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EPL Mask Modeling Update

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					                                                                       EPL Masks
                                        Presented at MNE 2000



                 EPL Mask Modeling Update
       Modeling Mask Fabrication and Pattern
     Transfer Distortions for EPL Stencil Masks
                                            Phil Reu

                             Computational Mechanics Center
                           Department of Mechanical Engineering
                             University of Wisconsin - Madison


                                   Acknowledgements
                      Chris Magg, NGL Mask Center of Competency
                     Michael Lercel, NGL Mask Center of Competency

                                        December 7, 2000


              This research has been supported by SEMATECH, SRC, and DARPA
Mask Fabrication and Pattern Transfer          1                         P. Reu
                                                                      EPL Masks




                   Motivation for the Research

 •   EPL stencil technology is one of the principal candidates for Next-Generation
     Lithography (NGL). The creation of a low distortion mask is essential to reach
     the sub-100 nm nodes.

 •   This research focuses on the distortions induced during mask fabrication and
     pattern transfer for the 4-in. format stencil mask. A comparison between the
     wafer and membrane flow manufacturing processes was completed.

 •   A parametric study was conducted by varying the stress levels of the films to
     determine their effect on in-plane distortion (IPD).

 •   Predictive studies were performed on both a uniform isotropic pattern and the
     IBM Falcon SRAM pattern.



Mask Fabrication and Pattern Transfer   2                               P. Reu
                                                                         EPL Masks


                 Mask Fabrication Process Flow
                          Processing Steps in FE Simulation

Membrane Flow                                       Wafer Flow
                      1. Membrane layer,                         1. Membrane layer,
                         Backside layer, and                        Backside layer, and
                         Hardmask layer formation                   Hardmask layer formation

                      2. Backside litho/etch                     2. Backside litho/etch


                      3. Membrane etch                           3. Resist coat


                      4. Resist coat                             4. Resist pattern


                      5. Resist pattern                          5. Hardmask etch


                      6. Hardmask etch                           6. Stencil etch


                      7. Stencil etch                            7. Membrane etch



 Mask Fabrication and Pattern Transfer         3                            P. Reu
                                                                    EPL Masks




                 EPL Stencil Modeling Parameters

                                          Parametric Study
      Layer(s)          Thickness (µm)                        Nominal Stress (MPa)
                                         Stress Range (MPa)

    Silicon Wafer            625.0              N/A                   N/A

Membrane / Backside           2.0              1 – 20                 20

     Hardmask                 0.5              1 – 20                 20

       Resist                 0.4               N/A                   20




       • Parametric study indicates the stress ranges used in the FE
         simulations.
       • Material properties were taken from published data.



Mask Fabrication and Pattern Transfer    4                            P. Reu
                                                               EPL Masks




                         EPL Stencil Test Mask

• Finite element simulations were                                 17×17 Window
  performed for each of the                                       Array

  fabrication process steps. The
  element birth/death option was        A                          A
  employed to simulate the
  deposition or removal of a given
  layer.

                                                                   ∅ 100 mm
• Equivalent modeling techniques
  were used to simulate pattern-
  specific distortions, including the   1.1 mm                  0.984 mm
  IBM Falcon SRAM pattern.
                                                               0.1087 mm
                                                 Section A-A




Mask Fabrication and Pattern Transfer   5                        P. Reu
                                                                                                            EPL Masks



                     IBM Falcon Pattern Layout

• Two patterns were modeled for the mask     1.1 8.8   8.8   8.8   7.7   7.7   12.1
                                                                                      110.0


                                                                                              12.1   7.7   7.7   8.8   8.8   8.8 1.1

  fabrication and pattern transfer:




                                                                                                                                       4.5
                                             0    0     0     0    0      0     0              0      0    0     0     0     0   0



    – A uniform isotropic pattern




                                                                                                                                       9.1
                                             0    6    50    20    14    20     5              5     20    14    20    50    6   0



    – IBM Falcon SRAM pattern

• The Falcon SRAM layout is shown on
  the right. The center region consists of




                                                                                                                                       51.4
                                                                                                                                              78.7
                                             0                               Orthotropic - Like                                  0
                                                                         Pattern Coverage ~ 25 %
  orthotropic-like pattern features. The
  second row from the top and bottom have
  a uniform isotropic region with the
  percent void fractions as indicated.




                                                                                                                                       9.1
                                             0    6    50    20    14    20     5              5     20    14    20    50    6   0




                                                                                                                                       4.5
                                             0    0     0     0    0      0     0              0      0    0     0     0     0   0

• The Falcon pattern was modified to fit
  within the 4-in. format stencil mask.
                                                              Ref.: C. Brooks, IBM



Mask Fabrication and Pattern Transfer   6                                                                        P. Reu
                                                                       EPL Masks



                           IBM Falcon Features
                         SEM of Orthotropic-Like Pattern




     • Pattern is characterized by directionally-dependent stiffness/compliance.
     • These properties are essentially orthotropic.
     • FE models of a “unit cell” were used to numerically determine the
       equivalent modulus values for simulating pattern transfer distortions.

