Abstract:
The invention relates to a method of adjusting the critical dimensions of a poly-silicon or amorphous silicon gate in an MOS transistor structure. In an example embodiment, there is a method for controlling critical dimensions on a wafer substrate, the wafer substrate comprising a silicon layer, an oxide layer, a poly-silicon layer, and an organic bottom anti-reflective coating (BARC) layer. The method comprises defining features on the organic BARC layer with a masking layer, the features having masking critical dimensions. With a first etch, unmasked areas on the organic BARC layer are etched until the poly-silicon layer is exposed. The first etch defines after-etch critical dimensions of the features.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to semiconductor process. More particularly the invention relates to critical dimension (CD) control of printed features on a wafer substrate.  
         BACKGROUND  
         [0002]    The electronics industry continues to rely upon advances in semiconductor technology to realized higher-function devices in more compact areas. For many applications, realizing higher-functioning devices requires integrating a large number of electronic devices into a single silicon wafer. As the number of electronic devices per given area of the silicon wafer increases, the manufacturing process becomes more difficult.  
           [0003]    A large variety of semiconductor devices has been manufactured having various applications in numerous disciplines. Such silicon-based semiconductor devices often include metal-oxide-semiconductor (MOS) transistors, such as p-channel MOS (PMOS), n-channel MOS (NMOS) and complementary MOS (CMOS) transistors, bipolar transistors, BiCMOS transistors.  
           [0004]    Each of these semiconductor devices generally includes a semiconductor substrate on which a number of active devices are formed. The particular structure of a given active device can vary between device types. For example, in MOS transistors, an active device generally includes source and drain regions and a gate electrode that modulates current between the source and drain regions.  
           [0005]    One important step in the manufacturing of such devices is the formation of devices, or portions thereof, using photolithography and etching processes. In photolithography, a wafer substrate is coated with a light-sensitive material called photo-resist. Next, the wafer is exposed to light; the light striking the wafer is passed through a mask plate. This mask plate defines the desired features to be printed on the substrate. After exposure, the resist-coated wafer substrate is developed. The desired features as defined on the mask are retained on the photo resist-coated substrate. Unexposed areas of resist are washed away with a developer. The wafer having the desired features defined is subjected to etching. Depending upon the production process, the etching may either be a wet etch, in which liquid chemicals are used to remove wafer material or a dry etch, in which wafer material is subjected to a radio frequency (RF) induced plasma.  
           [0006]    Often desired features have particular regions in which the final printed and etched regions have to be accurately reproduced over time. These are referred to as critical dimensions (CDs). As device geometry approaches the sub-micron realm, wafer fabrication becomes more reliant on maintaining consistent CDs over normal process variations. The active device dimensions as designed and replicated on the photo mask and those actually rendered on the wafer substrate have to be repeatable and controllable. In many situations, the process attempts to maintain the final CDs equal to the masking CDs. However, imperfections in the process or changes in technology (that may be realized in a given fabrication process, if the process were “tweaked”) often necessitate the rendering of final CDs that deviate from the masking CDs.  
         SUMMARY OF THE INVENTION  
         [0007]    There is a need for a photolithographic process that enables the user to adjust the final CDs in features printed on a wafer substrate that deviate from the masking CDs. In an example embodiment, there is a method for controlling critical dimensions on a wafer substrate, the wafer substrate comprising a silicon layer, an oxide layer, a poly-silicon layer, and an organic bottom anti-reflective coating (BARC) layer. The method comprises defining features on the organic BARC layer with a masking layer, the features having masking critical dimensions. With a first etch, unmasked areas on the organic BARC layer are etched until the poly-silicon layer is exposed. The first etch defines after-etch critical dimensions of the features. A feature of this embodiment is that the first etch may be selected to bias the after-etch critical dimensions in either a positive direction or a negative direction.  
           [0008]    In another example embodiment, there is a method for adjusting critical dimensions on a poly-silicon gate structure. The gate structure comprises a silicon substrate, an oxide layer, a poly-silicon layer, and a BARC layer. The method comprises defining features that have printed critical dimensions, on the BARC layer of the poly-silicon gate structure with a mask layer. The mask layer has critical dimensions. With an etch to adjust the printed critical dimensions, the BARC layer is etched with an etch. The etch is comprised of at least one of the two etches. A first BARC etch biases the printed critical dimensions greater than the mask layer critical dimensions. A second BARC etch biases the printed critical dimensions less than the mask layer critical dimensions. 
