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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Scanning Probe Metrology

Working Group 5.25

High resolution probing systems

 

PTB's development of probing systems based on SPMs (SPM: scanning probe microscope) is aimed at constructing and optimizing these measuring heads for use in dimensional nanometrology. Needless to say that the sensor systems described cannot only be used for metrological applications, but are of general interest for scanning probe microscopy and coordinate measuring techniques. 

In addition to the properties important from the viewpoint of metrology such as stability, sensitivity and noise behaviour, different other aspects have been incorporated into device development:

  • Combination of the SPM measuring heads with optical microscopes: here, the optical function extends from visualization to quantitative dimensional or analytical methods
  • The use of different detection principles: the movement and position of the measuring tip is measured by an external optical procedure or via an intrinsic electrical measuring principle
  • The use of different measuring tip materials: in recent developments, special diamond tips are used in addition to silicon and silicon nitride tips

Figure 1: Sketch of a scanning force microscope (SFM) with cantilever probe and beam deflection detection

Sensor objective with beam deflection detection

 

As the name already suggests, the concept of the so-called " sensor objective" directly takes up the combination of microscope objective and sensor, the sensor in this case working as a scanning probe microscope. The special feature of this sensor head development is that existing optical standard microscopes are used as a basis: due to the compact geometry and the special design, the sensor objective (composed of SPM module and imaging optics) can be directly screwed into the turret of an optical microscope. This allows two microscopy worlds to be ideally combined.

In measuring operation, the advantages of the combined system become obvious. Firstly, the well-proven conventional light microscopy is used for fast and extensive surface investigation. The spectrum of tasks extends from the orientation on the measurement object to quantitative optical measurements (see Sensor head for combined scanning probe and interference microscopy). Then local measurement is performed with the slower serial scanning probe procedure in the measurement area defined for calibration or, generally, at those points of the sample which require a high resolution.
Figure 2 shows a version of the sensor objective screwed into a standard microscope. During measurement the sample is scanned with a piezo scanning table whose position is controlled with capacitive sensors. In this objective design, the optical part of the measuring head consists of a combination of mirror optics. These allow the cantilever with the integrated measuring tip to be viewed simultaneously with the sample surface. This clearly improves user-friendliness as far as the adjustment and the selection of the measurement area are concerned. When optionally operated as optical near-field microscope, the same optics can be used for coupling and/or for collecting of optical radiation into or from the near-field probe. This allows very local optical investigations and spectroscopic surface characterizations to be performed even on sub-microscopic structures.
In the topography mode (SFM mode), even single atomic levels of a GaAs substrate wafer (step height: approx. 0.28 nm) have been resolved with this measuring setup despite the relatively large measuring circle (sample, microscope body, granite stand, positioning tables – cf. Fig. 2). These measurements were performed in a dynamic SFM mode using conventional silicon cantilever probes. Traditional beam deflection technique was applied to detect the bending of the cantilever. All opto-electronical elements of the beam deflection system have been arranged outside the measuring head, since a spatial integration was not intended when this version of the measuring head was constructed. This arrangement can be optimized, in particular with respect to its mechanical stability. The further objective of the PTB development went, however, beyond the integration of the beam deflection system into the measuring head. This is why measuring heads based on probes with monolithically integrated deflection detection have been developed (see Sensor objective with piezolever module, Sensor objective with tuning fork module).

Figure 2: Conventional standard microscope with screwed-in sensor objective:
The version shown here allows the device to be operated as optical near-field microscope in addition to scanning force and optical microscopy. The enlarged image section in addition shows a diagrammatic representation of the beam path inside the objective.

Sensor objective with piezolever module

 

One possibility of integrating the deflection detection system into the sensor probe (i.e. as near as possible to the measuring tip) consists in utilizing the piezoresistive effect of the cantilever material (here: silicon). Comparable to the strain gauge principle, the movement of the cantilever can thus be directly converted into a measurable electrical signal. This means that an adjustment of a light beam on the cantilever is not necessary. This improves user-friendliness of the system and avoids possible errors as a result of inexact adjustment. Step height measurements have, for example, shown that scattered light or reflections from the surface can lead to disturbing interference patterns or that the roughness of the rear side of the cantilever affects the measurement when optical methods are used for deflection detection. These error sources are avoided by monolithically integrated deflection sensors.


