Abstract:
A method is disclosed for etching a polysilicon material in a manner that prevents formation of an abnormal polysilicon profile. The method includes providing a substrate with a word line and depositing a polysilicon layer over said substrate and word line. An organic bottom antireflective coating (BARC) layer is then deposited over said polysilicon layer. A ladder etch is performed to remove the BARC layer and a portion of the polysilicon layer. The ladder etch consists of a series of etch cycles, with each cycle including a breakthrough etch and a soft landing etch. The breakthrough and soft landing etches are performed using different etchant gases, and at different source and bias powers, pressures, gas flow rates, and periods of time. The ladder etch results in a smooth polysilicon surface without abrupt steps.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a etching method for use as part of word line poly planarization, and more particularly to a ladder etching method for use as part of embedded flash cell word line planarization. 
       BACKGROUND OF THE INVENTION 
       [0002]    Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that has typically been used in memory cards, and USB flash drives (thumb drives, handy drive), which are used for general storage and transfer of data between computers and other Digital products. Flash memory costs far less than EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid-state storage is needed. Examples of applications include laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in the game console market, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data. 
         [0003]    Embedded flash technology consists of flash memory built directly onto a processor. For example, a graphics chip may have embedded memory instead of using separate memory chips. With the continuing growth of consumer and mobile electronics markets, chip makers are racing to make ever smaller features, which increasingly require more advanced embedded flash technologies as they begin to include more functionality. 
         [0004]    In addition to consumer and mobile electronics markets, the use of embedded flash technology is also becoming more prevalent in high density applications that require low power such as microcontroller cores, high-speed ASICs (application-specific integrated circuits) and multimedia ICs (integrated circuits). 
         [0005]    The production of embedded flash chips is not without difficulty. For example, due to the high topography of the typical flash cell structure, however, it is a challenge to perform word line etching without damaging the individual flash cells. To avoid such damage, word line polysilicon planarization may be performed using a chemical-mechanical polishing (CMP). A problem with using such CMP process is that an 800 Angstrom step height (see  FIG. 2 ) may still exist after poly-CMP. This step height can cause an abnormal SiON thickness (see  FIG. 3 ) that will induce an abnormal polysilicon profile during subsequent poly etching. An abnormal polysilicon profile can result in undesirable variations in etched channel length as well as reduced control over etched channel depth. 
         [0006]    Thus, it would be desirable to provide a planarization process that eliminates undesirable step heights associated with prior techniques, thereby minimizing or eliminating subsequent abnormal SiON thicknesses that can induce abnormal polysilicon profiles. 
       SUMMARY OF THE INVENTION 
       [0007]    To solve the aforementioned problem, a ladder etching process is disclosed for reducing step height and to obtain a smooth poly surface, that will reduce the risk of word line poly abnormalities. 
         [0008]    A method of word line etching is disclosed, comprising the steps of: (a) patterning a word line; (b) depositing a layer of polysilicon over said word line; (c) depositing a layer of bottom antireflective coating (BARC) material over said layer of polysilicon; (d) etching said BARC layer and said polysilicon layer using a ladder etch, said ladder etch removing the BARC layer and a portion of said polysilicon layer; (f) depositing a dielectric layer of a top surface of the etched polysilicon layer; and (g) applying a mask layer over said dielectric layer and etching at least one feature into said polysilicon layer; wherein said ladder etch comprises a series of breakthrough etch steps and soft landing etch steps. 
         [0009]    An etching method is disclosed, comprising the steps of: (a) providing a substrate with a plurality of word lines formed thereon; (b) depositing a layer of polysilicon over said plurality of word lines; (c) depositing a layer of organic spin-on material over said layer of polysilicon; (d) etching said organic spin-on material layer and said polysilicon layer using a ladder etch, said ladder etch removing the organic spin-on material layer and a thickness of said polysilicon layer so that said polysilicon layer does not overlie a top surface of each of the plurality of word lines; (f) depositing a dielectric layer of a top surface of the etched polysilicon layer; and (g) applying a mask layer over said dielectric layer and etching at least one feature into said polysilicon layer; wherein said ladder etch comprises a series of breakthrough etch steps and soft landing etch steps. 