       
    
    
       [0009]    The above summaries of the present invention are not intended to represent each disclosed embodiment, or every aspect, of the present invention. Other aspects and example embodiments are provided in the figures and the detailed description that follows.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a flowchart outlining the process according to an embodiment of the present invention;  
         [0012]    [0012]FIG. 1A is a flowchart outlining the process of FIG. 1 with a focus on the etching of the BARC layer for CD adjust;  
         [0013]    [0013]FIG. 2A illustrates a cross-section of a substrate to be processed according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 2B illustrates the cross-section of FIG. 2A with a mask having features with a critical dimension (CD);  
         [0015]    [0015]FIG. 2C illustrates the enlarging of an after etch CD with respect to the mask CD according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 2D illustrates the reducing of a after etch CD with respect to the mask CD according to another embodiment of the present invention;  
         [0017]    [0017]FIG. 2E illustrates a final CD with respect to the mask CD after the removal of the masking layer and BARC layer of FIG. 2C;  
         [0018]    [0018]FIG. 3A is a plot of Pre-Etch and Post-Etch CDs and the Δ Pre/Post Etch CD with a first BARC etch chemistry according to the present invention;  
         [0019]    [0019]FIG. 3B is a plot of Pre-Etch, Post Etch CDs v. Increased Overetch with a second BARC etch chemistry according to the present invention; and  
         [0020]    [0020]FIG. 3C is a plot of ΔCD v. Overetch Time with a combination chemistry of the first BARC etch chemistry and the second BARC etch chemistry according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The present invention has been found to be useful in the rendering of final CDs that differ from those on the photo mask. The present invention has been found to be particularly useful in situations in which changes in fabrication processes result in CD changes make it not cost-effective to replace photo masks in response to those changes.  
         [0022]    An organic BARC (Bottom Anti-Reflective Coating) is applied to a wafer substrate upon which poly-silicon or amorphous silicon (α-silicon) has been deposited. The features to be printed are masked. The mask has particular CDs to be rendered onto the poly-silicon. The wafer substrate is plasma etched for a predetermined time. The type of etch depends upon which direction the final CDs are biased, either up or down in relation to the mask CDs. During plasma etch, the gases react with photo resist and other compounds on the wafers to form long molecular chains (polymers) containing carbon, hydrogen, and other elements. These polymers deposit on the sidewall of the poly-silicon lines being etched. Depending upon the type of gases used in the reaction, these polymer chains are either formed or are removed. Organic BARC is applied like photo resist, with a spin-on process. There are different organic BARC formulations supplied by different vendors, but essentially they have the same optical and etch properties. Similar etch chemistries will etch them all.  
         [0023]    Refer to FIG. 1. In an example process according to the present invention, there is a process  100 . On a silicon substrate, upon which a thin dielectric is applied (usually a silicon oxide), poly-silicon (or α-silicon) is applied to the substrate  105 . A BARC layer is applied on the poly-silicon  110 . Next, photo resist is applied onto the BARC layer  115 . With a mask layer, the features are defined  120 . Usually, the photo resist coated wafer substrate is loaded into a wafer stepper and the features of a photo mask are printed thereon. The wafer substrate is developed to render the features to be etched. Often, as a means of monitoring process quality, the user measures the CDs of the defined features  125  after the photo resist is developed. The desired CDs of the defined features are calculated  130 . For the CD adjust  135 , the BARC layer is etched for a predetermined time. After BARC etch, the poly-silicon is etched for another predetermined time  140 . The BARC and resist is removed  145 . For measuring process quality, the final CDs may be measured  150 .  
         [0024]    In an example embodiment according to the present invention, the CDs may be adjusted upward (i.e., a positive bias) with a first etch.  
