Technische Ausstattung

Technical equipment



Combinatorial thin film deposition

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The Chair for MEMS Materials has specialized in applying the combinatorial technique to make and screen novel materials systems. Therefore, thin films are deposited by physical vapor deposition (PVD). There are currently 3 PVD chambers available (DCA, Finnland; AJA, USA), all designed to handle 100mm (4") silicon wafer substrates:

(1) K1 is a cryopumped UHV linear chamber, where 6 magnetron cathodes (3x DC/bipolar pulsed DC and 3x RF) are arranged along a straight arm that is translated back and forth to sequentially deposit layered films on a substrate. Shutters located near to the substrate surface can be positioned to confine the coated area by shadowing, or moved during the deposition to create wedge-shaped thickness gradients across the substrate.This chamber has a substrate table that can be RF biased, heated up to 1000°C and can have a magnetic field imposed at the substrate level during deposition or annealing.

(2) K2 is a cryopumped UHV circular chamber with 5 confocal cathodes (1x DC/bipolar pulsed DC, 3x RF). When the substrate is rotated about an axis through the focal point, atomic-scale mixtures of the sputtered materials are deposited. This yields nonequilibrium solid solutions and metastable alloy phases having identical surface and bulk composition without annealing, which would lead to thermodynamically stable phases, diffusion, segregation, and possible changes in bulk and surface compositions. Alternatively, substrate rotation can be stopped, leading to thickness and composition wedges coming from the deposition profile of the cathodes being used. The substrate in this chamber can be heated up to 1000°C during deposition or annealing, and either RF or DC biased.


(3) K3 is a turbopumped HV chamber with 3 confocal cathodes (1x each DC, RF, bipolar pulsed DC). This chamber is designed for depositing films of oxides and nitrides, and has a heater capable of going to 850°C in oxyxgen or nitrogen environments, as well as RF bias on the substrate.

A multipin stage can be mounted to any of the deposition chambers, which has 100 independent, spring-loaded electrical contacts arranged to press against the substrate around its periphery. This has been used for example to measure resistivity in situ during film growth, and for powering arrays of micro-hotplates during deposition or annealing.

A cooling stage can also be mounted to any of the deposition chambers. Liquid nitrogen can flow through this stage, with the temperature monitored by a built-in PT100 thermocouple.
Integrated with the UHV central handling transfer chamber, is a mask and storage module. Up to 6 wafers and/or shadow masks can be stored in low 10-9 mbar vacuum conditions. A handling mechanism allows the shadow masks to be placed, removed or rotated (in 90° increments) on a substrate wafer without leaving UHV.

Each deposition chamber is independently software controlled, so that for instance 2 chambers can be in use while target change or maintenance work is going on with the 3rd. Wafer and mask movement is computer controlled for chambers K1 and K2, while it is manually done for K3.




High throughput test stand (HTTS)

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The high throughput test stand (HTTS) is a custom made unit, specially designed for the rapid characterization of thin film materials libraries which in our case are commonly deposited on 4” Si-wafers.

Depending on the installed measurement module, electrical or magnetical properties of the materials library can be measured. A x-y table allows the scanning of the library with a laterial resolution down to a few tens of microns in a defined coordinate system, so different measurements at the same locations can be linked to each other. The samples are placed on a temperature controlled stage, so all measurements can be done over a wide temperature range, ranging from minimum temperatures of -100°C (using liquid nitrogen) up to a maximum temperature of +300°C. To avoid frost on the sample at low temperatures, the measurement table is enclosed in a box which is flooded with nitrogen during the measurement.

Electrical properties are measured using custom made four-point probeheads with spring loaded pins. The distance between the pins is 0.5mm, so the lateral resolution of the measurement is below 1mm. Special probeheads have up to 20 pins which can be addressed via a switching matrix to achieve even higher throughput. Temperature-dependent resistance measurements are, for example, ideally suited for measuring the transformation behaviour of shape-memory alloys.


Magnetic measurements are being performed using a MOKE setup with an electromagnet having a strength of approximately 0.3T. During these measurements, temperature-cycling is also possible. Since the electromagnet can be used simultaneously with the four-point probeheads, magnetoresistance measurements can also be done with the system.

When using a laser based distance sensor (resolution of 5µm) temperature-dependent measurements of materials libraries on cantilevers can be performed, where the deflection of the cantilevers during temperature cycling is monitored. The cantilevers’ deflection is due to stress which builds up for example during a phase transformation.

All measurements are automated; programming has been done using LabView.



Scanning electron microscopy & energy dispersive X-ray spectroscopy

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The JEOL 5800 SEM is a sophisticated scanning electron microscope that was designed to operate in either high or low vacuum mode. The microscope is controlled by an internal computer through a menu-driven software interface. The motorized stage (-62.5 mm < X < 62.5 mm, -50 mm < Y < 50 mm, 48mm < Z < 7 mm, -10° < Tilt < 90° and Rotate 360° endless) is operated from a joy stick, and the single joystick serves several functions which are set by buttons on the keyboard.
The instrument's computer can also receive instructions and be controlled by other independent computers through RS232 and SCSI ports. The chamber allows scanning over the total area of 4 inch wafers. The maximum loadable wafer size is 8 inch. The SEM is equipped with detectors for imaging of secondary electrons (SE), backscattered electrons (BSE) and x-rays. The SE detector is a highly efficient Everhart-Thornly detector for imaging at all scan rates in high vacuum. The Everhart-Thornly detector cannot be used in the low vacuum mode, and its function is replaced by the BSE detector.