         [0010]    An etching method is disclosed, comprising the steps of: (a) providing a flash cell on a substrate, the flash cell comprising a word line; (b) depositing a layer of polysilicon over said word line; (c) depositing a dielectric layer over an area peripheral to said flash cell; (d) depositing an organic BARC coating over said polysilicon layer; (e) performing a ladder etch of said BARC coating, said dielectric layer, and said polysilicon layer to a level below a top surface of said word line; (f) depositing a dielectric layer over said etched polysilicon layer and said word line; (g) applying a photo mask over said dielectric layer; and (h) etching the dielectric layer and the polysilicon layer to create an etched feature; wherein said ladder etch comprises a series of breakthrough etch steps and soft landing etch steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiment of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
           [0012]      FIGS. 1 through 4  show cross section views of a conventional process for poly word line planarization; 
           [0013]      FIG. 5  shows a cross section view of a plurality of word lines formed on a substrate and a polysilicon layer formed thereover; 
           [0014]      FIG. 6  shows the structure of  FIG. 5  subsequent to deposition of a BARC layer over the polysilicon layer; 
           [0015]      FIG. 7  shows the structure of  FIG. 6  subsequent to an etch back of the BARC layer and a portion of the polysilicon layer; 
           [0016]      FIG. 8  shows the structure of  FIG. 7  subsequent to the application of a mask layer; and 
           [0017]      FIG. 9  shows the structure of  FIG. 8  subsequent to etching of the polysilicon layer and removal of the mask layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    According to an embodiment of the present invention, disclosed herein is a method for ladder etching to reduce a poly profile step height and to obtain a smooth poly surface, thereby reducing the risk of word line poly abnormalities 
         [0019]    Referring to  FIG. 1 , a substrate  10  is provided upon which a plurality of word lines  12  are fabricated. A polysilicon layer  14  is then formed over substrate and word lines using known methods, such as chemical vapor deposition (CVD) or the like. A planarization process, such as chemical-mechanical planarazation (CMP) is then performed to remove the portion of the polysilicon layer  14  residing directly above the word lines  12 , with the resulting structure shown in  FIG. 2 . As can be seen, where the total thickness “T” of the polysilicon layer is less than the height “H” of the word lines, a step region  16  is formed in the polysilicon layer during CMP. In some cases, where the polysilicon layer is deposited to a thickness “T” of about 1,500 Angstroms (Å), this step height “H” can be about 800 Å. As shown in  FIG. 3 , during subsequent deposition of dielectric  18 , this step height “H” can cause an abnormally thick region “TR” of dielectric  18  to be formed in the step region  16 . In one example, where the dielectric  18  is deposited to a thickness “DT” of about 660 Å over the word lines  12 , the thickness “DTS” of the dielectric  18  in the step region  16  can be nearly twice that thickness (e.g., 1300 Å). When a mask layer  20  is applied and etching performed, this thicker layer of dielectric  18  in the step region  16  causes an abnormal polysilicon profile “AP” to be formed. 
         [0020]    Now referring to  FIGS. 5-9 , the inventive process will be described.  FIG. 5  shows a substrate  22  upon which are formed a plurality of floating gate and control gate pairs  24 . The control gate (i.e., the uppermost layer) may comprise a continuous polysilicon strip that forms a word line for the memory device. For convenience, item  24  will be described hereinafter as the “word line.” A polysilicon layer  26  is formed over the substrate and gates/wordlines  24  using known methods, such as chemical vapor deposition (CVD) or the like. This polysilicon layer  26  may have a thickness of from about 1000 Å to about 3000 Å, and preferably about 1500 Å, while thickness of the gate layer  24  may be from about 3000 Å to 3600 Å. An organic bottom antireflective coating (BARC) layer  28  may then be provided over the polysilicon layer. It will be appreciated that other easily removable materials may be used in lieu of BARC, such as a spin-on organic material (photoresist). The BARC layer  28  may have a thickness of up to about 1600 Å. The BARC layer  28  may be formed using appropriate spin-on techniques. 
         [0021]    The BARC layer  28  and a portion of the polysilicon layer  26  (to a point below the top of the word lines  24 ) may then be removed using a ladder etching process to achieve the profile shown in  FIG. 7 . The ladder etching process may be performed follows: 
         [0022]    Etch step  1  may comprise a Breakthrough Etch (BT1) using CF 4  as the etchant gas. BT1 may be performed at a pressure of about 4 milli-Torr (mT), a source power of from about 100 Watts (W) to about 500 W, preferably about 300 Watts (W), a bias power of from about 30 W to 150 W, preferably about 45 W, an etchant gas flowrate of from about 30 standard cubic centimeters per minute (sccm) to about 150 sccm, preferably about 50 sccm, and for a period of about 30 seconds. 
         [0023]    Etch step  2  may be a Soft Landing Etch (SL1) using a combination of HBr and HeO 2  as etchant gases. SL1 may be performed at a pressure of about 5 mT, source power of from about 100 W to about 500 W, preferably about 350 W, bias power of from about 20 W to about 100 W, and preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 15 seconds. 
         [0024]    Etch step  3  may comprise a second Breakthrough Etch (BT2) using a combination of HBr and HeO 2  as etchant gases. BT2 may be performed at a pressure of about 4 mT, source power of from about 100 W to about 500 W, preferably about 350 W, bias power of from about 30 W to about 150 W, preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 13 seconds. 
         [0025]    Etch step  4  may comprise a second Soft Landing Etch (SL2) using a combination of HBr and HeO 2  as etchant gases. SL2 may be performed at a pressure of about 5 mT, source power of from about 100 W to about 500 W, preferably about 350 W, bias power of from about 20 W to about 100 W, preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 15 seconds. 
         [0026]    Etch step  5  may comprise a third Breakthrough Etch (BT3) using 50 CF 4  as the etchant gas. BT3 may be performed at a pressure of about 4 mT, source power of from about 100 W to about 500 W, preferably about 300 W, bias power of from about 30 W to about 150 W, preferably about 45 W, CF 4  flowrate of from about 30 sccm to about 150 sccm, preferably about 50 sccm, and for a period of about 13 seconds. 
         [0027]    Etch step  6  may comprise a third Soft Landing Etch (SL3) using a combination of HBr and HeO 2  as etchant gases. SL3 may be performed at a pressure of about 5 mT, source power of from about 100 W to about 500 W, preferably about 350 W, bias power of from about 20 W to about 100 W, preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 15 seconds. 
         [0028]    Etch step  7  may comprise a fourth Breakthrough Etch (BT4) using CF 4  as etchant gas. BT4 may be performed at a pressure of about 4 mT, source power of from about 100 W to about 500 W, preferably about 300 W, bias power of from about 30 W to about 150 W, preferably about 45 W, CF 4  flowrate of from about 30 sccm to about 150 sccm, preferably about 50 sccm, and for a period of about 13 seconds. 
         [0029]    Etch step  8  may comprise a fourth Soft Landing Etch (SL4) using a combination of HBr and HeO 2  as etchant gases. SL4 may be performed at a pressure of about 5 mT, source power of from about 100 W to about 500 W, preferably about 350 W, bias power of from about 20 W to about 100 W, preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 15 seconds. 
         [0030]    Etch step  9  may comprise a fifth Breakthrough Etch (BT5) using CF 4  as etchant gas. BT5 may be performed at a pressure of about 4 mT, source power of from about 100 W to about 500 W, preferably about 300 W, bias power of from about 30 W to about 150 W, preferably about 45 W, CF 4  flowrate of from about 30 sccm to about 150 sccm, preferably about 50 sccm, and for a period of about 13 seconds. 
         [0031]    Etch step  10  may comprise a fifth Soft Landing Etch (SL5) using a combination of HBr and HeO 2  as etchant gases. SL5 may be performed at a pressure of about 5 mT, source power of from about 100 W to about 500 W, preferably about 300 W, bias power of from about 20 W to about 100 W, preferably about 36 W, HBr flowrate of from about 100 sccm to about 300 sccm, preferably about 200 sccm, HeO 2  flowrate of from about 10 sccm to about 30 sccm, preferably about 23 sccm, and for a period of about 10 seconds 
         [0032]    Etch step  11  may comprise a sixth Break Through Etch (BT6) using CF 4  as etchant gas. BT6 may be performed at a pressure of about 4 mT, source power of from about 100 W to about 500 W, preferably about 300 W, bias power of from about 30 W to about 150 W, preferably about 45 W, CF 4  flowrate of from about 30 sccm to about 150 sccm, preferably about 50 sccm, and for a period of about 13 seconds. 
         [0033]    The results of ladder etch steps  1 - 11  can be seen in  FIG. 7 , in which the BARC layer  28  and a portion of the polysilicon layer  26  (to a point just below the top of the word lines  24 ) are removed. The ladder etch can be seen to create a smooth curved profile. Referring to  FIG. 8 , a thin dielectric layer  30  is then applied over the etched polysilicon layer  26 . In one embodiment, the dielectric layer  30  comprises SiON deposited to a thickness of about 660 Å. Acceptable alternative dielectric materials for use as layer  30  may be SiN, CVD amorphous carbon, or the like. A masking layer  32  is then applied over the dielectric layer  30 . The polysilicon layer  26  is then etched using a dry etching technique (e.g., hardmask etch, in situ O 2  ashing, then poly etching) to provide a trench  34  between adjacent word lines  24  ( FIG. 9 ). As can be seen, the dielectric (SiON) layer  30  and polysilicon layer  26  adjacent to the trench  34  (identified as area “PP”) are free from abnormalities. 
         [0034]    In a further step, a layer of SiN (i.e., an SiN cap layer) may be applied to protect the peripheral logic elements on the chip. In one embodiment, this SiN cap layer may be about 1600 Å thick. If this step is performed, then the SiN layer would be applied after the step of depositing the polysilicon layer  26 . This SiN layer would be removed after the ladder etching steps are performed. 
       ADVANTAGES OF THE INVENTION 
       [0035]    The inventive process is simple and thus can be implemented at low cost. Further, a word line polysilicon layer having a curved surface is easy to be identified. That is, the curved surface created by the inventive technique is distinguishable from the flat-plate type profile that typically results from CMP processes. Additionally, the inventive process solves the problems associated with abnormal poly profiles in embedded flash memory cells. 
         [0036]    While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope and range of equivalents of the appended claims.