A Novel In-situ Methodology to Characterize Bond Pad and by fdh56iuoui

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									      A Novel In-situ Methodology to
   Characterize Bond Pad and Dielectric
Mechanical Behavior during Wafer Level Test



      Cheryl Hartfield             Tom Moore, Ph.D.            Jerry Broz, Ph.D.
 Member, Group Technical Staff           President                Director R & D
   Texas Instruments, Inc.            Omniprobe, Inc.         Point Technologies, Inc.
 13536 N. Central Expressway     7738 Forest Lane, Ste. 426    6859 N. Foothills Hwy
      Dallas, TX 75243               Dallas, TX 75230           Boulder, CO 80302




                        SouthWest Test Workshop - 2003
            Outline
                   Objectives
                   Background
                   Approach
                   In-situ Methodology Development
                   Application of Methodology
                   Summary




Hartfield et al.                                     08-MAY-2003
SWTW - 2003                                               Page 2
            Objectives
                   Develop a methodology to assess the in-situ
                   mechanical behavior of a bond pad metal stack
                     Evaluate the elastic and plastic deformation of bond pads
                     during wafer level testing
                     Determine the deformation limits of the low-k dielectric layers
                        Impact of bond pad reinforcement structures
                        Impact of various bond pad metals

                   Visualize the probe process in real-time as the
                   probe tip scrubs across a bond pad

                   Correlate the probe scrub action with contact
                   resistance, overtravel, and applied strain

Hartfield et al.                                                                  08-MAY-2003
SWTW - 2003                                                                            Page 3
            Background
                   Semiconductor device development and scaling
                      Conversion from aluminum to copper traces and from SiO2 to lower
                      dielectric constant materials
                      Metal stack is a complex multi-layered “sandwich” of metal
                      conductor traces and insulating dielectric materials

                   Potential for damage during fabrication, probe, and
                   assembly may cause long term reliability issues
                      Low-k materials tend to be fragile and susceptible to damage
                         FSG, k~3.3: Modulus 50GPa
                         HSQ, k~3.1: Modulus 4GPa

                   Knowledge of the dielectric / metal stack characteristics
                   and acceptable damage limits are critical
                      Defining wafer level test practices of advanced IC technologies
                      Synergy between test and assembly, i.e., optimized probe practices
                      facilitate improved assembly yield

Hartfield et al.                                                                      08-MAY-2003
SWTW - 2003                                                                                Page 4
            Background (cont.)
                   Overtravel is required to reduce contact resistance (CRES) to
                   an acceptable level during test
                   At the end of overtravel, a small contact area imparts large
                   stresses on the bond pad and the dielectric stack
                      Applied stress can be 75-400 MPa range for various tip sizes (0.6mil –
                      1.2mil) using 64um overtravel, 1.75BCF card

                   Probing an Al-Bond Pad                                         CRES During Probe




                                                      Contact Resistance (ohms)
                                   Scrub

                   Probe Mark



                                                                                   Probe Force / Scrub Length
                             Metal Stack
Hartfield et al.                                                                                                08-MAY-2003
SWTW - 2003                                                                                                          Page 5
            Background (cont.)
                   In-situ visualization of the “scrubbing action” during wafer
                   test is extremely difficult
                      Previous work used an SEM approach with an embedded probe


                                                         PROBE TIP




                       Kister, et al., 2000


                   Want to correlate REAL-TIME in-situ “scrubbing action”
                   with CRES
Hartfield et al.                                                                  08-MAY-2003
SWTW - 2003                                                                            Page 6
            Background (cont.)
                   Current methods of assessing dielectric damage
                      “Lab Tests” that vary the probe conditions
                       PROS
                              High volume, statistically relevant sample size easily generated
                              Easy to generate data on variety of materials (full flow, blanket films)
                              Easy to extrapolate data to test floor
                       CONS
                              Data analysis tedious and time-consuming
                              Dielectric cracks can be difficult to identify on fully processed
                              material and correlate to probing conditions
                              “Production” may have weaknesses not uncovered by “lab tests”
                              High % uncertainty due to wide std .dev. (wide range probe specs)
                      FEM
                       PROS
                              Can run new “test conditions” without generating new Si
                       CONS
                              Model only as good as the input variables.
                              Often relies on data provided by mechanical tests of dielectric films