For realization of the piezoresistive cantilevers (briefly referred to as "piezolevers"), the piezoresistive elements were arranged in the form of a complete Wheatstone bridge and incorporated into the silicon cantilevers by ion implantation. This work was performed in cooperation with NanoSensors GmbH, Forschungszentrum Jülich and Surface Imaging Systems (S.I.S.) GmbH. As a special option, one of the Wheatstone resistors is realized as an electrically controllable resistor which allows the measuring bridge to be nulled. 

During the design of our very compact SFM measuring head, which is based on these piezolevers, special attention was directed towards the requirement for detachable contacts of the cantilever chips. In the piezolever SFMs so far realized, the cantilever chips were glued on small ceramic boards and the contacts were bonded. To avoid these complex additional process steps, the cantilever chips should be directly clamped and, at the same time, electrically contacted. To achieve this spring contacts were used which are made of gold-plated platinum beryllium ( see Figure 3b). hese "fingers" are arranged on a steel spring which is pressed-on or flapped-back with the aid of a very small cam to allow the probes to be exchanged. The complete holder must be exactly pre-adjusted and work free from mechanical play in order to contact the electrodes on the rear side of the chip reproducibly with the fingers, to exert enough force on the chip and to achieve good contacting. As can be seen in Figure 3b, the contacts which are only 50 µm apart from one another. The latter emphasizes the desired mechanical precision of the contacting mechanism. 

The dimensions of the whole SFM module which comprises both, a piezo element for the dynamic excitation of the cantilever and the electrical connections for the sensor signals, were reduced to 4 mm x 3.5 mm x 35 mm only (see Figure 3c). This compact design allows the combination with different measuring heads and measuring objectives. Topographic measurement results obtained with this piezolever module are described and shown in the interference-optical measurements.(Figure 6)

A great advantage of the mirror optics used in the sensor objective version described above was the fact that the dimensions and the optical parameters could be calculated by optical computational programs and manufactured with diamond turning machines. This finally allowed the whole sensor objective to be designed and constructed at our own options and the space required for the SFM module and the positioning mechanics to be taken into account. As described, the compact piezolever module does not require so much space. This is why these aspects are no longer important and combination with a commercial microscope objective as shown in Figure 3a furnishes a solution which is more universal. This combination - microscope objective and SPM module - has been realized for all measuring head versions so far developed (cf. also Figure 4a).

-Figure 3: 
Piezolever module combined with a standard microscope objective Figure 3b shows the finger contacts for fastening and electrical contacting

Sensor objective with tuning fork module

 

Another possibility of integrating the deflection detection system into the measuring probe consists in using a cantilever arm made of quartz. In operation, this quartz is - just like the tuning fork in a quartz clock - excited to oscillate after an electrical voltage has been applied. Measurement of the distance between probe and surface and thus imaging the surface is performed by recording the current flowing through the quartz. This signal is proportional to the lever arm vibration and reacts very sensitive to changes of the damping when the distance between tip and surface varies. Diamond tips designed at PTB are fastened on these tuning forks to allow high lateral resolution of the measurement (tip radii < 100 nm). Figure 4b shows a quartz probe with tip. The selection of diamond as tip material is based on both, the mechanical properties (stability and resistance to abrasion) and the optical properties which are important for the future use of the probes in optical near-field microscopy. To test the efficiency of the tuning fork measuring head, topographic measurements were performed on structures with dimensions in the nanometer range. The samples used here are made of self-organized, punctual InAs quantum dots on a GaAs substrate. These quantum dots have pyramidal geometries (width: approx. 20-30 nm, height: approx. 4-6 nm). The mechanical stability of the whole microscope is sufficiently high to image such nanostructures. Investigations of the noise resulted in values of less than 0.6 nm (root mean square value) on a profile 2 µm in length. Due to their extremely slim construction and their adjustment-free deflection detection, the tuning fork sensors can be tilted relative to the surface without any problem. This also allows measurements to be performed on object areas difficult to access such as structure edges or inclined areas. These properties allow these as well as the piezolever sensors to be used as sensitive probes in a coordinate measuring machine. Relevant developments have already been initiated at PTB.

