energy has been one approach utilized to encourage the solvent removal of photoresist. For example, the sonic energy may be in the form of ultrasonic (i.e., greater than 20,000 hertz) vibration as the, for example, wafer  710  including a photoresist layer  400  is submerged in a photoresist solvent. However, it is difficult to apply sonic energy uniformly to the substrate as it is difficult to tune or focus the sonic energy evenly over the entire surface of the substrate. Further, sonic energy is directional. The same sonic energy directionality that promotes photoresist removal, however, tends to also shear the underlying ferroelectric polymer. Further, the entire substrate is exposed to the sonic energy, potentially damaging otherwise interior layers.  
      The solvent spray or sprays of an embodiment, in addition to exposing the photoresist layer  400  to a solvent, adds mechanical energy to the solvent substantially perpendicularly to the surface of wafer  710 . The spray parameters (e.g., pressure, nozzle size and configuration, solvent type, solvent temperature, and duration of spray), in combination with the motion of both the wafer  710  and, if applicable, the motion of the high pressure chemical spray arm  760  can be adjusted to substantially uniformly expose the surface of the wafer  710  to the wet photoresist removal chemistry. Further, the same parameters, or a subset thereof, can be adjusted to increase or decrease the mechanical energy experienced by the surface of the wafer  710  to remove the photoresist layer  400  substantially without damaging the underlying polymer layer  320 .  
      Once the wafer  710  has been exposed to the high pressure spray, the low pressure chemical spray manifold  750 , including a plurality of low pressure nozzles  780 , sprays the surface of the wafer  710  with the wet photoresist removal chemistry as illustrated by  FIG. 13 . In an embodiment the wafer  710  is spinning in the fixture  720  to aid uniformity in wet photoresist removal chemistry coverage. The pressure of the wet photoresist removal chemistry is approximately between 10 and 100 pounds per square inch. In an embodiment, the wet photoresist removal chemistry pressure is approximately 90 pounds per square inch. The wet photoresist removal chemistry has a temperature of approximately between 20° C. and 90° C. In an embodiment, the wet photoresist removal chemistry temperature is approximately 70° C. The low pressure chemical spray manifold  750  sprays the wafer  710  with the aforementioned parameters for approximately between 10 and 200 seconds. In an embodiment the low pressure chemical spray manifold  750  sprays the wafer  710  for approximately 90 seconds. During the spray, the wafer  710  may be spun in the fixture  720  at approximately between 25 and 1500 revolutions per minute. In an embodiment, the wafer is spun in the fixture  720  at approximately 50 revolutions per minute. During the low pressure spray of an embodiment, a high pressure chemical spray arm  760  is withdrawn, swung aside, or otherwise moved so as to not interfere with the spray from the low pressure chemical spray manifold  750 . The second low pressure spray substantially removes any remaining photoresist  400  from the wafer  710 . In an embodiment, the second low pressure spray may be omitted as substantially all of the photoresist  400  is removed by the first low pressure spray and the high pressure spray.  
      Following photoresist  400  removal, the wafer  710  may be rinsed with, for example, deionized water to remove the wet photoresist removal chemistry from the surface of the wafer  710 . In an embodiment, the deionized water rinse is preceded by a rinse with ethylene glycol to prevent a reactive solvent from interacting with the water to the extent that the solvent, for example, precipitates solute or leaves a residue on the wafer  710  surface. The wafer  710  may further be spun dry. In an embodiment, the wafer  710  is spun for approximately 180 seconds at approximately 1500 revolutions per minute. In an embodiment, the rinse and spin-dry is performed by photoresist removal tool  700  so as to avoid transferring or transporting a wet wafer. During the spin-dry, the chamber  770  may be opened to the ambient atmosphere (i.e., dry air) to facilitate drying.  
      As noted, the resulting rinsed and dried wafer  710  has had the photoresist  400  removed in the presence of exposed ferroelectric polymer  320 . An embodiment removes the photoresist  400  without substantially damaging the ferroelectric polymer  320  as the ferroelectric polymer  320  is neither exposed to a damaging solvent nor exposed to an oxygen-containing plasma during the photoresist  400  removal. Further, the metal  330  has not been substantially damaged by exposure to the solvent. In an embodiment, the result is a substantially intact layer of ferroelectric polymer  320  combined with a substantially intact layer of metal  330  that has been patterned to form, for example, bit lines  250 - 280 .  
      It is to be understood that the wafer may be spun in the fixture either clockwise or counter clockwise relative to a reference direction. For example, in an embodiment the wafer is spun in one direction for the first low pressure spray and the high pressure spray and in the other direction for the second low pressure spray. However, the spin orientation may be altered differently. Altering the spin direction may aid the uniformity with which the wet photoresist removal chemistry removes photoresist  400 . For example, it may be that spinning the wafer  710  in the same direction for all sprays creates a leeward side to the photoresist  400  topology and non-uniform photoresist  400  removal.  
      One skilled in the art will recognize the elegance of the disclosed embodiment in that it mitigates one of the limiting factors of fabricating polymer ferroelectric memory devices. By avoiding ashing (i.e. exposure to oxygen-containing plasma) photoresist removal steps, an embodiment substantially avoids damaging the ferroelectric polymer during photolithographic patterning steps during which the ferroelectric polymer is exposed.