Structure and method for monitoring a semiconductor process,...

Details Structure and method for monitoring a semiconductor process,... Structure and method for monitoring a semiconductor process,...

2000-07-24

2004-10-26

Wilczewski, Mary (Department: 2822)

Semiconductor device manufacturing: process

With measuring or testing

C438S149000, C438S423000, C438S424000, C438S691000

Reexamination Certificate

active

06808944

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to semiconductor manufacturing processes, and more particularly to methods and structures for monitoring such processes.
BACKGROUND OF THE INVENTION
Semiconductor circuit devices are typically manufactured by a sequence of steps, each of which can deposit and/or modify a layer formed in or on a wafer. While it would be ideal for each step of a process to be absolutely repeatable, in most cases, manufacturing steps can vary over time. Consequently, it may be desirable to monitor particular steps. Thus, if a particular step begins to vary, the process step can be adjusted to provide the desired result.
One common form of process monitoring is to provide monitoring wafers. A monitoring wafer may be run through a set of predetermined manufacturing steps and subsequently examined to determine how one or more of the steps are performing. Because data provided from a monitoring wafer could be used to adjust one or more process steps, it follows that the more reliable and/or accurate a monitoring wafer is, the better a process step can be adjusted.
Spacings, such as trenches or the like, can be an important feature to monitor. As but one example, substrate trenches formed in shallow trench isolation process steps may affect various device features. However, because such substrate trenches can be subject to uncontrollable variation, it can be difficult to monitor particular features that may vary with substrate trench depth.
As another example, silicon-on-insulator (SOI) processes can include spacings (trenches) that separate one semiconductor island (mesa) from another. During SOI process development it can be desirable to characterize how a particular feature can be formed on, and with respect to, such islands. However, an SOI development process monitoring step is typically destructive, in that a wafer may have to be destroyed. Thus, it may be very expensive to develop/monitor/characterize physical features in a SOI process, as many SOI wafers may have to be destroyed.
To better understand the present invention, a first conventional process monitoring method and structure will now be described.
A method of forming a structure that may be used to monitor a process step is shown in
FIGS. 5A
to
5
D.
FIGS. 5A
to
5
D show a series of cross sectional views of a shallow trench isolation (STI) formation process. In the conventional example, a chemical-mechanical polishing (CMP) step can be monitored by measuring a step height of a material that is removed by the CUT step. More particularly, a step height of an insulating material deposited in a substrate trench is measured.
FIG. 5A
shows a semiconductor device
500
that may include a substrate
502
on which a trench etch mask
504
may be formed. A trench etch mask
504
can selectively expose portions of a substrate
502
where a trench is to be formed. A substrate
502
may be formed from monocrystalline silicon. A trench etch mask
504
may be formed from a layer of deposited silicon nitride that is patterned with conventional photolithographic techniques. A thin layer of silicon dioxide
505
may be formed over a substrate
502
.
FIG. 5B
shows trenches
506
that may be formed in a substrate
502
by etching with a trench etch mask
504
. Each trench
506
may have a trench depth, one of which is shown by measurement
508
. It is noted that a trench etching step may be subject to some variation. Consequently, a trench depth (such as
508
) may vary across a wafer and/or between different lots. This will be discussed in more detail below. A trench
508
may be formed by an anisotropic plasma etch, as but one example.
FIG. 5C
shows a trench insulating material
510
that has been formed in trenches
506
. Such a trench insulating material
510
can provide substrate isolation for particular circuit devices formed in a substrate
502
. A trench insulating material
510
may include silicon dioxide deposited with a high density plasma (HDP). As shown in
FIG. 5C
, following the deposition of a trench insulating material
510
, a resulting top surface of a semiconductor device can be uneven. It may therefore be desirable to planarize such a surface.
FIG. 5D
shows semiconductor device
500
following a planarizing step. A planarizing step may include chemical mechanical polishing. A resulting structure such as that shown in
FIG. 5D
may then be used to monitor the planarization process. In particular, a trench insulator step height, shown as
512
and residual nitride thickness may be measured. Such a measurement, ideally, could give an indication as to the rate and/or amount of trench insulating material being removed by a planarization step, or the like.
Thus, a monitoring wafer could be removed at this point, and a trench insulator step height
512
and residual nitride thickness could be measured by taking a cross sectional view of the wafer, or the like.
It has been found that a resulting trench insulator step height can vary according to trench depth. Unfortunately, as noted above, trench depth may vary across a wafer and/or across a lot. Consequently, a given monitoring wafer (or wafer portion) may provide different measurements for a trench insulator step height depending upon a depth of the corresponding trench.
One conventional way of attempting to compensate for such variation is illustrated in
FIGS. 6A and 6B
.
FIGS. 6A and 6B
are graphs showing experimentally determined relationships between trench depth and resulting trench insulator step height. In particular,
FIG. 6A
shows a relationship between trench depth, trench insulator step height, and wafer position.
FIG. 6B
shows a relationship between trench depth and trench insulator step height. In both of the graphs, as trench depth decreases, step height increases.
FIGS. 7A and 7B
provide various examples of such relationships in side cross sectional views.
FIG. 7A
shows the range of values in FIG.
6
A.
FIG. 7B
shows the range of values in FIG.
6
B.
FIGS. 7A and 7B
include many of the same structures shown in
FIGS. 5A
to
5
D. To that extent like structures will be referred to by the same reference character.
FIG. 7A
shows a maximum trench depth and corresponding minimum step height
512
and a minimum trench depth and corresponding maximum step height
512
′ for the values shown in FIG.
6
A. Similarly,
FIG. 7A
shows a maximum trench depth and corresponding minimum step height
512
and a minimum trench depth and corresponding maximum step height
512
′ for the values shown in FIG.
6
B. As shown in
FIG. 6B
, a minimum step height
512
can be a negative value.
A drawback to the above conventional approach is that a monitoring measurement may not be a direct measurement, but rather an indirect measurement. That is, simply measuring a trench insulator step height does not necessarily give an accurate indication of how a CMP step is performing, as the corresponding trench depth must also be measured. The two values may then be applied to an experimentally determined relationship, such as those shown in
FIGS. 7A and 7B
.
Still further, it is not clear that experimental results may accurately represent a CMP step for all cases. As but a few examples, if a trench width or trench insulator material is changed, a new experiment may have to be run to determine a relationship between trench depth and trench insulator step height.
In light of the above described background art and example, it would be desirable to arrive at an improved way of monitoring a manufacturing step, where the monitoring includes measuring a feature that may vary according to trench depth.
Having described a first conventional process monitoring method and structure, a second conventional example will now be described.
FIG. 8
shows a conventional SOI monitoring structure. A SOI monitoring structure
800
may include a portion of a SOI wafer having desired features formed thereon. Because a monitoring structure
800
is formed on an SOI wafer, a monitoring structure may include a substrate
802
, a substrate i

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