         [0025]    In another example embodiment according to the present invention, the CDs may be adjusted downward (i.e., a negative bias) with a second etch. Refer to FIG. 1A. Details of the BARC etch are shown in step  135 . A first BARC etch is used to bias the CD in a positive direction  135   a.  If the CD is too low  135   b,  the etch may continue. If at desired CD, the process verifies whether the CD is too high  135   c.  If too high, a second BARC etch is used to bias the CD in the negative direction  135   d.  Process resumes to that illustrated in FIG. 1. Some example CD etch processes are illustrated in tables that follow.  
         [0026]    A series of figures, illustrates the process according to the present invention. The gate of an MOS transistor is being defined. Refer to FIG. 2A. A substrate  200  has a silicon layer  210 . Upon the silicon layer  210 , there is a dielectric layer  220 , usually an oxide. Upon the dielectric layer  220 , poly or (α-silicon) is applied  230 . The BARC layer  240  is on the poly-silicon layer  230 . The critical dimensions involve the poly-silicon gate of an example MOS transistor that is fabricated.  
         [0027]    Refer to FIG. 2B. A mask layer  250  is applied to the BARC layer  240 . The mask layer has and example critical dimension, so labeled as CD Mask . The process  100  of FIG. 1 etches the BARC layer  240 , poly-silicon layer  230 , and dielectric layer  220 . These layers from a stack  260 . Depending upon the bias of the etch process, the profiles of either FIG. 2C or FIG. 2D are attained.  
         [0028]    Refer to FIG. 2C. The unmasked areas  270  have the stack  260  removed. In that the bias is positive, the final feature size  250   a  is greater than the mask feature size  250 . CD After Etch  is greater than CD Mask . The CD After Etch  features are depicted with dashed lines.  
         [0029]    Refer to FIG. 2D. Refer to FIG. 2C. The unmasked areas  270  have the stack  260  removed. In that the bias is negative, the final feature size  250   a  is less than the mask feature size  250 . CD After Etch  is less than CD Mask . Again, the CD After Etch  features are depicted with dashed lines. After the desired after etch CD is obtained, the photo resist layer  250  and organic BARC layer  240  are removed. The final critical dimension of etch, CD Final  may or may not be equal to the CD After Etch . However, this difference may be characterized for the given fabrication process.  
         [0030]    Refer to FIG. 2E. The mask layer  250  and the BARC layer  240  are removed. The substrate  200  has the remaining features of a poly silicon layer  230  over a thin oxide layer  220 . In an example process, these features define the gate regions of a MOS transistor. The CD Final  measurements of the gate regions are taken after the process removes the photo resist and BARC layer that defined them.  
         [0031]    Refer to Table 1. To increase the CDs, the substrate undergoes CF 4  etch. The substrate is placed into a plasma etch apparatus. The reactant gas CF 4  is released into the chamber at about 7 m Torr. After 30 seconds at Step 1, the etch begins and proceeds until a fixed time or endpoint is reached. The fixed time or endpoint would be determined by the process parameters particular to a given manufacturing environment. The CDs are adjusted upward by the over etch of the BARC. The time T 1  to produce the required CDs is characterized for a given fabrication process.  
                                                           TABLE 1                           First Recipe to Increase CDs                1st Recipe   Gas                   CD Increase   stability   Etch   Over Etch           Step   Step 1   Step 2   Step 3                            Pressure (mTorr)   7   7   7           RF_Upper (W)   0   250   250           RF_Lower (W)   3   170   170           Gap (cm)   6.03   8.1   8.1           Cl2 (sccm)   0   0   0           He/O 2  (sccm)   0   0   0           HBr (sccm)   0   0   0           O 2  (sccm)   0   0   10           He           CF 4     100   100   100           SF 6             He-Clamp   8           Completion   Stabilize   Fixed Time   %                   or Endpoint   Overetch           Time (sec)   30       T1                      
 
         [0032]    Refer to Table 2. To decrease the CDs, the substrate undergoes an etch in HBr and O 2 . The substrate is placed into a plasma etch apparatus. The reactant gas is released into the chamber at about 7 m Torr. After 30 seconds at Step 1, the etch begins and proceeds until a fixed time or endpoint is reached. The fixed time or endpoint would be determined by the process parameters particular to a given manufacturing environment. The CDs are adjusted downward by the over etch of the BARC. The time T 2  to produce the required CDs would be characterized for a given fabrication process.  
