Matthias Plötner

Photopatterning of thermally sensitive hydrogels useful for microactuators

Abstract Hydrogels based on PNIPAAm [poly-( N -isopropylacrylamide)] are an attractive working material for microactuators because of their swelling with thermally induced phase transitions leading to a very high volume expansion. By the employment of co-polymerization with a chromophore, a specially designed polymer has been developed that can be photocrosslinked to a gel and by this photopatterned. The focal point of this work is the preparation and characterization of mircopatterns using this co-polymer. The efficiency of the photochemical reaction is quite good and patterning results down to 20 μm spaces have been achieved. The swelling of dot-like patterned hydrogel films on Si/SiO 2 substrates in water occurs with similar temperature dependency and a swelling ratio of 3 to 3.5 in the same order of magnitude compared to bulk PNIPAAm-based gels, but the transition time of 2 to 8 s measured at the gelous dots is much shorter than that of macroscopic objects, making the material interesting for microactuator performance. The transition temperature decreases from 32 to 34°C at pure PNIPAAm to 18 to 20°C at the photopatterned gels of the copolymer.

Characterization of Cu chemical mechanical polishing by electrochemical investigations

Abstract In order to investigate the chemical etching and passivating effects of Cu in the polishing slurry and to give some insights of the polishing process itself, the influence of the oxidizing agent (H 2 O 2 ) was studied. The interaction between the Cu surface and the slurry was investigated by potentiodynamic measurements applied during the polishing process as well as under static conditions. It was shown that with increasing H 2 O 2 concentration the wet etch rate and also the polish rate of Cu decreases. That can be explained by changes in the structure of the passivating layer and the dominating role of the dynamic repassivation during polishing. In addition, WTi used as a barrier/adhesion layer in Cu chemical mechanical polishing (CMP) was also investigated in terms of electrochemical interaction with the polishing slurry and Cu. The observed acceleration of the WTi polish rate in the presence of Cu ions is caused by the galvanic interaction between Cu and WTi.

Microfabrication of thermoelectric generators on flexible foil substrates as a power source for autonomous microsystems

A flexible thermoelectric generator with overall dimensions of 16×20×0.05 mm has been fabricated using a unique low-cost procedure as part of our ongoing research efforts on developing economical and reliable energy sources for autonomous microsystems. The generator consists of a multiplicity of micro Sb-Bi thermocouple strips embedded in a 50 µm thick flexible epoxy film and is capable of generating a voltage of 0.25 V at a temperature difference of 30 K. Its fabrication involves only a few steps, such as foil lithography, electroplating, embedding and wet chemical etching. The exposure of aqueous photoresist on flexible foil substrates is carried out using a specially constructed mask aligner by Karl Suss GmbH. The thermocouple strips, with a cross section of 40×10 µm and a length of 20 mm, are electroplated galvanostatically from their associated acidic electrolytes into the patterned AZ 111XFS template on a 50 µm thick copper foil. After a top-over embedment of the electroplated structures with an epoxy film, the original copper substrate is removed completely by wet chemical etching, leaving the generator module to be embedded in the epoxy film. This process is proven to be cost-effective, easily manageable and highly reliable. It is now being applied to our current fabrication of more efficient thermoelectric generators based on n- and p-type Bi2Te3 compound materials.

Characteristics in chemical-mechanical polishing of copper : comparison of polishing pads

Abstract A systematic study of Cu CMP in terms of the effect of polishing pad properties on the process characteristics has been performed. The IC 1000 and IC 1000/SUBA IV polishing pads were compared with regard to the polish rates, across-wafer uniformity, planarity and pattern sensitivity of the CMP process. Polishing with the IC 1000/SUBA IV pad, a better uniformity and higher polish rates were achieved. No differences in the pad influence on the geometry effects (Cu dishing and SiO2 thinning) have been found, which can be explained with the same near-surface layer affecting the interaction between pad and wafer. The hardness of the IC 1000 allows the material to planarize across wide Cu areas with minimal dishing and good planarity, but it is also essential that the SUBA IV bottom layer of the IC 1000/SUBA IV polishing pad improves the resilience and compressibility of the pad and enhances the global uniformity in the polish removal. It was demonstrated that uniform material removal during polishing is one of the fundamental concerns in the CMP technique affecting the tolerances deliverable by the CMP process.

Chemical-mechanical polishing of copper for interconnect formation

Abstract A systematic study of Cu chemical-mechanical polishing (CMP) in terms of process parameters influence, planarization ability of the process and pattern sensitivity of the polish rate was performed. We examined the effects of Cu dishing and SiO 2 thinning and the reasons for them. Both were found to be sensitive to the pattern geometry (line width and pattern factor) and the overpolishing time. The influence of the within-wafer nonuniformity in the polish removal on the polishing performance (planarity, line thickness) was also studied. Different optimization concepts in terms of the barrier/adhesion layer (Ta and WTi) were examined. CMP of Cu for damascene patterning was demonstrated and evaluated by electrical measurements as a promising technique for forming Cu lines.

