The sol-gel solution of PZT obtained from Alfa Aesar (A Johnson Mattey Company) was spin-coated (WS-400 from Laurell Technology Corporation) on Pt/Cr/SiO2/Si wafer at 1000rpm for 10s and at 3000 rpm for 30s. The PZT films were subsequently prebaked at 150◦C for 10 min and at 350◦C for 30 min on a hot plate. The SiO2 layer, as a barrier layer, is deposited by furnace to prevent the diffusion of Cr and Pt. Cr and Ti are both candidates for adhesion layer. However, the Ti is ruled out due to the possible etching away, while lithographic process of the device. Pt/Cr film was deposited by the dual e-gun evaporation system (Ulvac EBX-10C) and then annealed by RTA (HEATPLUSE 610) at 650◦C for 5 min.
The Hydrothermal treatment was carried out in a microwave reactor (CEM MARS-5) by immersing the substrate in 30 ml KOH + Pb(NO3)2 solution of various concentrations. The concentration of KOH was varied from 0.5M, 1M to 2M, and Pb(NO3)2 was varied from 0.05M, 0.1M, 0.15M and 0.2M. The substrate with PZT film was put in a Teflon beaker and covered by working solution in the microwave vessel. Prior to microwave heating, the vessel was purged under N2 atmosphere. The Teflon beaker was open with 3 holes to afford the paths for ions and to prevent the reaction between the films and with the precipitation of the solution. It should be noted the PZT film would be oxidized to PbO2 and lost if the hydrothermal annealing was not under N2
atmosphere. The setup of the hydrothermal annealing is shown in Fig. 3-1. For each experiment, the microwave reactor was sealed and heated to the working temperature of 160◦C in 10min, and the system was maintained for 30min. The system was cooled after the hydrothermal treatment, and the samples were
rinsed in boiling deionized water. Finally the sample is and dried by a hot plate.
The film quality was analyzed by SEM (HITACHI S-4000), AFM (Veeco Dimension 5000 Scanning Probe Microscope), XRD (Shimadzu XD-5), and EDX (JEOL JEM 3000F).
Fig. 3-1 The setup of hydrothermal annealing in microwave system:
(a) substrate, (b) solution, (c) Teflon beaker, (d) vessel (e) N2.
The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. The SEM uses electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by heating the metallic filament. The electron beam follows a vertical path through the column of the microscope. This design makes a better way through electromagnetic lenses which focus and direct the beam down towards the sample. Once it hits the sample, other electrons (backscattered or secondary) are ejected from the sample. Detectors collect the secondary or backscattered electrons, and convert them to a signal that is sent to a viewing screen similar to the one in an ordinary television, producing an image.
The atomic force microscope (AFM) or scanning force microscope was
invented in 1986 by Binnig, Quate and Gerber. Like all other scanning probe microscopes, the AFM utilises a sharp probe moving over the surface of a sample in a raster scan. In the case of the AFM, the probe is a tip on the end of a cantilever which bends in response to the force between the tip and the sample. Figure 3-2 illustrates the working principle of AFM work. As if the cantilever flexes, the light from the laser is reflected onto the split photo-diode.
By measuring the signal difference (A-B), various changes in the bending of the cantilever can be measured.
Fig. 3-2 The working principle of AFM.
There exist three main types of interaction, such as contact mode, tapping mode and non-contact mode. Contact mode is the most common method of operation for the AFM technique. As the name implies, the tip and the sample remain in close contact as the scanning proceeds. The so-called "contact"
means the repulsive regime of the inter-molecular force curve. One of the drawbacks of remaining in contact with the sample is that there exist large lateral forces on the sample as the drip is "dragged" over the specimen.
Tapping mode is the next most popular mode used in AFM. When operated in air or other gases, the cantilever is oscillated at its resonant frequency (often hundreds of kilohertz) and positioned above the surface.
