Measuring and testing – Surface and cutting edge testing – Roughness
Reexamination Certificate
1999-05-18
2003-02-18
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Surface and cutting edge testing
Roughness
Reexamination Certificate
active
06520005
ABSTRACT:
BACKGROUND OF THE INVENTION
Stylus profilers are used for obtaining surface profiles of samples. The stylus of the profiler follows the surface under a small contact force, and the resulting motions of the stylus are measured with a sensor assembly. The sensor assembly includes a stylus, a mechanical linkage (usually a stylus arm) connecting the stylus to a flexure pivot, and a transducer. When the stylus is scanned across the surface of the sample, the force exerted by the sample surface on the stylus causes a rotation of the stylus arm about the flexure pivot. The vertical displacement of the stylus is converted by the transducer into an electrical signal which indicates the profile of the sample surface.
Advanced profilers also include a force control mechanism, such as an electromagnetic actuator, for maintaining a constant contact force between the stylus and the sample surface as the stylus is scanned across the surface. To maintain a constant contact force between the stylus and the sample surface, the spring action of the flexure pivot is calibrated and the force control magnetic actuator is controlled to counteract the change in the force applied by the flexure spring on the stylus caused by rotation of the stylus arm. Thus, a constant force is exerted by the stylus against the sample surface, as the stylus is scanned across the surface. As an example of a profiler which has been used in the semiconductor and disk drive industries, please see U.S. Pat. Nos. 5,705,741 and 5,309,755; both patents are incorporated herein in their entirety by reference.
As the semiconductor industry progresses to smaller dimensions with each new generation of products, there is an increasing need for scanning instruments that can measure sub-micrometer scale surface features. While the depths or vertical dimensions (dimensions normal to the plane of the wafer surface) of the features such as trenches, or via holes, in semiconductor wafers, commonly exceed one micrometer, the lateral dimensions (dimensions in the plane of the wafer surface) have been continually reduced. At the current state of the art, the lateral dimensions of features such as trenches are less than 0.5 micrometer. With the continual reduction of the lateral dimensions of features such as trenches and via holes in the surface of semiconductor wafers, the ratio of depth to the lateral dimension of such features, also known as the aspect-ratio, is continually increased.
In order to measure such high aspect-ratio features, a very sharp, thin but long (high aspect-ratio) stylus must be used. However, a sharp, thin but long stylus is fragile and may easily break, especially when subjected to lateral forces (forces in directions in, or parallel to, the plane of the sample surface). Thus, when a high aspect-ratio stylus contacts a steep feature, such as the side wall of a trench or via hole, the contact force has a relatively large lateral component and a relatively small vertical component. Stylus profilers, such as the profilers described in the two patents referenced above, are designed such that motion of the stylus is constrained to one degree of freedom, namely, rotation about the flexure pivot. This degree of freedom is substantially normal to the sample surface. The stylus arm is relatively stiff in all other degrees of freedom. Consequently, the lateral forces generated when the high aspect-ratio stylus encounters a steep wall can easily break the stylus and damage the sample being measured.
The stylus arm in a profiler has a single degree of freedom, which comprises rotations about a pivot. The stylus or sensing tip travels along a path normal to a radial line passing through the center of rotation at the pivot and the tip. Since the sensing or stylus tip must be located “below” or at a lower elevation than the pivot to ensure that the tip and not the body of the sensor assembly contacts the sample, the motion of the stylus or sensing tip is not truly normal to the plane of the sample surface, but is in the shape of an arc. While the main direction of travel of the tip is downwards, it nevertheless also travels in the lateral direction in the plane of the sample surface. This lateral motion is also known as parasitic motion of the sensing tip. The parasitic motion of the sensing tip may hamper or even preclude the sensor assembly from measuring relatively deep and narrow features.
It is therefore desirable to provide an improved surface measurement system which overcomes the above drawbacks.
SUMMARY OF THE INVENTION
The above-described difficulties can be overcome by allowing the sensing tip of the profiler to contact the sample surface without substantially rotating the stylus arm about the pivot. Instead, a distance between the sample and the sensing tip of the profiler is reduced until the tip touches the sample, without moving the tip and the sample laterally relative to each other. By avoiding lateral relative motion between the tip and the sample before the tip touches the surface, the above-described problems are avoided. When such a scanning process is used, thin and long (high aspect-ratio) styli can be used to penetrate high aspect-ratio features for measurement. Data related to the height of the sample may then be measured with the tip stationary and in contact with the sample. After the measurement, the tip and the sample are separated and moved laterally relative to each other to measure the sample surface at a different location.
With minor modifications, the above-described scanning process may also be applied to other scanning instruments, such as the scanning probe microscope, which includes atomic force microscopes and scanning tunneling microscopes.
As described above, the feature on the sample surface may be found and measured by repeatedly causing the sensing tip (of the profiler or scanning probe microscope, for example) and a sample to repeatedly contact at different locations of the sample surface. In this process, the sensing tip and the sample are brought together substantially without lateral relative motion between them until they contact, separated again substantially without lateral relative motion between them, and moved laterally relative to each other until the tip is at a location above a different portion of the sample. This process is repeated at different locations of the sample. If the separation between the tip and the sample during such lateral motion is less than the change in height of the sample surface, the lateral motion will cause the sensing tip to contact the sample surface laterally, thereby causing damage to the sensing tip. To reduce the probability of such damage, the separation may be increased to a large value before lateral relative motion is initiated. If no knowledge of the height variation or distribution of the sample surface is available, such value should be large enough that it exceeds any probable height variations of the sample surface one may encounter. The resulting process can be quite time consuming, especially if the sample surface is to be measured at many different locations. This difficulty can be avoided by separating the tip and the surface by just enough to avoid such lateral contact.
A number of techniques may be employed to assure that the sensing tip and the sample are separated by an adequate distance so that the sensing tip will not contact the sample surface during the subsequent lateral relative motion. In the preferred embodiments, if certain height information is provided concerning the sample surface or a portion thereof (such as within a target area), then the sensing tip may be positioned at or close to the portion of the sample having the highest elevation. If the height information of the sample surface or a portion thereof is not readily available, such information can be acquired quickly by actually measuring the height of the surface at several sampling locations. Yet another technique that can be employed is to actually measure data related to the height of the sample when the tip and the sample come into contact as the tip i
Eaton Steven G.
McWaid Thomas
Panagas Peter
Samsavar Amin
Wheeler William R.
Hsue James S.
KLA-Tencor Corporation
Larkin Daniel S.
Skjerven Morrill LLP
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