Driven by the continued demand for increased functionality in integrated circuits, the trend toward ever-higher levels of integration has resulted the use of increasingly three-dimensional structures in order to gain more functions out of a given area. This is evident in several areas of both device and interconnect technology. One example is the continuing trend toward higher aspect ratios in vias, contacts and memory capacitor structures. Another example is the adoption of vertically-oriented transistor channels within FinFET logic devices at advanced technology nodes. On a much larger length scale, the use of through-silicon via (TSV) structures to create 3D interconnects enables vertical stacking of multiple dies. Each of these three developments leads to new challenges in process control and metrology, and a common theme is required between them to measure profiles and depths of etched structures. While diagnostic techniques, such as SEM, AFM and SPM play an important role in characterizing processes, optical metrology methods are highly desired, because they provide rapid measurements on product wafers, enabling routine monitoring and advanced process control.
Figure 1. 3D array of square trenches in silicon (left) and corresponding simulated infrared spectra (right), illustrating agreement between RCWA and EMA calculation methods in the long wavelength limit.
Model-based infrared reflectometry (MBIR) combines a photometrically accurate FTIR measurement system with a spectral range of 1-20 μm, and model-based analysis of reflectance spectra from multilayered films and structures. The infrared wavelength range provides unique advantages for the measurement of 3D-etched structures, which have already been exploited in applications of MBIR for various structures, including memory capacitors, power device trenches and isolation trenches.
One principle advantage of the infrared metrology is the simplified modeling of complex periodic structures. At wavelengths, greater than the pitch of a structure, the light propagates through the structure as if it was a homogeneous medium with an effective refractive index. This can be calculated from the geometry of the structure and the refractive indices of its component materials by using an effective medium approximation (EMA). If the structure parameters, such as trench width vary with depth, it is modeled as a stack of multiple layers, each characterized by its own effective refractive index. Therefore the problem of modeling the optical response of a complex etched structure can be reduced to the simpler problem of modeling a multilayer stack.
To illustrate this point, see the example presented in Figure 1. The figure shows simulated spectra for 45 degrees incidence and S-polarization for an array of square trenches etched in silicon. The trenches are 1 micron deep, have a pitch of 0.25 microns and have a width of 0.125 microns. The figure shows both an exact calculation using rigorous coupled-wave analysis (RCWA) and a simplified calculation using a Maxwell-Garnett type EMA. A vertical line corresponding to about 10,000 cm-1 (that is a wavelength of 1 micron) indicates the “diffraction threshold”, WHERE the first transmitted diffraction order emerges. In the short-wavelength range to the right of the threshold there is a rather complex spectrum typical for scatterometry. In the infrared range to the left of the threshold, the diffraction is absent. The spectrum comprises a regular pattern of interference fringes arising from the interference of light reflected from the top and bottom of the trench structure. As per the figure, the fringes can be well approximated by using the EMA method. Parameters of the structure, such as trench depth and width are readily determined from the period and amplitude of the fringes. Thus the long-wavelength approach greatly simplifies the measuring of 3D-etched structures. Note that the advantage is not only in the ease of modeling, but also in the fact that infrared spectra themselves are rather simple and their relationship to the parameters of the structure can be readily understood.
The maximum obtainable magnification with a conventional optical microscope is approximately 800-1000×, because of the nature of light. For further magnification Scanning Electron Microscopes (SEMs) are used, and among these, the Transmission Electron Microscopes (TEMs) can show single atoms and thus provide the highest possible magnification. Why then do we have the Scanning Probe Microscope, SPM as yet another type of microscope?
One of the reasons is that a sample to be examined in a transmission electron microscope must be sliced thinly and thus becomes ruined. The SPM technique images surface structures with atomic (height) resolution without the necessity of damaging the sample. A further reason is the kind of imaging offered by SPM microscopes, the results are shown as a kind of three-dimensional image (also in cases where only two-dimensional information is evaluated). As this is the case with an optical microscope, it is very difficult to investigate the surface structure of a sample with an electron microscope. In order to measure a surface profile with the highest resolution, it is necessary to slice the sample. Moreover, an SPM operates without vacuum, and unlike to an electron and optical microscopes it can measure other physical effects. This includes electrical properties, such as Kelvin probe force microscopy (KPM / KPFM) or magnetic properties (Magnetic force microscopy, MFM).
The most basic AFM operation mode is the so-called DC or contact mode. The most basic AFM operation mode is the so-called DC, or contact mode. A force is applied to the cantilever (sensing) tip, when the cantilever and the sample surface are close to each other. This leads to the bending of the cantilever, which changes the reflectance angle of the detection laser. The deflection of the laser is measured by a position-sensitive photo detector. When the cantilever is moved towards the sample during the approach, attractive forces are applied to the cantilever. These negative forces can be used to perform a surface scan.