Hartfield et al.                                                                                  08-MAY-2003
SWTW - 2003                                                                                            Page 7
            Approach
                   Utilize the capabilities of the Omniprobe Analytical Tool
                      Micromanipulator enabled for vacuum environments
                      Configured for mechanical tests using various tungsten probe tips
                      Electrical test resistance and contact resistance
                      Real time video image capture in a Focused Ion Beam (FIB) instrument
                      Applied strain sensing / load monitoring
                     Omniprober Model 100.5        Omniprober mounted on FEI DB235 FIB




Hartfield et al.                                                                         08-MAY-2003
SWTW - 2003                                                                                   Page 8
            Approach (cont.)
                   Video capture and still images
                      Scanning Electron Microscope (SEM) videos synchronized with
                      overtravel experiment
                      Experiments conducted within single beam FIB instrument
                      High resolution SEM images at critical points
                   Overtravel and Fixturing
                      Precise 3-D translational control
                      Customizable “test probe” holding fixtures
                   Electrical Resistance
                      Electrical continuity detection
                      Contact resistance monitoring
                   Strain sensor
                      Applied probe force monitoring
                      High sensitivity to loading changes
                   In-Situ “Lift-Out” for Transmission Electron Microscope
                   (TEM )sample prep and inspection
                      Determine presence of material cracking
                      Reliable, site-specific capability of the Omniprobe tool
Hartfield et al.                                                                    08-MAY-2003
SWTW - 2003                                                                              Page 9
            Approach (cont.)
                    In-Situ Lift-Out to assess dielectric stack cracking

       65um wide section removed by lift-out method       TEM Inspection




                   Optical image        FIB image           TEM image



Hartfield et al.                                                           08-MAY-2003
SWTW - 2003                                                                    Page 10
            Approach (cont.)
                   Overview of Experimental Hardware

                                Vacuum Chamber
                                                                   Synchronized
                                  Electron or                      Video
                                   Ion beam
                                  pole piece
             Secondary
              electron                                                     z
              detector                                    CRES         y
                                                                           x
                                                                   Overtravel
                                                          Strain

              Electron or Ion beam at 0-52o relative to
                plane of the probe tip for best view

Hartfield et al.                                                                08-MAY-2003
SWTW - 2003                                                                         Page 11
            Approach (cont.)
                   Approximate probe orientation in FIB chamber
                                                   Bent probe needle
                                                    in holding fixture
                                                                                   Beam Angle within
                                                                                    Holding Fixture
                                                                                     ~ 5 to 12-deg




                        Substrate Sample


                                           Probe is “overtravelled” into wafer while applied
                                               load and electrical contact is monitored


Hartfield et al.                                                                                 08-MAY-2003
SWTW - 2003                                                                                          Page 12
            Proof of Concept Applications
                   Evaluation of inter-layer dielectrics covered by
                   thin film blanket Cu
                     Three low-k dielectric materials with different mechanical
                     and material properties


                   Evaluation of aluminum capped test die
                     Bond pads with a double thick dielectric
                     Bond pads with dense metal structures


                   Multiple touchdowns on a blanket low-K dielectric
                   material layered on top of silicon


Hartfield et al.                                                                  08-MAY-2003
SWTW - 2003                                                                           Page 13
            Methodology Details
                   Electrochemically polished tungsten probes were
                   mounted into the Omniprobe holding fixture.
                     The probes were bent with a 8-mil tip length
                     The probe tips were electrochemically radiused
                     Tip diameter is less than 1 mil

                   Synchronized data collection
                     Real-time scrubbing action correlated with overtravel, strain,
                     and electrical resistance
                     Wafer is stationary, probe z-axis height adjusted to apply
                     overtravel
                     Surface contact detected with strain gauge and verified with
                     in-situ visual observation
                     Overtravel is initiated 0.1um above the surface

Hartfield et al.                                                                  08-MAY-2003
SWTW - 2003                                                                           Page 14
            Results - ILD Materials (cont.)

