Browse Prior Art Database

SEM Slope Compensation Device

IP.com Disclosure Number: IPCOM000102540D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 119K

Publishing Venue

IBM

Related People

Via, G: AUTHOR

Abstract

In VLSI, both process development and subsequent wafer manufacturing are assisted by SEM (scanning electron microscope) observations: either made in-line, while the wafers are being processed or in a post-mortem failure-analysis mode. Photoresist, polysilicon, polyimide and metal are evaluated in the SEM usually in the secondary electron emission mode: often, low voltages of the order of 1,000 volts are used to minimize surface charging and avoid damage to the features being observed. Seldom do the lines to be measured have vertical walls, substantial positive and negative slopes can be present, making the measurement difficult to the interpretation, due to edge effects which result in signal blooming leading to poor image definition.

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SEM Slope Compensation Device

       In VLSI, both process development and subsequent wafer
manufacturing are assisted by SEM (scanning electron microscope)
observations: either made in-line, while the wafers are being
processed or in a post-mortem failure-analysis mode.  Photoresist,
polysilicon, polyimide and metal are evaluated in the SEM usually in
the secondary electron emission mode: often, low voltages of the
order of 1,000 volts are used to minimize surface charging and avoid
damage to the features being observed.  Seldom do the lines to be
measured have vertical walls, substantial positive and negative
slopes can be present, making the measurement difficult to the
interpretation, due to edge effects which result in signal blooming
leading to poor image definition.

      VLSI processes do require measurements of the slopes, as it is
the case of metal lift-off process where negative slopes are needed.
Presently, the slopes are measured using cross-sectional techniques:
the wafer is sacrificed, the chips separated, cleaved, mounted on a
special holder and observed at edge-wise at 90-degree tilt.  This
method requires the sacrifice of a wafer, extensive sample
preparation without the guarantee of success, due to the uncertainty
of the location of the cross-sectioning breakage.

      This article introduces a new investigating technique,
performed with a novel piece of hardware, that can be added to a
standard electron microscope stage.  Once in use, this apparatus
permits the direct observation/measurement of slopes, independently
from thickness of the topography, and without requiring the
destructive analysis of the wafer. One of the immediate benefits is
the assessment of the slope uniformity along a linewidth, measurement
formerly impossible due to the nature of the fracture method which
gives only one slope value on a single geometric cross-section.  In
addition, the wafer processing can be continued to assess the
influence of a specific slope to the subsequent process parameters
and device performance.

      Other benefits are the direct correlation of measured slopes
with slope values predicted by modeling of electron interaction with
solids, the verification of predicted charging effects and precise
edge location for linewidth measurements.  Models of interaction can
be verified and improved, and specific SEM tools responses can be
tailored in function of accelerating voltages and structural stacks
of different materials, so that precise linewidths measurements can
be made using video threshold techniques. Direct observation of the
slopes permits the evaluation of the validity of existing models
dealing with predicted interaction of energetic electrons with matter
of different atomic number: thus, new model...