Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
1999-04-30
2001-05-29
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
With measuring or testing
43, C382S151000
Reexamination Certificate
active
06238939
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to semiconductor device fabrication and, more particularly, to a method of fabrication quality control, based on monitoring the alignment of two layers deposited successively on a semiconductor wafer.
Semiconductor devices, such as processor chips and memory chips, are fabricated by the deposition of successive layers of substances such as polysilicon, silicon dioxide and various metals on a silicon wafer substrate. As shown in
FIG. 1
, the devices are fabricated as dies
10
, separated by scribes
12
, on a wafer
14
. After each layer is deposited, it is covered with photoresist. The photoresist is exposed to a preselected pattern of light. Depending on the type of photoresist, a portion of the photoresist, either the portion that was exposed to the light or the portion that was not exposed to the light, is removed, usually by dissolution, exposing the layer beneath the photoresist. The thus exposed part of the new layer is either totally or partially removed, for example by etching, to provide the layer with its desired geometry.
The exposure of the photoresist to the light pattern is effected using a tool called a “stepper”. The desired two-dimensional geometric pattern of the layer is embodied in a reticle, either in a transparent portion of the reticle or in an opaque portion of the reticle. Collimated light is directed through the reticle, and focused on a portion of the wafer. The portion of the wafer upon which the light is directed at any one time is known in the art as a “field”. The fields in
FIG. 1
are demarcated by dotted lines
16
. In this example, each field spans four dies
10
.
It is critical that successive layers be aligned accurately. For this purpose, the reticle includes alignment key portions in the portion of the reticle corresponding to scribes
12
.
FIG. 2
shows, schematically, a typical reticle pattern
18
, including die portions
20
corresponding to four dies
10
, scribe portions
22
corresponding to scribes
12
that separated dies
10
, and, in scribe portions
22
, two alignment key portions
24
corresponding to Cartesian x and y axes. After the corresponding layer on wafer
14
has been provided with its desired geometry, as described above, the layer includes, in scribes
12
, and as shown in
FIG. 1
, alignment keys
26
whose geometry matches the geometry of alignment key portions
24
. The geometry of alignment key portions
24
is designed so that the two-dimensional pattern of alignment keys
26
shows up both in the layer in which alignment keys
26
are fabricated and in the immediately succeeding layer, so that wafer
14
can be positioned accurately by the stepper, relative to the reticle of the succeeding layer, for the accurate patterning of the succeeding layer relative to the layer that bears alignment keys
26
. Because alignment keys
26
are used specifically by the stepper to effect this alignment, alignment keys
26
are termed “stepper keys” herein.
To verify the accuracy of the mutual alignment of two layers on wafer
14
, reticle
18
also includes overlay key portions
28
that typically are square as shown. Overlay key portions
28
for successive layers have different sizes, with the overlay key portion of the lower of a pair of successive layers (termed herein the “first” layer) being larger than the overlay key portion of the upper of a pair of successive layers (termed herein the “second” layer). The two layers, when finally formed with their desired geometries, include corresponding square portions that are termed “overlay keys” herein. If the second layer is aligned accurately with the first layer, then the overlay keys of the second layer are at their nominal (designed) positions relative to the overlay keys of the first layer, i.e., exactly in the centers of the corresponding overlay keys of the first layer. Deviations of the actual positions of the overlay keys of the second layer, relative to the overlay keys of the first layer, from this central positioning are diagnostic of misalignment between the first and second layers.
FIG. 3
shows a microphotograph of an overlay key
30
of a second layer of photoresist centered in an overlay key
34
of a first layer
36
of metal.
The stepper positions wafer
14
, for exposure to light through the reticle corresponding to the second layer, by searching for and locating geometric patterns of stepper keys
26
that are located approximately in the expected (“nominal”) positions of stepper keys
26
. Usually, stepper keys
26
are not located precisely in their nominal positions, but instead are slightly displaced relative to their nominal positions. The stepper constructs a mathematical model of this displacement and positions wafer
14
accordingly to adjust the locations of fields
16
for the second layer. This model is termed herein the “stepper model”.
After the second layer is fabricated, wafer
14
is transferred to an overlay measurement tool for evaluating the accuracy of the mutual alignment of the first and second layers, as represented by the accuracy of the mutual alignment of the first and second overlay keys. Like the stepper, the overlay measurement tool constructs its own mathematical model, termed herein the “overlay model”, of the misalignment of the second layer relative to the first layer.
FIG. 4
shows six ways in which fields
16
′ of the second layer can be misaligned with respect to fields
16
of the first layer.
FIG. 4A
shows scaling: the overall size of the geometric pattern defined by fields
16
′ is larger than the overall size of the geometric pattern defined by fields
16
. Isotropic scaling is shown in FIG.
4
A. Scaling can also be anisotropic, with different magnifications in the x and y directions.
FIG. 4B
shows orthogonal x-rotation: fields
16
′ are skewed horizontally relative to fields
16
.
FIG. 4C
shows rigid rotation of fields
16
′ relative to fields
16
.
FIG. 4D
shows magnification of individual fields
16
′ relative to corresponding fields
16
.
FIG. 4E
shows rotation of individual fields
16
′ relative to corresponding fields
16
as a consequence of reticle rotation.
FIG. 4F
shows vertical translation: fields
16
′ are shifted vertically relative to fields
16
. For illustrational clarity, only nine fields
16
and
16
′ are shown in each of
FIGS. 4A
,
4
B,
4
C,
4
D,
4
E and
4
F. The six modes of misalignment shown in
FIG. 4
, plus horizontal translation, are the degrees of freedom of the overlay model. (Orthogonal y-rotation can be expressed as a combination of rigid rotation and orthogonal x-rotation.) The same modes of misalignment, with the exception of magnification and reticle rotation, are the degrees of freedom of the stepper model.
The residuals of the overlay model, ie., the differences between the actual Cartesian (x,y) coordinates of the overlay keys of the first layer, in a coordinate system defined by the second layer, and the Cartesian coordinates of the overlay keys of the first layer that are predicted by the overlay model, are diagnostic of the quality of the overlay model. Large absolute values of the residuals indicate that the model fails to explain the misalignment. This failure is in turn diagnostic of problems in the fabrication process. Similarly, the residuals of the stepper model, ie., the differences between the measured Cartesian coordinates of the stepper keys and the coordinates of the stepper keys that are predicted by the stepper model, are diagnostic of the quality of the stepper model. In practice, only the overlay model residuals are used for quality control of the fabrication process, because only the overlay model is directly related to the actual degree of misalignment of the first and second layers. Although the stepper calculates stepper model residuals for each wafer
14
processed therein, these stepper model residuals are used only for calibrating the stepper during installation and for stepper maintenance.
SUMMARY OF THE INVENTION
Cohen David
Wachs Amir
Friedman Mark M.
Niebling John F.
Stevenson André C
Tower Semiconductor Ltd.
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