Patent Description:
The present document relates to scanning probe microscopy systems (SPM), such as atomic force microscopy systems (AFM). In conventional scanning probe microscopy systems for investigating the surfaces of samples, such as wafers, the sample is held in place by a sample support structure during scanning of the probe. For example, if the sample is a wafer, the wafer may be clamped onto a chuck. The chuck is mounted on a coarse positioner, which positions the wafer such that the area to be investigated on the sample surface is arranged underneath the scanning probe. Next, the probe tip approaches the surface and scanning commences.

Although the above works well for many applications, it does provide some drawbacks that limit application of the technique in particular circumstances. For example, one of these drawbacks relates to thermal expansion of components, which puts constraints on the achievable accuracy. However, not only the system itself, but also the sample is subject to thermal expansion. This particularly becomes problematic for larger samples, e.g. <NUM> wafers. As may be appreciated, if structures on the surface of the sample are to be mapped on nanometer scale, even the smallest temperature variation may result in an unacceptable inaccuracy in the measurement.

Reference is made to Chinese application <CIT>, international application <CIT> and US patent <CIT>.

It is an object of the present invention to provide scanning microscopy system wherein the abovementioned disadvantages have been overcome, and which allows for highly accurate sensing on nanometer scale.

To this end, there is provided herewith a scanning probe microscopy system according to claim <NUM> for mapping nanostructures on a surface of a sample, the system comprising a metrology frame, a sample support structure for supporting a sample, a sensor head including a probe tip, and an actuator for scanning the probe tip relative to the sample surface for mapping of the nanostructures, wherein the system comprises a clamp for clamping of the sample, and wherein the clamp is fixed to the metrology frame and arranged underneath the sensor head, wherein the clamp is arranged for locally clamping of the sample in a clamping area underneath the probe tip, the clamping area being smaller than a size of the sample such as to clamp only a portion of the sample.

In accordance with the present invention by locally clamping the sample in a clamping area underneath the sensing head, the center of thermal expansion will be known and fixed to the clamping area. Because the clamp is fixed to the metrology frame, this center point for thermal expansion is likewise fixed with respect to the metrology frame. Hence, thermal expansion is excluded as a source of disturbance within the system because the metrology loop (i.e. from the measuring area via the metrology loop and the sensing head to the probe) will include this center for thermal expansion.

In accordance with some embodiments, the clamp is arranged underneath the sensor head such that a measurement axis through the probe tip and transverse to the sample crosses the clamping area. This ensures that the clamping area always is included by the metrology loop. The clamping area preferably is of relatively small size as compared to the wafer. In particular, the desired accuracy for the system and the thermal expansion coefficient of the material of the sample will set an upper limit for the size of the clamping area. Preferably, the clamping area will be of a size well below this upper limit. In general, it has been found that preferably the clamp has a size such that the clamping area on the sample is of a same size as an area to be scanned on the sample surface in use for mapping of said nanostructures. Alternatively, or in addition, the clamp may have a size such that the clamping area on the sample has a diameter in cross section within a range between <NUM> millimeter and <NUM> millimeter, preferably between <NUM> millimeter and <NUM> millimeter, more preferable between <NUM> millimeter and <NUM> millimeter, such as <NUM> millimeter. These ranges are also typically (but not exclusively) applicable where the sample is a silicon wafer, for example.

In accordance with the invention, the system further comprises a sample support structure for supporting a sample, the sample support structure comprising a plurality of support struts, wherein the clamp is arranged for clamping at a first lateral stiffness, and wherein the support struts are arranged for supporting the sample at a second lateral stiffness lower than the first lateral stiffness. This invention is advantageous for large samples. The sample will be clamped by the clamp underneath the sensing head, while being further supported by the struts on the support structure. However, as will be appreciated, the struts must be designed in such a manner relative to the clamp, that they do allow the sample to expand without causing the center of thermal expansion to shift. Therefore, as defined above, the clamp is arranged for clamping at a first lateral stiffness, whereas the support struts are arranged for supporting the sample at a second lateral stiffness lower than the first lateral stiffness. This causes the center of thermal expansion to remain fixed to the clamping area.

According to the invention, the support struts are arranged on the sample support structure such as to support the sample outside the clamping area, e.g. for allowing large samples to be supported (e.g. <NUM> wafers). The support struts may include at least one element of a group comprising: support burls, support knobs, or flexible or rigid support poles. The struts may for example include a flexible material, or may include a rubber or flexible tip. In other embodiments, the ends or tips of the struts may be smooth and rigid to allow lateral slipping of the sample. A combination of these measures is also possible, e.g. flexible poles having smooth rigid tips.

