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Circular Thin Film Resistors

IP.com Disclosure Number: IPCOM000120789D
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 195K

Publishing Venue

IBM

Related People

Moksvold, T: AUTHOR

Abstract

With the circular thin film resistors (CTFR) design described in this invention, "fraction of a square" resistors can be made without paralleling. It enables the thin film resistor (TFR) SiCr sheet resistance, film thickness and film composition to be selected and optimized based on process, laser trim (if used), quartz integrity, stability, yield and ease of implementation considerations and not by the need to cover low value resistors. "Low sheet resistance/parallel resistors" is the prior-art approach to making low value - less than one square - resistors. Thus, conventional TFR design requires process and design compromises if a wide range of resistor values are to be covered. CTFR enables low value resistors to be made from a wide range of sheet resistances as shown in the table below for the resistors in Fig.

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Circular Thin Film Resistors

      With the circular thin film resistors (CTFR) design
described in this invention, "fraction of a square" resistors can be
made without paralleling.  It enables the thin film resistor (TFR)
SiCr sheet resistance, film thickness and film composition to be
selected and optimized based on process, laser trim (if used), quartz
integrity, stability, yield and ease of implementation considerations
and not by the need to cover low value resistors.  "Low sheet
resistance/parallel resistors" is the prior-art approach to making
low value - less than one square - resistors.  Thus, conventional TFR
design requires process and design compromises if a wide range of
resistor values are to be covered.  CTFR enables low value resistors
to be made from a wide range of sheet resistances as shown in the
table below for the resistors in Fig. 1:
                                  Sheet Resistance (ohm/square)
          rl   r2   r2/rl         200       300        400   500
          (um) (um)               -----Resistance  (ohm)-----
CTFR4     175  250  1.429         11.4      17.0      22.7   28.4
CTFR3     150  250  1.667         16.3      24.4      32.5   40.7
CTFR2     125  250  2.0           22.1      33.1      44.1   55.2
CTFR1     100  250  2.5           29.2      43.8      58.2   72.9
CTFR Design: (see Fig. 1)

      The resistor is defined by two third level metal contacts which
have concentric circular leading edges causing the current to flow
radially through the SiCr annulus formed by the contacts which
overlap the SiCr film. Contact to the circular inside CTFR contact is
made by standard second to third level via.  Contact to the outside
CTFR contact is made by standard third level wiring.
CTFR Model:  (see Figs. 1 and 2)

      The simplicity and symmetry of the design and the radial
current flow makes the modeling of the CTFR straightforward.  The
nominal resistance is given by:
   R  = (RS/2PI) x 1n(r2/rl) (ohm)
where  RS = sheet resistance in ohm/square
       PI = 3.1415926..
       1n = natural logarithm
       rl = radius of the leading edge of the inside contact
       r2 = radius of the leading edge of the outside contact

      The resistor tolerances are a function of the sheet resistance
of the resistiv...