Figure 4a: "Tuning fork" module with positioning mechanics and adapter ring for the microscope objective Figure 4b: a micrograph of the tuning fork lever arm with the diamond tip

Sensor head for combined scanning probe and interference microscopy

 

Up to now, imaging optics in SFMs only served as visualization tools to determine the area of interest for the measurement and to aid during probe alignment. In the sensor head realization described in this chapter, the functionality has been considerably progressed. The combination of SFM and interference microscope allows an optical measuring technique to be integrated into the measuring head which can be traced back to the SI unit "metre". 

This measuring system is based on the developments of the compact SFM measuring heads so far described and has been conceived so that it can be operated in different interference microscopes. For the PTB measuring setup, a commercial interference microscope (MicroMap, Nikon) was selected as the basic instrument. Due to the identical mechanical connecting plate, the newly developed sensor only replaces the exchangeable interference objective (see Figure 5). The basic instrument makes use of both, the evaluation software and the displacement mechanics for phase-shifting interferometry or white light interferometry.

 

Figure 5: View of the combined SFM and interference microscope composed of sensor head and commercial basic instrument

For the realization of the sensor head, two possibilities came into consideration:

  1. Modification of a commercial interference objective by adding a SFM module with the aid of an adapter or
  2. new internal development of the whole interferential sensor head with additional SFM module.

A solution according to 1) can directly be achieved by adapting the adapter ring shown in Figure 3a. In view of the planned improvement of the optical properties of the objective, which will be explained in the following, preference has, however, been given to the internal development of the measuring head.Core of the newly developed sensor head is a Michelson interferometer in the case of which illumination is not performed via the internal, filtered microscope white light lamp, but via external laser sources coupled to optical fibres. This way, an essential heat source is removed from the measuring setup and the mechanical stability is improved. Even more important is the fact that - due to the small illumination aperture of the optical fibre - aperture correction becomes negligible in the interference-microscopic evaluation. This clearly reduces the measurement uncertainty. At present, an HeNe laser (λ= 632,80 nm) or a frequency-doubled Nd-YAG laser (λ= 532,26 nm) can optionally be used as external laser sources in the measuring setup. If desired, this allows operation in the multi-wavelength interferometry mode by which, compared to operation with only one wavelength, the range of unambiguous measurements of the interference microscope is extended. 

Figure 6: Topography image of an 80 nm step-height standard (H80) ;
left: recorded in the interference-optical mode (range: 900 µm x 900 µm) - The SFM cantilever can be seen in the circle marked on the upper left corner
right: Section measured with the integrated SFM module (range 40 µm x 20 µm)

For combination with a scanning probe microscope, the compact SFM module with piezolevers was mounted on the sensor head below the beam splitting cube. The cantilever can be seen in the image section of the optical microscope (both, in the "live image" and in the interference-microscopic image; see Figure 6: on the left above) so that measurement area selection is very user-friendly. The interference-optical measurement (e.g. in phase-shifting mode) is performed simultaneously over the whole image section; in the current configuration, the optical measuring range amounts to approx. 900 µm x 900 µm. It can, however, also be varied by using different optical systems. In the case of an optical enlargement it is, however, to be taken into account that the depth of focus is reduced and the advantage of an optical survey image is no longer given. In a second step, the object area to be investigated with a high lateral resolution is moved below the SFM measuring tip with the aid of a manually-operated linear x, y- table. After the surface has been successfully approached to the tip, the measurement object is moved line by line with a position-controlled piezo scanning table (max. operating range: approx. 100 µm x 100 µm). The extraordinary advantage of this combined measuring system consists in the possibility of a direct z-calibration of the SFM. As soon as SFM and interference microscope are measuring at the same place of the sample, the interference-optical result can be used as calibration value for the SFM. Special step-height standards are suited to be used as precise standards for heights from a few nanometers up to some micrometers. Comparison measurements performed at PTB with the newly established measuring system and the reference interference-optical microscope showed for step-height measurements on 80 nm and 260 nm calibration standards deviations of less than one nanometer. Figure 6 shows a comparison of the results of measurements performed on an 80 nm standard in the interference-microscope mode and in the SFM mode. Another advantage of this combined device becomes obvious in the case of heterogeneous objects. As soon as the optical constants of substrate and measurement structure differ, the optical wave in the interference microscope experiences different phase jumps upon reflection. This leads to a measurement error as long as the relevant optical constants are not taken into account in the interference-microscopic evaluation. Determination of these constants for thin layers in the nanometer range is, however, quite time-consuming and often imprecise so that this correction is only conditionally possible. This is different in the case of the device on hand: here, the measured value of the interference microscope is corrected by the SFM module which had before been calibrated. It is worthwhile pointing out that the SFM calibration was, as already described, performed with the same interference microscope, although on a sample with homogeneous surface. This example shows the complementary properties of the two independent measuring principles combined in one measuring instrument.

Assembled Cantilever Probes (ACP)

 

Figure 1:
The ACP micro probe measures the vertical flank of a micro gear. Thereby, the movement of the probe is read out optically. The figure shows the holding chip with the cantilever and the vertical probe shaft with the measuring tip.

For three-dimensional determination of the topography of measurement objects (e.g. micro gears or other complex components of micro system technology), different design variants of so-called Assembled Cantilever Probes (ACPs) have been developed at PTB. These ACPs consist of a horizontal SFM cantilever (L1) to which one or several vertical cantilever(s) (L2, L3, ...) are affixed by adhesion (simplified representation in Figure 1). As cantilevers, commercially available scanning probe microscope probes can be used and, if required, complemented by additional micro components such as micro wires and micro spheres.

a)b)
Figure 2: Example of the arrangement of an ACP
(a: construction principle; b: SEM recording of a mounted stylus tip)

Compared to conventional SFM stylus tips, this design has two important advantages which can be seen, for example, when it is used for characterization of the side wall:

  1. The stylus tip is extended decisively in horizontal direction. Side walls can thus be scanned in their normal direction, which leads to an increase in the measurement sensitivity and repeatability.
  2. The extension of L1 by L2 allows measurements to be performed on side walls with depths of some hundred micrometers (as a function of the length of L2), without cantilever L1 touching the surface of the measuring object.

In contrast to conventional coordinate measuring machines (CMM) and micro CMMs, the lateral resolution of the ACP tip is in the case of side walls considerably higher due to their much smaller tip radius (r around 20 nm). The selection of special cantilever types for L2 as, for example, SFM types with hydrocarbon nanotubes ("nanotubes") or tips sharpened by ion beam attack (e.g.  SuperSharpSilicon™ of the NanoWorld company), allows the resolution during measurements on structures with large aspect ratio to be further increased.
The small stylus probing forces which in the case of the SFM measuring technique amount to only a few nanonewton even in the contact mode, prevent scratching of the surface when soft materials are used.The ACP detection system can be operated as scanning sensor or in the contacting trigger mode (as in the case of "trigger probes" in CMMs). Within the control system it is possible to change the position control for the detection system used in normal SFM operation from the z-axis to one of the lateral axes. Thus, a correct measurement is possible on the side faces, i.e. the scanning process is performed vertically to the vertical surfaces.In the contacting trigger mode, the ACP is approached to the surface of the object to be measured at each discrete measuring point. A contact with the surface releases a trigger signal, by which the measurement values are synchronously read in by the individual sensors. After that, the next position measurement is performed at a different place of the object. Deflection or torsion of the ACP probe systems is detected on cantilever L1 by means of the optical lever principle known from SFM technology.