In accordance with some embodiments, the system further comprises a sample positioner for positioning of the sample on the clamp, wherein the sample positioner is external to a metrology loop, wherein the metrology loop is a virtual path going from the clamping area via the clamp and the metrology frame to the sensing head and the probe tip. The use of a sample positioner which is arranged outside the metrology loop further excludes sources of inaccuracy. In particular, the external sample positioner allows to position the sample onto the clamp, which is fixed to the metrology loop. Conventional SPM systems in many cases apply a coarse positioner underneath a substrate table, which thereby becomes part of the metrology loop. By this any inaccuracies or sources of disturbance within the coarse positioner likewise become part of the metrology loop; for example differences in thermal expansion characteristics between different parts of the coarse positioner, play between parts, or vibrations caused by operation of the positioner. Such sources of inaccuracy are excluded by applying the external positioner that may pick up the sample and position it onto the clamp. Moreover, the use of an external sample positioner that is external to the metrology loop renders the direct fixing of the clamp onto the metrology frame (for making it part of the metrology loop) more easy. In a further embodiment thereof, the sample positioner is external to the sample support structure and separated therefrom. Moreover, the positioning arm may even be completely separate from the metrology frame, e.g. being a completely separate and individual part of the system. In yet other such embodiments the sample positioner comprises a robotic arm for positioning of the sample on the sample support structure. However, as may be appreciated, various alternative designs for the sample positioner may be suitable, including for example - but not limited to - one or more (or a system of) robotic arms, or a movable table or support that is retracted underneath the sample upon clamping to the clamp. In a further embodiment, the sample support structure comprises one or more lifting pins for enabling lifting of the sample such as to allow gripping of the sample by the sample positioner or robotic arm. Lifting of the sample using lifting pins would allow an end-effector of a robotic arm to move underneath the sample. In some embodiments, one or more of the earlier described support struts form the lifting pins, although the system may alternatively or in addition be equipped with dedicated lifting pins.

In other embodiments of the invention, the clamp is of a material having a same or similar thermal expansion coefficient as the sample, or wherein the clamp is a silicon carbide clamp. Silicon carbide clamps are a preferential class of clamps in case the samples to be analyzed are mostly or exclusively silicon wafers. The thermal expansion coefficient of the clamp is the same as that of the wafer, and as a result, variations in the size and dimension of the clamp as a result of temperature variations during scanning are the same as the size variations within the clamping area on a silicon wafer sample. Hence, the location of the center of thermal expansion remains fixed and no stress is exerted on the sample as a result of differences in thermal expansion. Of course, the present invention is not limited to application in the field of analysis of silicon wafers, but may be applied in any type of scanning probe microscopy system for any purpose, in particular where highly accurate mapping of structures on nanometer scale is desired. In general, if a particular type of sample material is used, the material of the which the clamp is made may be selected to match the sample material in the sense that the coefficient of thermal expansion of the clamp material is similar (preferably equal) to that of the sample material. This may not always be possible, as will be appreciated. In some embodiments, the clamp may be fixed to the metrology frame in such a manner that it may be easily replaced between use of the system to match the clamp with the sample to be scanned.

In other embodiments of the invention, the clamp is a suction clamp for clamping the sample by means of suction. The use of a suction clamp, for example in combination with a robotic arm type external positioner provides a convenient implementation of the invention that allows placement and replacement of the sample on the clamp in a fast and non-destructive manner. Moreover, suction force can be accurately controlled by controlling the pressure within the clamp during clamping. This allows to control the lateral stiffness provided by the clamp such as to ensure that it is larger than, for example, the lateral stiffness of supporting the sample by the support struts of the support structure, as described above.

In other embodiments of the invention, the sample support structure is external to the metrology frame. In addition to maintaining the lateral stiffness provided by the clamp to be larger (controlling it to be larger) than the lateral stiffness provided by the support structure, excluding the sample support structure from the metrology loop or even from the metrology frame provides a further decoupling of parts in the system that excludes sources of inaccuracy from the measurement. For example, due to the separation of these parts, vibrations in the support structure or thermal expansion of the structure is not passed on to the metrology frame, but is excluded. Advantageously, the sample support structure will not be part of the metrology loop (e.g. sample area - clamp - metrology frame - sensor head - probe). Preferably, however, the support structure is not fixed to the metrology frame at all to prevent passing disturbances on via the frame.

A system in accordance with the present invention is schematically illustrated in a cross sectional side view in <FIG>. The scanning probe microscopy system <NUM> comprises a metrology frame <NUM> comprising, as illustrated, at least an upper arm <NUM> and lower arm <NUM>. The upper arm <NUM> comprises a sensor head <NUM> mounted on an actuator <NUM>. The actuator <NUM> enables scanning of the probe <NUM> of the sensor head <NUM> across the surface of a sample <NUM>. For example, with reference to the coordinate system <NUM> schematically illustrated in <FIG>, the actuator may allow scanning of the probe <NUM> of the sensor head <NUM> in the x and y direction. Additionally, the actuator <NUM> may be arranged for lowering the probe <NUM>, in particular the probe tip <NUM> thereof, towards the surface of the sample <NUM>. In that case, the actuator <NUM> is also arranged for moving the sensor head <NUM>, or at least the probe <NUM> thereof, in the z direction.