Mask Fabrication and Pattern Transfer    7                               P. Reu
                                                         EPL Masks



             FE Model of the EPL Stencil Mask




                            Quarter-Symmetry Test Mask

Mask Fabrication and Pattern Transfer   8                 P. Reu
                                                                                                                 EPL Masks


                                 Out-of-Plane Distortion Results
                                 Membrane vs. Wafer Flow – Falcon Pattern
                                 5000


                                 4500


                                 4000


                                 3500
              Maximum OPD (nm)




                                 3000


                                 2500


                                 2000


                                 1500
                                                          Membrane Flow
                                 1000                     Wafer Flow

                                 500


                                   0
                                        Step 1   Step 2   Step 3        Step 4        Step 5   Step 6   Step 7
                                                                   Fabrication Step

     • Maximum OPD for the entire mask was tracked for each of the
       processing steps outlined on Slide 3. Nominal mask parameters utilized.
     • The max OPD for the membrane flow is greater than that of the wafer
       flow due to the etching of the windows before removal of the resist and
       the hardmask.
Mask Fabrication and Pattern Transfer                                   9                                         P. Reu
                                                                                                                EPL Masks


                                       In-Plane Distortion Results
                                  Membrane vs. Wafer Flow – Falcon Pattern
                                  70



                                  60



                                  50
               Maximum IPD (nm)




                                  40



                                  30



                                  20


                                                          Membrane Flow
                                  10
                                                          Wafer Flow


                                   0
                                       Step 1   Step 2   Step 3        Step 4        Step 5   Step 6   Step 7
                                                                  Fabrication Step


    • Maximum pattern area IPD was tracked for each of the processing steps
      outlined on Slide 3. Nominal mask parameters utilized.
    • The IPD results are dominated by the wafer bow throughout the
      manufacturing process.

Mask Fabrication and Pattern Transfer                                 10                                         P. Reu
                                                                                                                                                     EPL Masks


                                                    Stress Sensitivity Study
                                          Falcon Pattern – No Magnification Correction
                            30                                                                                        30




                            25                                                                                        25




                                                                                          Pattern Transfer IPD (nm)
Pattern Transfer IPD (nm)




                            20                                                                                        20
                                                                     Membrane Flow                                                                          Membrane Flow

                                                                     Wafer Flow                                                                             Wafer Flow
                            15                                                                                        15




                            10                                                                                        10



                             5                                                                                        5



                             0                                                                                        0
                                 1          5         10        15            20                                           1     5         10          15                20
                                                 Stress (MPa)                                                                         Stress (MPa)

                                     Membrane/Backside Layer Variation                                                         Hardmask Layer Variation

            • Pattern transfer IPD is defined as the difference between the modeling solution of
              resist patterning and the mask solution.
            • The stress levels of the membrane/backside layers and the hardmask layer were
              varied independently over a range of 1 to 20 MPa (while the other parameters
              were at nominal values).
            • The wafer flow distortion decreases with increased stress due to the domination
              of the OPD effects on IPD.
Mask Fabrication and Pattern Transfer                                                11                                                               P. Reu
                                                                  EPL Masks

                           Pattern Transfer IPD
                        Membrane Flow – Falcon Pattern




       Uncorrected IPD                           Corrected IPD
• The Falcon pattern IPD showed strong radial characteristics allowing for the
  effective use of magnification correction. The IPD was reduced from 27 nm to 5
  nm by magnification correction (mask level). Nominal stress values were used.
• The correction scheme used was an independent x and y axis best fit linear
  interpolation scheme.

Mask Fabrication and Pattern Transfer   12                          P. Reu
                                                                      EPL Masks

                           Pattern Transfer IPD
                      Membrane Flow – Isotropic Pattern




        IPD Contour Plot                            IPD Vector Map
 • The uniform isotropic pattern used a void fraction of 25% over all of the window
   areas. The nominal stress values were used for this model.
 • The FE contour plot of the pattern area shows a maximum pattern transfer IPD of
   27 nm (mask level). FE scale is in cm.
 • Distortions are strongly radial in nature and with magnification correction the
   final max IPD is reduced to 3 nm (mask level).
Mask Fabrication and Pattern Transfer   13                              P. Reu
                                                                     EPL Masks


                       Summary of FE Results
                       Pattern Transfer IPD – Mask Level

                                        Membrane Flow         Wafer Flow
               Case
                                  Uncorrected Corrected Uncorrected Corrected

         Isotropic Pattern              27 nm        3 nm   8 nm      1.8 nm

          Falcon Pattern                27 nm        5 nm   8 nm      1.9 nm
         Falcon Pattern
       Membrane/Backside                22 nm        2 nm   14 nm     0.1 nm
        Stress of 1 MPa
          Falcon Pattern
            Hardmask                    16 nm        4 nm   4 nm      1.8 nm
         Stress of 1 MPa

        • Nominal mask parameters have been used unless otherwise indicated.


Mask Fabrication and Pattern Transfer           14                    P. Reu
                                                                           EPL Masks




                    Summary and Conclusions
 • In general, IPD is caused primarily by the OPD (or wafer curvature) induced
   during mask fabrication.

 • The radial nature of the distortions allow for effective use of magnification
   correction.

 • The Falcon pattern, when broken up with no gradients within a window, yields
   only slightly larger distortions as compared with the isotropic patterns.

 • In general, lower film stresses yield lower pattern transfer distortions.

 • The wafer flow process yields lower pattern transfer distortions in all cases, for
   both the stress variations and the different patterns.

 • Future modeling efforts will include the effects of gravitational loading as well as
   mounting.



Mask Fabrication and Pattern Transfer      15                                  P. Reu

				
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