This detector consists of an annular ring mounted under the objective lens and another flat detector a short distance from the lens, creating a parallax from which topographic information is derived in the low vacuum mode.
The Oxford analytical system consists of the EDX detector (Inca X-act) and two interface boxes (X-stream and Mics) and a standard pc. The EDX has an ultra-thin polymere window (d = 200nm) installed and can therefore detect elements from B to U. The detector is controlled from the Oxford interface, which contains the electronic hardware for the EDX signal and image acquisition, along with microscope automation hardware. The interface passes information to and receives commands from the user through the Oxford software on the PC computer. Image and EDx acquisition is accomplished through Oxford Windows software. The software is modular in design, and many of the available modules have been installed. There are four primary modules: Analyzer, Point & ID, Mapping and Automation. Qualitative x-ray identification, quantitative analysis, and x-ray mapping are available through the Analyzer module. The EDX can be utilized in both vacuum modes, but requires a working distance of approximately 10 mm (+/- 1 mm).



X-ray diffraction

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The PANalytical X’Pert PRO X-ray diffraction system is used for the high-throughput characterization of material libraries. Phase analysis of thin films and bulk samples can be performed, as well as reflectometry of thin layers and analysis of residual stresses. The system is equipped with a PiXcel detector that permits the acquisition of diffraction patterns in short times. Also, the phase analysis of batches of samples is possible. The analysis of small spots can be performed via using a microcapillary having 800 µm in diameter.

The system is equipped with a heating/cooling stage that enables the analysis of changes in the crystalline structure as a function of temperature in the range of -100 °C to 900 °C.



Cleanroom

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The cleanroom at the Chair for MEMS Materials is class 1000, and includes tools for photolithography and further microstructuring.

In the photolithography section of the cleanroom there is a spincoater which is used to coat the wafers with different photoresists. After a short prebake, the samples can be exposed to UV light by a special laser exposure unit (Heidelberg Instruments) or a mask aligner (Suess, Ma6/Ba6; Fig. right) for patterning. The laser exposure device has a resolution of approximately 5µm. Designs to be written can be easily loaded using a special software which makes it very flexible for rapid writing of unique patterns. This system is also used to produce patterned Cr-masks for the use in the mask aligner.


The mask aligner has a higher resolution (down to 1µm) but it is not as flexible since Cr-masks are needed for the exposure. Higher throughput of samples can be achieved with the mask aligner since the whole wafer is exposed at once, and not in a serial way as with the laser exposure unit.

The exposed samples are developed and then used for lift-off processes or etching steps. For the etching of Si a special KOH-etching bath is used where the KOH is heated to a constant temperature (80°C) for higher and stable etching rates.

MEMS structures are being produced by a multitude of repeated processing steps like deposition of thin films, etching and photolithography. Examples would be microcantilevers or microhotplates.

For the characterization of film thicknesses or the measurement of etching rates, a profilometer (Ambios, XP2; Fig. left) is used which is also situated inside the cleanroom.



Cantilever test stand

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The picture on the right shows a vacuum high-throughput test stand for the characterization of the actuator response of thin film SMA (including hot/cold plate, liquid nitrogen (LN2) and electrical feeds). Stress change measurements can be conducted in this high vacuum system equipped with a heating/cooling plate (HCP604SCV, Instec Inc., 4”x 4”, -100 °C up to 600 °C, temperature stability ± 0.2 °C, temperature uniformity ± 0.2 °C/inch). Water condensation at low temperatures or oxidation at elevated temperatures is prevented by the vacuum. The change of the temperature-dependent film stress (Δσ) at heating/cooling rates of typically 5 K/min is monitored by the deflection of each cantilever, as detected by a laser beam (parallel line optics) reflected at the free ends onto a screen and recorded by a camera. The stress can be calculated by the well known Stoney-Equation.

The test stand is a powerful tool to determine the reversible phase transformation behavior of thin films under substrate-induced stress.



High-throughput Automated Gourmet café System

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This tool was implemented immediately adjacent to the laboratory, to fulfill a critical support requirement of virtually all experimental and theoretical work. The high-throughput, one-touch automated Café gourmet chemical reactor (Phillips NV, Eindhoven NL) enables rapid fabrication of small-batch samples with extremely good uniformity and reproducibility. Solid source precursors are mixed in one chamber, while hydrogen hydroxide is thermally brought to the liquid-gas phase transition boundary (T = 373 K) in an upstream-adjacent pretreatment chamber. The gas, liquid and solid reagents are then directed into a reaction manifold, at the exit of which the products are once again separated, with the synthesized liquid being recovered in a downstream collector.

Suitably trained personnel can achieve the entire process in a few minutes, although relativistic time-dilation effects occuring during early morning conditions can give the perception of extending this quite considerably. Additional post-processing experiments can be conducted ex situ with natural or synthetic crystalline solid disaccharide, various grades of milk, or even cream.