The system further comprises a sample support structure <NUM>. The sample support structure <NUM>, as illustrated in <FIG>, is arranged for supporting the sample <NUM> across the surface thereof. To this end, the sample support structure <NUM> comprises a plurality of burls <NUM> that support the sample <NUM> from underneath. Preferably, the sample <NUM> is supported by the burls <NUM> without clamping or fixing thereto. In particular, the sample <NUM> may be supported by gravitation of force only.

In accordance with the present invention, a clamp <NUM> is fixed to the lower arm <NUM> of the metrology frame <NUM>. The clamp <NUM> locally clamps the sample <NUM> to the metrology frame <NUM>. The clamping of the sample <NUM> is only performed locally underneath the sensing head <NUM>. In particular, a measurement axis <NUM> that goes straight through a clamping area provided by the clamp <NUM> and through the sensing head <NUM>, illustrates that the clamping is only achieved in a direct vicinity of this measurement axis <NUM>. A large part (i.e. most) of the sample <NUM> is not clamped. Optionally, as is done in <FIG>, this unsupported part of the sample <NUM> may be supported by further supporting means; e.g. such as burls <NUM>, a gas bearing or a table.

The clamp <NUM> may for example comprise a suction clamp, and clamping is achieved by suction from outlet <NUM> to create a low pressure area underneath the sample <NUM> within the clamp <NUM>. The use of a suction clamp <NUM> allows convenient clamping of the sample <NUM>, but also allows to control the clamping force provided by the clamp <NUM> by controlling the pressure inside the clamp <NUM>. In accordance with the teachings of the present invention, the lateral stiffness provided by the clamp <NUM> (i.e. the stiffness of the clamping with respect to the directions parallel to the sample surface <NUM>) is larger than the lateral stiffness provided by the burls <NUM>. In the invention including such burls <NUM>, this lateral stiffness requirement is important to ensure that the center of thermal expansion is arranged within the clamping area provided by the clamp <NUM>. The center of thermal expansion is the midpoint of the thermal expansion, i.e. the point that remains fixed at all times regardless of any temperature change of the sample <NUM>. As will be appreciated, if the temperature of the sample <NUM> rises, the material will expand in accordance with the thermal expansion coefficient. As a result, and particularly detectable of a nanometer scale, the expansion will move any point on the surface of the wafer <NUM> radially outward relative to the center of thermal expansion. The magnitude of the displacement of each point is dependent on the distance between the respective point and the center of thermal expansion. By using a local clamp <NUM> which locally clamps the sample <NUM> underneath the sensor head <NUM> and the probe tip <NUM>, the center of thermal expansion can be fixed to the clamping area provided by the clamp <NUM>. As a result, any displacement within the measurement area (which is directly in the vicinity of the center of thermal expansion) will at utmost be of limited magnitude. Thus, within the measurement area, due to the absence of errors due to thermal expansion, a higher measurement accuracy is obtainable. Displacement of any points that are remote from the clamp <NUM> may be much larger, however these points lie outside the measurement area and therefore have no relevancy to the measuring by the probe.

Further illustrated in <FIG> is the external positioner system <NUM> comprising a robotic arm <NUM> and a gripper <NUM>. The gripper <NUM> allows to pick up the sample <NUM> and place it differently underneath the sensor head to allow scanning of different measurement area. In the embodiment in <FIG>, use is made of an external positioning system <NUM>. The external positioning system <NUM> comprises a robotic arm <NUM> equipped with a gripper <NUM> for picking up the sample <NUM> and repositioning it on the clamp <NUM>. By using an external positioner, which is at least external to a metrology loop through the metrology frame <NUM>, any sources of disturbance that relate to the positioning system can be excluded from the measurement. Moreover, by using the external positioning system <NUM>, the sample <NUM> may be clamped directly onto the metroframe by using the clamp <NUM> mounted on the metrology frame <NUM>.

A further source of error that is excluded from the measurement as a result of clamping the sample <NUM> directly underneath the measurement area, and only in the direct vicinity thereof, comes from the fact that the length of the metrology loop through the upper arm <NUM> of the metrology frame <NUM> to the probe tip <NUM> is equal to the length of the metrology loop through the lower arm <NUM> of the metrology frame to the clamp <NUM>. As a result, thermal expansion of the upper arm <NUM> of the section between the right portion <NUM> of the metrology frame towards its distal and where the sensor head <NUM> is mounted, is equally compensated by thermal expansion of the lower arm <NUM>. Moreover, the metrology frame may be an integral part of a homogeneous material. The material of the frame may be such as to be low sensitive to temperature gradients. In particular, such material comprise a relatively low thermal expansion coefficient while having a high thermal conductivity. Examples of such suitable materials are lithium aluminosilicate glass-ceramics such as Zerodur® (manufactured by Schott AG of Mainz, Germany), silicon carbide, nickel-iron alloys such as Invar (FeNi36 (invented by Charles Édouard Guillaume in <NUM>)), and aluminum. However, the invention is not limited to either one of these examples, and other material may be likewise suitable for the specified purpose.

A top view of a scanning probe microscopy system in accordance with the present invention is schematically illustrated in <FIG>. In <FIG>, the positioning system <NUM> with the robotic arm <NUM> and gripper <NUM> are schematically illustrated. A sample support structure <NUM> comprising a plurality of burls <NUM> supports wafer <NUM>. The wafer <NUM> rests on the tips of each of the burls <NUM> underneath it. For simplicity, only few of the burls have been indicated by means of reference numerals <NUM>. The lower arm <NUM> of the metrology frame <NUM> is schematically illustrated comprising the clamp <NUM> defining the clamping area <NUM>. The clamping area <NUM> only covers a small portion of the total wafer surface. The center of thermal expansion <NUM> is located within the clamping area <NUM> as a result of the local clamping by the clamp <NUM>. Any expansion (schematically illustrated by arrow <NUM> and <NUM>) will only be of very limited magnitude in the clamping area <NUM>. Therefore, as a result of the local clamping, error caused by thermal expansion of the wafer <NUM> is sufficiently kept within limits to allow the measurements to be performed at high accuracy.

<FIG> is a schematic illustration of the metrology loop <NUM> in the embodiment of <FIG>. In <FIG>, only the metrology frame <NUM> with upper arm <NUM> and lower arm <NUM> is illustrated. The external positioner <NUM> and a sample support structure <NUM> are not illustrated. The metrology loop <NUM> runs from the probe tip <NUM> via the clamp <NUM> and the lower arm <NUM> towards the upper arm <NUM> of the metrology frame <NUM>, through the actuator <NUM> and the sensor head <NUM> and through the probe arm of probe <NUM> back to the probe tip. Clamping of the wafer <NUM> by means of the clamp <NUM> directly to the metro frame <NUM> prevents a number of sources of inaccuracy in the lower arm <NUM> of the metrology frame <NUM>. Also, as a result of clamping the wafer <NUM> directly underneath the probe tip <NUM> and providing only local clamping in the direct vicinity of the measurement area, fixes the center of thermal expansion to the clamping area provided by the clamp <NUM>, and excludes errors due to thermal expansion of the metrology frame (thermal expansion in the upper arm <NUM> is compensated by an equal amount in the lower arm <NUM>). Using a plurality of burls <NUM>, the clamping force is controlled such that the lateral stiffness of the clamp <NUM> is larger than the lateral stiffness of the wafer <NUM> resting on the burls. To this end, the tips of the burls may for example be made of a smooth and rigid material to decrease their lateral stiffness with respect to the wafer. The burls <NUM> will in that case only support the wafer in the z-direction (gravitational) but not in the x and y directions (slipping is allowed in these directions). By at the same time clamping with a sufficient suction force through clamp <NUM>, the wafer <NUM> is, at clamp <NUM>, fixed in each direction x, y and z with sufficient lateral stiffness to prevent slipping with respect to the clamp <NUM>. This fixes the sensor of thermal expansion.

Claim 1:
Scanning probe microscopy system (<NUM>) for mapping nanostructures on a surface of a sample (<NUM>), the system (<NUM>) comprising a metrology frame (<NUM>), a sensor head (<NUM>) including a probe tip (<NUM>), and an actuator (<NUM>) for scanning the probe tip relative to the sample (<NUM>) surface for mapping of the nanostructures,
wherein the system (<NUM>) comprises a clamp (<NUM>) for clamping of the sample (<NUM>), and wherein the clamp is fixed to the metrology frame and arranged underneath the sensor head (<NUM>), wherein the clamp (<NUM>) is arranged for locally clamping of the sample (<NUM>) in a clamping area underneath the probe tip, the clamping area being smaller than a size of the sample (<NUM>) such as to clamp only a portion of the sample (<NUM>);
the system (<NUM>) further comprising a sample support structure (<NUM>) for supporting the sample (<NUM>), the sample support structure (<NUM>) comprising a plurality of support struts arranged on the sample support structure (<NUM>) such as to support the sample (<NUM>) outside the clamping area, wherein the clamp (<NUM>) is arranged for clamping at a first lateral stiffness, and wherein the support struts are arranged for supporting the sample (<NUM>) at a second lateral stiffness lower than the first lateral stiffness, wherein the lateral stiffness relates to a stiffness in a direction parallel to the surface of the sample in use.