Therefore, it only taps the surface for a very small fraction of its oscillation period. When the imaging of contact mode is restricted by the unstable or soft samples, tapping mode may be a second choice than contact mode for imaging.
Non-contact operation is another method which may be employed when imaging by AFM. The cantilever must be oscillated above the surface of the sample at such a distance that we are no longer in the repulsive regime of the inter-molecular force curve. This is a very difficult mode to operate in ambient conditions with the AFM.
X-ray Powder Diffraction (XRD) is an efficient analytical technique used to identify and characterize unknown crystalline materials. Monochromatic x-rays are used to determine the interplanar spacings of the unknown materials.
Samples are analyzed as powders with grains in random orientations to insure that all crystallographic directions are "sampled" by the beam. When the Bragg conditions for constructive interference are obtained, a "reflection" is produced, and the relative peak height is generally proportional to the number of grains in a preferred orientation. The x-ray spectra generated by this technique, thus, provide a structural fingerprint of the unknown. Mixtures of crystalline materials can also be analyzed and relative peak heights of multiple materials may be used to obtain semi-quantitative estimates of abundances. A glancing x-ray beam may also be used to obtain structural information of thin films on surfaces. In addition, changes in peak position that represent either compositional variation (solid solution) or structure-state information (e.g.
order-disorder transitions, exsolution, etc.) are readily detectable. Peak positions are reproducible to 0.02 degrees of glancing angle.
Energy Dispersive X-ray analysis (EDX) is often used in conjunction with
SEM or TEM and is an efficient surface science technique. An electron beam strikes the surface of a conducting sample. The energy of the beam is typically in the range 10-20keV. This causes X-rays to be emitted from the point where the material is irradiated by the primary electron beam. The energy of the X-rays emitted depends on the material for examination. The X-rays are generated in a region about 2µm in depth, and thus EDX is not an actual surface science technique. The detector used in EDX is the Lithium drifted Silicon detector. This detector must be operated at liquid nitrogen temperatures.
When an X-ray photon strikes the detector, it will generate a photoelectron within the body of the Si. As this photoelectron travels through the Si, it generates electron-hole pairs. The electrons and holes are attracted to opposite ends of the detector with the aid of a strong electric field. The size of the current pulse thus generated depends on the number of electron-hole pairs created, which in turn depends on the energy of the incoming X-ray. Thus, an X-ray spectrum can be used to acquire the giving information on the elemental composition of the interest material under examination.
3-2 Process of PZT Sensors
Figure 3-3 shows the process of a PZT sensor. The SiO2 500nm and Si3N4
300nm were deposited on Si wafer by a furnace. The purpose of Si3N4 layer is to prevent the possibility of device damage by KOH while the hydrothermal annealing. The adhesion layer (Cr) and the bottom electrode (Pt) were sequentially deposited on the lithographic pattern of SiO2/Si3N4/Si wafer. Then, the resist is removed by the specific solvents. The sample is annealed by RTA (HEATPLUSE 610) at 650◦C for 5min. The sol-gel solution of PZT obtained from Alfa Aesar (A Johnson Mattey Company) was coated on the Pt/Cr/Si3N4/SiO2/Si wafer by spin-coater (WS-400 from Laurell Technology Corporation) at 1000rpm for 10s and at 3000rpm for 30s. This pattern is designed by the exposure on photoresist, and etched away by HCl. The PZT films then were treated by the specific hydrothermal annealing in the microwave system. Finally, we also use the lift-off process to define the the top electrode (Au) and adhesion layer (Cr) prior to deposite the electrode by thermal coater, the photoresist pattern is defined. The developed PZT sensors were used to detect the organic vapor, including of the alcohol, toluene and acetone. The gas molecules were adsorbed on the sensors by only the physical adsorption. The frequency shifts were detect by the frequency counter (Agilent 53132A) and recorded by computer in real-time.
Fig. 3-3 The process of a PZT sensor.
Chapter 4 Results and discussion