Influence of process parameters on chemical-mechanical polishing of copper

Chemical-mechanical polishing (CMP) appears to be the most promising technology for global planarization of device topography and metal patterning in the damascene technique. Cu has been recently studied as a candidate material for future integrated circuit metallization because of its low resistivity and better electromigration resistance than current Al alloy interconnects. In order to achieve a highly reliable CMP process for Cu delineation it is necessary to examine the limitations of the process. Integrating Cu CMP into an interconnect processing sequence requires a detailed understanding of how process parameters affect different aspects of the CMP process and therefore the quality of the patterned lines.

Photocrosslinking of thin films of temperature‐sensitive polymers

The use of polymeric materials with temperature-dependent degrees of swelling (especially polymers that exhibit lower critical solution temperature (LCST) behaviour in aqueous solutions) in microsystems requires the preparation and patterning of layers in the µm range. Copolymers based on N-isopropylacrylamide were modi-fied with a stilbazolium salt chromophore to yield photocrosslinkable temperature-sensitive polymers. The chromophore and the polymers were characterized by UV, IR, 1H-NMR (nuclear magnetic resonance) and 13C-NMR spectra. The resulting polymers showed LCST behaviour, which was measured by differential scanning calorimetry. The photocrosslinking properties were studied by UV irradiation of the thin films and measurement of the changes in the UV absorption spectra. By irradiation of thin films through a mask it was possible to obtain patterned networks in the µm range (20 µm space width and ≥50 µm line width). The resulting patterned networks showed temperature-dependent swelling properties in aqueous media. Copyright

The interaction between different barrier metals and the copper surface during the chemical-mechanical polishing

Abstract The copper polishing rate in a weak acid slurry with H 2 O 2 as a oxidizing agent is determined by a two-step process of the formation of copper oxides followed by its mechanical abrasion. The electrochemical potential is a criterion for the driving force of oxidation-reductions reactions that occur during the Cu dissolution process. Using the experiment a corrosion potential ( E corr ) was determined. It depends on the potential of the cathodic reaction, the Tafel slopes of each reaction and the control of the reaction by Tafel kinetics or concentration polarisation. Potentiodynamic measurements were performed with different H 2 O 2 contents in the slurry. The dissolution current density ( i corr ) is correlated to the oxidation-reduction reaction rate. In the presence of a barrier metal, such as WTi, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H 2 O 2 . There is an increase in the barrier wet etch rate when copper is present. In the case of polishing both TiW and Cu simultaneously a dramatic increase of the barrier polish rate was observed. The acceleration of the W or Ti polish rate can be explained by the galvanic coupling between Cu and the barrier metal. By the use of TiN as a barrier metal no galvanic interaction has been observed. The removal rates of TiN have not been influenced by the presence of Cu.

Endpoint detection method for CMP of copper

A novel method to detect the endpoint during Cu-CMP has been developed. It is based on the determination of the Cu concentration within the slurry on the pad that has just polished the wafer. The measurement of the ion concentration is performed using a capillary and an ion-selective electrode. The endpoint of the CMP process is detected by the decrease of Cu ion concentration, which is displayed by an decreased potential at the electrode. An experimental set-up has been established which can be applied to a commercial polishing tool. The method has been tested under various process conditions. The new endpoint detection system revealed to work independently of the polishing tool and the wafer size.

Miniaturized gas monitoring system employing several SAW sensors

We have developed a compact gas monitoring system which is able to run up to four different SAW sensors. Additionally, four conductivity type sensors can be integrated. Besides the sensors themselves a major part of the device is an ASIC managing system control and data acquisition, i.e. both the handling of several RF signals and the control of the measuring conditions. A PCMCIA interface. has been included providing a standardized connection to a laptop for readout. It enables the data transfer and quick changes in the measuring procedure. The set-up of the system, especially the control of the sensor temperature and the sensor data acquisition, is discussed. At present we have integrated two sensors with differently deposited copper phthalocyanine films on self-designed 80 MHz-SAW filters with additional conductivity probe patterns differently responding to O/sub 3/ and NO/sub 2/. Additionally, a similar SAW sensor with a hyperbranched polyester film is able to measure NH/sub 3/ concentrations. The sensor design and preparation together with the resulting gas detecting properties of these devices are explained. The sensors proved to be capable to detect those gases down to the ppb concentration range. Results obtained with a test device of the gas monitoring system as a whole are discussed. The device comes out as an opportunity for an easy and handy multi-component gas analysis.