Fabrication of the Chalcogenide Glass-Based Thin-Film Sensor Array
The chalcogenide glasses, which are needed as targets for the subsequent PLD process, were synthesized from the base elements of high purity in evacuated quartz ampoules at a pressure between 0.1-0.01 Pa and at temperatures of 1000-1200 K for 5-12 h. The ampoules with the melt were air-quenched in the 400-600 K temperature range, with an average cooling rate of 4-6 Ks
-1 [
7,
11]. In this way prepared chalcogenide glasses served as target materials for the thin-film deposition.
For the fabrication of the sensor arrays, a p-doped (Bor) 4” single-crystal silicon wafer (Wacker-Chemitronic) with <100>-orientation, specific resistance of >1000 Ωcm and thickness of 381±25 µm has been used. The complete fabrication steps to realize the thin-film sensor array are summarized in
Figure 1.
Figure 1.
Survey on the fabrication steps of the chalcogenide glass-based thin-film sensor array (I: thermal oxidation, II: spin-coating, III: exposure, IV: chemical etching, V: electron-beam evaporation, VI: lift-off process, VII: ONO passivation, VIII: RIE, IX: PLD).
Figure 1.
Survey on the fabrication steps of the chalcogenide glass-based thin-film sensor array (I: thermal oxidation, II: spin-coating, III: exposure, IV: chemical etching, V: electron-beam evaporation, VI: lift-off process, VII: ONO passivation, VIII: RIE, IX: PLD).
The first fabrication step is the generation of a SiO2 layer with a thickness of about 500 nm as insulating layer between the substrate and the electrodes as well as the electrodes (fabrication step I). This SiO2 layer was grown by means of thermal oxidation (oxidation oven, Tempress). To structurize the respective thin-film sensors as sensor array on the silicon wafer, photolithographic patterning has been performend (fabrication steps II + III). Therefore, a positive photoresist (AZ 5214, Hoechst) was spin-coated on the silicon substrate. Hexamethyldisiloxan was firstly used to improve the adhesion between the photoresist and the silicon substrate. For the exposure of the photoresist through a mask, which consists of the structure of the sensor array, a mask aligner (MA 6, Suss-microtec) was used. After developing the photoresist (AZ 312 MIF developer, Clariant), the structure of the sensor electrodes was chemically wet-etched by hydrofluoric solution (AF-91-09-HF, Riedel de Haen) as shown in fabrication step IV. In this way, trenches have been etched into the SiO2 layer, in order to embed the contact layers for the sensor structures. Fabrication step V depicts the deposition of the contact layers by means of electron-beam evaporation (PLS 500, Balzers). The contact layers are built-up of a layer system of Ti:Pt:Au with a thickness of 30:175:300 nm.
The deposition of the contact layers has been finished by a lift-off process in acetone that allows to remove all photoresist-coated metallic areas (see fabrication step VI). To prevent a later corrosion of the silicon substrate and the contact layers, an oxide-nitride-oxide (ONO) passivation layer system has been deposited by means of a plasma-enhanced chemical vapour deposition process (UDP 80, Plasma Technology) [
12]. This ONO-layer system was made of SiO
2:Si
3N
4:SiO
2 with a thickness of 130:530:130 nm (see fabrication step VII). By means of a reactive ion-etching process (AMR, Oxford Instruments), only the areas which serve as contact for the sensor layers as well as the contact pads have been opened, as shown in fabrication step VIII.
Subsequent to the described process steps, the heavy metal-selective chalcogenide glass materials such as Pb-Ag-As-I-S, Cd-Ag-As-I-S and Cu-Ag-As-Se have been deposited by means of an “off-axis” PLD-process (see fabrication step IX). A KrF-excimer laser (LPX 300, Lambda) with a wavelenght of 248 nm and a repetition rate of 10 s
-1 was used to deposit the individual materials of the respective chalcogenide glass target to the Si/SiO
2 substrate including the contact layers. An energy density of the laser of 5 Jcm
-2 was applied to the chalcogenide glass target at room temperature. The PLD process took place in a N2-atmosphere to prevent any oxidation of the deposited materials at a pressure of 2*10
-1 mbar. A process time of approximately 10 minutes was used to get thin films with a thickness of about 500 nm. To ensure the patterning of the different thin-film sensors on the silicon chip, a home-made Al mask has been used. The detailed PLD process is described elsewhere [
9,
10,
13]. After cutting the wafer into single sensor chips of 10 mm * 10 mm, each sensor chip has been glued on a printed circuit board (PCB), wire-bonded and encapsulated by an epoxy resin (EPO-TEK 87-GT, Polytec).
Figure 2 shows the cross-sectional view of a single sensor chip including the chalcogenide glass-based thin-film sensor array after connection to the PCB and encapsulation.
Figure 2.
Schematic cross-section of the chalcogenide glass-based sensor chip after bonding and encapsulation.
Figure 2.
Schematic cross-section of the chalcogenide glass-based sensor chip after bonding and encapsulation.
Physical and Electrochemical Characterization of the Chalcogenide Glass-Based Thin-Film Sensor Array
The thin films based on chalcogenide glass materials were physically studied using Rutherford backscattering spectrometry (RBS) to control the stoichiometry of the target and the deposited thin films. By means of transmission electron microscopy (TEM) as well as scanning electron microscopy (SEM) the morphology and topography of the prepared thin films have been proved. For the TEM characterization, see e.g. [
10].
The electrochemical properties of the sensor array were investigated by means of potentiometric measurements. The sensor array itself represents an electrochemical half-cell. To complete the measuring set-up for ion-selective potentiometry, it is necessary to utilize an additional reference electrode. In this experiment, we chose a Ag/AgCl double-liquid junction reference electrode with an inner solution of 0.1 mol/l KCl, and 0.1 mol/l KNO3 for the outer solution. The potentiometric response of the sensor array has been characterized in Pb2+-, Cd2+- and Cu2+-solutions in the concentration range of 10-6-10-2 mol/l for Pb2+- and Cd2+-ions, respectively, and 10-7-10-3 mol/l for Cu2+-ions. To guarantee a constant ionic strength of the test sample, a constant background electrolyte solution of 0.1 mol/l KNO3 was used. All chemicals applied were of reagent grade.
The sensor signal was recorded by a highly ohmic voltmeter (Type 2700, Keithley) controlled by a conventional personal computer via general purpose interface bus (GPIB); using a multiplexer offers the possibility to simultaneously measure and control up to 20 sensors. A schematic picture of the complete measurement set-up is shown in
Figure 3.
Figure 3.
Measurement set-up for the electrochemical characterization of the chalcogenide glass-based thin-film sensor array.
Figure 3.
Measurement set-up for the electrochemical characterization of the chalcogenide glass-based thin-film sensor array.