                                                           Summary of 70um Overtravel Scrubs
                                                9000
                                                8000

                                                7000

                                                6000                                      ILD1 CRes
                                                                                          ILD1 Strain




                                   CRes (Arb)
                                                5000                                      ILD2 CRes
                                                4000                                      ILD2 Strain
                                                                                          ILD3 CRes
                                                3000
                                                                                          ILD3 Strain
                                                2000

                                                1000
                                                  0
                                                       0      17      34       52    66
                                                                   Overtravel (um)




          Breaking through metal                       Comparison of 3 ILD Mat’l
               surface layer                              Strain and CRES
Hartfield et al.                                                                               08-MAY-2003
SWTW - 2003                                                                                        Page 15
                        Results - Excessive Overtravel Test
                                                  03-021 Strain                                                   Strain
                        Low-K Dielectric Film (1um) on Si: Strain Measurements
                                                                                                                  monitored while
                          4                                                                                       overtravel
                                                                                                                  applied in
                                                                                      holding at ~ 460um
  Strain Measurements




                        3.5
                                   Applied overtravel                                                             successive
                                                                                           Probe needle was
                          3                                                                visually deformed      forward steps.
                                                        ~400um                             at full overtravel
                        2.5
                                                                                                                  Tip is already
                                                                              Hypothesis: strain
                          2                             ~300um
                                                                              relaxation events occur             into Si well
                                                                              immediately after severe            before 100um
                        1.5                                                   deformation of tip
                                               200um                                                              overtravel.
                          1                                        Slope changes (red line to
                                    100um                          yellow line) – hypothesis:                     Strain data may
                        0.5                                        slope affected by probe tip
                                                                   deformation                                    be able to
                          0                                                                                       reveal when
                               0       100      200       300    400    500        600     700      800     900   probe tip
                        -0.5
                                                                                                                  deformation
                                                                                                                  occurs.

Hartfield et al.                                                                                                           08-MAY-2003
SWTW - 2003                                                                                                                    Page 16
            Results – Aluminum Bond Pads
                   High Res SEM images – 0 to 100um of overtravel




Hartfield et al.                                                    08-MAY-2003
SWTW - 2003                                                             Page 17
            Results – Aluminum Bond Pads
             Scrub mark visualization and CRES vs. Overtravel


                                                     3

                                                    2.5

                                                     2




                                       CRes (Arb)
                                                    1.5

                                                     1

                                                    0.5

                                                     0
                                                          0   0.6   1.2   1.8   2.4   3    3.6   4.2   4.8   5.4   6
                                                                                  Overtravel (um)




          Stable contact occurs at ~6um overtravel with a clean radiused tip

Hartfield et al.                                                                                               08-MAY-2003
SWTW - 2003                                                                                                        Page 18
            Results – Metallized Substrate (cont.)




                     Real-time probe scrub visualization
                   on a non-aluminum metallized substrate




Hartfield et al.                                            08-MAY-2003
SWTW - 2003                                                     Page 19
            Results – Metallized Substrate (cont.)




Hartfield et al.                                     08-MAY-2003
SWTW - 2003                                              Page 20
            Results – Copper Substrate (cont.)




                   Real-time probe scrub visualization on Cu
                   showing electrostatic debris interactions




Hartfield et al.                                               08-MAY-2003
SWTW - 2003                                                        Page 21
            Results – Copper Substrate (cont.)




Hartfield et al.                                 08-MAY-2003
SWTW - 2003                                          Page 22
            Results – Aluminum Bond Pads (cont.)




                   Real-time probe scrub visualization
                   on an aluminum capped bond pad




Hartfield et al.                                         08-MAY-2003
SWTW - 2003                                                  Page 23
            Results – Aluminum Bond Pads (cont.)




Hartfield et al.                                   08-MAY-2003
SWTW - 2003                                            Page 24
            Conclusions / Future Work
                   A methodology has been developed for in-situ probe
                   scrubbing action visualization combined with synchronized
                   force and electrical measurements.

                   Using this method a clearer understanding of probe effects
                   to the materials under test as a function of material (probe
                   or sample) composition, probe tip shape, etc., can be
                   developed.

                   Future work……
                      Calibration of strain
                      In-situ crack detection
                      Blade assemblies with variable probe force (0.5 – 3 g/mil)
                      Flat tip vs. radius tip shape comparison
                      Scrubbing behavior on Al, Cu, and Au pad comparison
                      Continuing Low-K studies

Hartfield et al.                                                                   08-MAY-2003
SWTW - 2003                                                                            Page 25
            Acknowledgements
                   Omniprobe
                     Aaron Smith
                     Rocky Kruger


                   Texas Instruments
                     Pat Jones, DFAB PFA
                     Greg Hotchkiss, SCPD
                     Jason Aronoff, PDT




Hartfield et al.                            08-MAY-2003
SWTW - 2003                                     Page 26

								
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