Precision optical spacers for image sensor filters

A process is described for fabricating spacers of a desired thickness of filters, the spacers to be used in separating the filter from an underlying image sensing device. The process includes the steps of forming a pattern of electrically conductive material on one surface of the filter, depositing dry resist to the desired thickness over all of the filter except on the electrically conductive pattern, depositing additional electrically conductive material on at least the electrically conductive pattern, and removing the dry resist.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to packages for integrated circuits, and in 
particular a process and the resulting structure for fabricating precision 
optical spacers of desired thickness to separate a window or filter from 
an image sensor such as a charged coupled device. 
2. Brief Description of the Prior Art 
Several techniques have been used to fabricate a precision optical spacer 
for separating an image sensing device from a filter or window. One prior 
art technique has been to utilize mechanical spacers, for example, 
precision machined material, to dispose between the image sensor and the 
overlying filter or window. Such techniques have typically been expensive, 
difficult to fabricate in applications where very thin spacers are 
required, and difficult to handle and align. 
Another prior art technique has been to deposit metal or other material 
directly on the surface of the integrated circuit and attempt to form the 
material to the desired thickness. Such techniques require several extra 
masks during the fabrication of the image sensor or other integrated 
circuit, resulting in a lower overall yield. Additionally, such techniques 
make back lapping the substrate on which the integrated circuit or image 
sensor is formed difficult, and frequently result in cracking of the 
sensor during back lapping. 
A third prior art approach has been to design precision packages for 
containing the image sensor. Such packages typically require extremely 
careful manufacturing processes, with accordingly high cost. 
SUMMARY OF THE INVENTION 
The process and resulting structure of this invention overcome the 
disadvantages of the prior art processes and structures. In particular, 
the process of this invention, by fabricating the spacing device directly 
on the window or filter, does not require precision manufactured 
mechanical spacers or precision packages. The difficulties of depositing 
metal or other material on the underlying image sensing device are also 
eliminated. In one embodiment a process for fabricating spacers of desired 
thickness for a filter to be used in separating the filter from an 
underlying integrated circuit structure comprises the steps of: forming a 
pattern of electrically conductive material on one surface of the filter, 
the electrically conductive material being substantially thinner than the 
desired thickness; depositing selected material over all of the filter 
except on the electrically conductive material, the selected material 
being deposited to substantially the desired thickness; depositing 
additional electrically conductive material on at least the pattern of 
electrically conductive material; and removing the selected material. 
In another embodiment a package for an image sensing device comprises: a 
filter, an image sensing device including bonding pads; an electrically 
conductive pattern disposed on the filter and separating the filter from 
the image sensing device, the pattern disposed on the filter being nowhere 
in contact with the bonding pads.

DETAILED DESCRIPTION 
FIG. 1 is a cross-sectional view of a typical integrated circuit package 
showing the substrate 10, a portion of electrically connecting pins 12, 
integrated circuit 15, bonding pads 17 and bonding wires 18. It is an 
object of this invention to provide a process for fabricating a structure 
to enable attaching a window or filter to the upper surface of circuit 15 
at a precise spacing above the upper surface of circuit 15. In the 
preferred embodiment circuit 15 will be an image sensing device, for 
example a charge coupled device. 
FIG. 2 is a top view of the structure shown in FIG. 1 with corresponding 
portions given corresponding designations. For purposes of explanation of 
the invention it is assumed that a window or filter having length X and 
width Y is desired to be disposed parallel to image sensor 15 and at a 
fixed spacing away from sensor 15. It is further assumed that the filter 
or window to be disposed above circuit 15 is not to contact the 
semiconductor surface. Contact with the semiconductor surface is 
particularly undesirable in conjunction with charge coupled devices which 
are known to be extremely surface sensitive. 
FIG. 3 shows an array of window or filter planforms chosen which have 
dimensions which fall within the bonding pad region of the circuit shown 
in FIG. 2. The array of windows or filters 20 will be an appropriate 
multiple of the X and Y dimensions such that a multiplicity of filters 20 
may be processed simultaneously. The convenient multiple may be chosen 
depending upon the dimensions of the process equipment used. As used 
herein, the terms filter and window are intended to be interchangeable. 
The terms are intended to refer to any piece of material which is desired 
to be disposed a fixed distance from an underlying integrated circuit or 
image sensor. 
Next, as shown in FIG. 4 an adherent electrically conductive pattern 23 is 
formed on the surface of the array of filters 20. The adherent 
electrically conductive coating may be any desired coating. In the 
preferred embodiment layers or films of chromium with copper and chromium 
with gold have been found suitable. The metal is typically deposited to a 
thickness of approximately 5000 to 10,000 angstroms, for example using 
evaporation or sputtering techniques. The pattern may be defined using any 
suitable method, for example, well known photolithographic masking 
techniques in conjunction with photoresist and etching processed, or 
resist lifting techniques. It is essential, however, that the metal 
pattern 23 be fabricated using a platable metal. 
After the metal grid 23 has been formed, a layer of dry film resist, for 
example, Riston manufactured by Dow Chemical Corporation, is deposited 
everywhere on each filter 20 except where the grid portion 23 is formed. 
The dry resist is deposited to whatever thickness is desired for the 
separation between filter 20 and intergrated circuit 15. In the preferred 
embodiment in conjunction with a charge coupled device, resist 25 will be 
approximately 0.5 to 1.0 mils thick. 
As also shown in FIG. 6, the grid 23 is electroplated to thicken the 
electrically conductive pattern to the desired thickness of the resist 25. 
The relatively uniform field created by grid 23 helps ensure a uniform 
current density, and thereby ensure that additionally deposited conductive 
material 28 will be substantially as thick as resist 25. We have 
discovered that the resist 25 has a self-limiting effect on the thickness 
of plating 28. That is, plating 28 will virtually stop when it reaches a 
thickness corresponding to the thickness of resist 25. 
As next shown in FIG. 7 resist 25 is removed from the surface of filter 20 
to lead the electrically conductive regions 28. This step may be 
accomplished using an appropriate solvent for the resist chosen. For 
example the Riston resist may be removed using acetone. 
As shown in FIG. 8 the array of filters 20 is then separated into 
individual filters using any well known technique. In the preferred 
embodiment the array of filters 20 is separated by scribing the filters 
and then breaking them apart. Depending upon the type of filter, however, 
sawing or laser cutting may also be used. 
As shown in FIG. 9 filter 20 may be attached to the surface of sensor 15 
using any desired technique, for example optical cement or epoxy. The 
effect of the electrically conductive material 28 is to space filter 20 
away from chip 15 by the desired distance. 
An expanded cross-sectional view of the structure shown in FIG. 9 appears 
at FIG. 10. This figure more clearly illustrates the spacing between 
filter 20 and sensor 15 caused by plated regions 28. 
This invention provides a method and structure by which precision spacers, 
providing a fixed optical distance between a semiconductor image sensing 
device and a filter, may be economically produced. The procedure utilizes 
known semiconductor metallurgical processes and circumvents the 
disadvantages inherent in prior art techniques. The invention is readily 
adaptable to batch processing, allows great flexibility in the format of 
the spacer and minimum tooling costs. The invention is particularly 
suitable for separating a filter for a charged coupled device from the 
device itself.