In order to prevent the cantilever from damages, most AFM microscopes are usually operated in the so-called AC mode instead of DC mode. In this mode, a very low force is used during the scanning, and a very little interaction occurs between the surface and the cantilever, without loosing resolution.
The cantilever is permanently vibrated with its resonance frequency. This oscillation leads to a periodic bending of the cantilever, which is measured by a reflected laser beam like in DC mode. When the cantilever is close to the surface and interacts with the surface atoms, the resonance frequency changes. (The resonance frequency increases, when the cantilever reaches the surface.) This leads to the dampening of the amplitude and the phase change of the cantilever oscillation. The tip-sample interaction force and the dampening of the cantilever oscillation are approximately proportional when the tip is close to the sample.
AC mode works with less interaction force, than DC mode, and has some advantages:
To be able to detect magnetic forces, one has to use a cantilever, which is covered with a magnetic coating. Standard MFM tips have a magnetic coating with a relatively large thickness of around 40 nm. The high thickness results a much longer tip radius of around 50 nm, which is much larger than the conventional AFM tips. With these standard tips, one can obtain lateral magnetic resolutions of about 100 nm. So the magnetic resolution is much less, than the resolution, which can commonly be expected from AFM topography measurements, which are a factor of 10 better.
When the MFM tip is close to the sample, mechanical force, as well as magnetic force contribute to the force that is measured by the cantilever. The magnetic force is much smaller than the mechanical force, but is effective over a long distance. To separate the two force sources, the MFM force must be measured at a certain distance from the sample surface, where the contribution from the mechanical force can be neglected.
This mode is a method to determine the chemical potential between a cantilever and a sample surface, and therefore gain information about the material and its state on a sample surface. The basic setup for a KPFM measurement is shown on Figure 6. The sample is electrically connected to the output of a frequency generator. This creates an oscillating electrical field between the cantilever and the sample, which can be measured by the position sensor of the AFM. This detected signal is then fed into a lock-in amplifier and its output is led into an integrator. The result is then added as a constant offset to the oscillation, which is applied on the sample. This acts as a feedback loop, which minimizes the electrically-induced cantilever oscillation.
This method is used to determine the relative changes of majority carrier concentration of flat semiconductor surfaces. The capacity measurement is done quite similar to the KPFM measurement. You can use the same measurement setup. The main differences are that the output is coming from the lock-in amplifier, not from the feedback signal, so one uses the force component F2ω, described above at KPFM, which is proportional to the capacity C between tip and sample surface.
The capacity measurement also results in relative information about the capacity. The absolute capacity between tip and a conducting sample is in the range of aF (atto farad = 10^-18 farad) where the capacity between cantilever body and sample is in the range of fF (femto farad = 10^-15 farad). These small dimensions and the large offset show that a direct measurement of the capacity is nearly impossible and also there, only relative values can be obtained.
A KPFM measurement of the electrostatic force without a nulling feedback regulator is called EFM, electrostatic force measurement. Here, the feedback regulator is left out and the output of a lock-in amplifier is directly monitored. This measurement shows, as seen in the equation Fω derived above for KPFM, a mixture of different information and is influenced by the chemical potential, the oscillation voltages, the capacity etc. Therefore the EFM method is less useful for determining physical properties than the KFPM method.
The STM mode has the most simple principle and works only with conducting samples. A metall tip perpendicular to the sample surface is used instead of a cantilever. The tip as well as the sample are connected to a voltage supply. Now the STM tip is brought so close to the sample that a current flows, a so-called tunnel current. There is an exponential connection between the tip to sample distance and the current flow.
There are two variations for scanning: In the constant height mode, the height of the tip and the voltage is kept constant, while the tunneling current changes. For an images, this changes are translated to topography information. In the constant current or feedback mode, the tip is moved up and down to maintain a constant tunnel current, thereby the tip is moving in a constant distance to the surface. The necessary up and down movements of the tip are electronically recorded and can be displayed as an topography image.
Even though this principle is very simple, it instantly delivers the maximum possible magnification and allows the investigation and manipulation of individual atoms. The reason for this is a kind of self-focusing effect: The tunnel current responsible for the imaging already flows over the atom on the tip which is closest to the sample surface. So even when using a tip cut just by an ordinary pair of pincers one quite often gets atomic resolution.
The voltages used for STM measurements are normally below 1 V, since the electric field between sample and tip created by the fine tip is very large. An applied voltage of 5 V already results in a strong electric field causing a displacement of the tip and the sample surface. Actually, a 5 V pulse is often used for "cleaning" the tip.