                                                           TABLE 2                           Second Recipe to Decrease CDs                2nd Recipe   Gas                   CD Reduction   stability   Etch   Over Etch           Step   Step 1   Step 2   Step 3                            Pressure (mTorr)   7   7   7           RF_Upper (W)   0   250   250           RF_Lower (W)   0   140   140           Gap (cm)   6.03   8.1   8.1           Cl 2  (sccm)   0   0   0           He/O 2  (sccm)   0   0   0           HBr (sccm)   20   20   20           O 2  (sccm)   20   20   20           He   0   0   0           CF 4     0   0   0           SF 6             He-Clamp   8   8   8           Completion   Stabilize   Fixed Time   %                   or Endpoint   Overetch           Time (sec)   30       T2                      
 
         [0033]    Refer to Table 3. Rather than have separate etches for increasing the CDs and decreasing CDs, the two etches may be combined into a single process. The first etch increases the CDs for a predetermined time, T 1 . After the first etch, the second etch “dials” in the desired CDs for a second predetermined time, T 2 . By printing and etching test wafers, T 1  and T 2  may be characterized. The test wafers have equivalent CD features similar to those used in product wafers having integrated circuit devices. Results of the characterization enables the user to derive a particular T 1  and T 2  for a given fabrication process.  
                                                           TABLE 3                           Combination Recipe to Dial in CDs            Combo Recipe   Gas   First   Gas   Second       CD Dial-In   stability   Etch   stability   Etch       Step   Step 1   Step 2   Step 3   Step 3                    Pressure (mTorr)   7   7   7   7       RF_Upper (W)   0   250   0   250       RF_Lower (W)   3   170   0   140       Gap (cm)   6.03   8.1   8.1   8.1       Cl 2  (sccm)   0   0   0   0       He/O 2  (sccm)   0   0   0   0       HBr (sccm)   0   0   20   20       O 2  (sccm)   0   0   20   20       He   0   0   0   0       CF 4     100   100   0   0       SF 6         He-Clamp   8   8   8   8       Completion   Stabilize   Fixed Time   Stabilize   %               or Endpoint       Overetch       Time (sec)   30   T1   30   T2                  
 
         [0034]    Refer to FIG. 3A. In an example process, with a first BARC etch chemistry, the pre-etch and post-etch critical dimensions (CDs) along with their corresponding differences have been characterized and are plotted. The etch time had been fixed to ensure complete removal of the BARC with the first recipe. In this process, the post etch CDs are greater than the pre-etch CDs. For the three sample data points, the difference between the pre etch and post-etch is about constant, about 0.02 μm. In a production process, many more data points are collected and analyzed. In an example manufacturing process, these data points may range into the hundreds over many wafer substrates processed over several weeks.  
         [0035]    Refer to FIG. 3B. In another example process, with a second BARC etch chemistry, the pre-etch and post-etch critical dimensions (CDs) along with their corresponding differences are plotted. However, unlike that depicted in FIG. 3A, in which BARC was completely removed, FIG. 3B shows the differences plotted against the percentage of over etch. The second etch decreases the post-etch CDs. For a 20% over etch, the difference between the pre and post-etch CD is about −0.015 μm. For an 80% over etch, the difference is about −0.045 μm.  
         [0036]    In yet another recipe according to an embodiment of the present invention, the recipes as depicted in Tables 1 and 2 and then combined in Table 3 may be characterized and plotted. Refer to FIG. 3C. The plot depicts the CD Change (ΔCD) v. Time. To derive this plot, the etch time for the first recipe was at a fixed T 1 . With the second recipe, the etch time T2 was varied. Thus, the increased CD After Etch  of about 0.012 μm is reduced to CD Mask  (ΔCD=0) and CD After Etch =CD Mask , after about 7 seconds. After about 12 seconds, the CD After Etch  is about −0.012 μm less than CD Mask . Of course, other plots may be derived depending upon particular process characteristics in a manufacturing environment.  
         [0037]    While the present invention has been described with reference to several particular example embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention, which is set forth in the following claims.