The tunnel currents are on the nA scale, and the distance between tip and sample are in the order of magnitude of a few nm. STM measurements require that the sample surface is atomically "clean". In air, it is only possible to obtain atomic resolution for certain samples. The easiest atomic grid to observe is that of an HOPG sample (Highly Oriented Pyrolytic Graphite) since the top layer from such a sample can be removed by means of a piece of adhesive tape, revealing for a short time an atomically "clean" surface. Therefore, such samples are often used as test structures for STM microscopes; all DME STMs are tested for atomic resolution with a HOPG sample as a part of our internal certificate of complience (CoC) test.
The STM scanning is very fast; for atomic resolution images you can usually achieve a scan rate of multiple images per second.
In this method also an conductive probe is used. Operating in DC mode, an DC bias is applied and the electric current between the tip and the sample is measured. The resulting resistance is caused by the spreading resistance and resistivity of the sample which also give information about the carrier distribution.
Note, that you have to scratch the surface of the sample with the tip to get a better electrical contact.
The typical radius of SPM sensors is in the order of 10 nm. So it is possible to perform lithography on a scale of only a few nanometers.
Doing a lithography means that physical or chemical properties of a sample's surface are being changed. This can be done in three different ways:
The main idea of the SPM program lithography function is to transfer the information of a bitmap via physical properties like a force, a voltage or a current to the surface. Depending in which mode (AFM-AC, AFM-DC, STM) the SPM is running, the physical property can be a bias or a current (STM), a force variation (AFM) or voltage at the auxiliary port of the controller (all modes). The Aux voltage can be converted into many other physical properties using external equipment.
Assume the surface of an AFM tip to about 100 nm2. If a tip with a force constant of 50 N/m is pressed 200 nm towards the surface, the resulting force equals about 10-5 N. The pressure applied to the surface is of about 100 GPa. This means that on almost any surface softer than the silicon tip, patterns may be scratched into the surface.
Almost all Scanning Probe Microscopes are capable of resolving atomic height steps.
With the Semilab AFM Proberstation series, we provide the ultimate unification of ease of use and performance. Decade-lasting experience in the Scanning Probe Microscopy application field and manufacturing are united in the Semilab AFM scanners to help the user achieve the best and most reliable results in the shortest possible period of time. The compact design of the Semilab AFM scanner guarantees outstanding stability and scan rates. The unique plug and play cantilever exchange secures the fast and safe operation of the instrument. An integrated optical axis in the AFM scanner provides total visual control during approach and positioning. The Semilab AFM scanner provides the facilities for all common and advanced AFM modes. The integrated electronics in the scan head guarantee the lowest noise values in electrical AFM modes. Semilab AFM Proberstation series allows the installation of the Semilab AFM scanner into the stages and other facilities, like nanoindenters or optical microscopes.
The Atomic Force Microscope - Proberstation 1000 provides the possibility of investigating nearly all samples by Scanning Probe Microscopy. From a splitter of a coated wafer to a whole nano-coated surgical implant, the compact granite stage will surprise with its flexibilty.
Thereby, the design makes no compromises concerning stability.
Scanning atomic layers on a standrand writing desk is possible. The open tip scanner design gives room for free sample access and variety of selectable XY translators and special measurement stages as a temperature control stage or liquide cell. For exotically shaped sample we can provide customised illumination, sample stages and sample holder for request.
This small granite stage is an ultimate AFM work horse with unlimited upgrade abilities.
Ease of use:
Atomic Force Microscope - Proberstation 2000 is an automated high-end Scanning Probe Microscopy platform for large samples.
Its long range XY translators allow the investigation of 200x250 mm samples at all positions and up to 300 mm on specific sample areas. The fully programmable sample positioning enables autonomous AFM measurements on multiple sample positions with automatic image analysis and automatic report generation. The combination with high end optics enable on automated preselection of sample sites for SPM investigation.
Thereby the Semilab AFM-2000 setup is the optimal solution for industry quality maintenance application and R&D environments. The unlimited space for custom or OEM integration of additional analytics equipment opens up new horizions for research application.
Ease of use:
Upgrade your light microscope to subatomic resolution with AFM Objective: the 15x objective which delivers nanometer resolution and 3D topography.
The AFM Objective in your optical microscope is the ticked for the world behind the defraction limit. The resolution is pushed far below sub-wavelength range to only a few nanometer or less. Additionally you benefit from the 3D topograpical data with a better resolution than CLSM ans STED microscopy.
Nano-manipulation and nano-lithography combined with the optical facilities of your microscope build a powerful combination. Because of the Semilab adaptor concept for the AFM Objective, the integration in all known microscope brands is possible.
Unchanged operation of hte light microscope with all its facilities is ensured.
Main application areas are microelectronics, material and biological sciences, etc.
Ease of use: