Análisis de casos prácticos

The in-situ measurements were performed using an existing ultra high vacuum (UHV) chamber with load lock. However, this chamber was not prepared to conduct electronic measurements and to evaporate organic material in a controlled way. Therefore, we had to customize the chamber by equipping it with an evaporation cell and a sample stage. Especially making contact using the load lock was challenging and will be explained in detail below. In the following, a short description of the evaporation chamber including the sample stage and sample holder, is given.

Evaporation chamber

A photograph of the evaporation chamber is depicted in Figure 6.2. For a better overview, the important parts of our measurement chamber for the in-situ measurements are sketched in Figure 6.3.

The evaporation chamber is equipped with a Perkin Elmer ion getter pump and a titanium sublimation pump. Originally, the base pressure was about 10−9_{mbar, but due to the}

electronic components and several feed throughs, that had to be added, it deteriorated about one order of magnitude to roughly 10−8_{mbar, which was measured by a Balzers full}

6.2 Experimental implementation 67

Figure 6.2: Photograph of evaporation chamber. Most relevant parts for the in-situ mea- surements are labeled.

range gauge. This pressure was sufficient for our experiments and is comparable to the pressure of the UHV evaporation chamber, that we used for the fabrication of our standard transistors (see Figure 4.1). We incorporated a tantalum bag, that was resistively heated by an electric current, as described in section 4.3. A new sample stage, which is explained in more detail below, was mounted at a manipulator rod, that was located in the center of the chamber. It was possible to rotate the sample stage enabling us to turn the samples in and out of the evaporation flux. Furthermore, two quartz crystals were installed adjacent to the sample stage at the manipulator rod in order to measure the thickness of the deposited pentacene. The sample stage could be transferred into the main chamber without braking the vacuum using a transfer rod. The load lock was evacuated by a turbo molecular pump, that worked in combination with a rotary vane pump. As soon as the pressure of the pre-vacuum chamber was below 10−5_{mbar, the plate valve, that separated the pre-vacuum}

chamber from the main chamber, could be opened to enable the transfer of the sample holder to the sample stage. The whole mounting procedure could be done within about ten minutes.

68 6. Electronic thickness dependent in-situ measurements

Figure 6.3: Schematic overview of the in-situ measurement chamber. For claritiy purposes, only the main components, which are relevant for the electronic in-situ measurements, are sketched.

Sample stage

A completely new sample stage for the in-situ measurements was built. The design was optimized to comply with the following demands:

• transfer of the sample holder and clamping it as stable as possible

• electrically connecting the samples to the feedthroughs of the evaporation chamber

• heating the samples and controlling the sample temperature

• measuring the deposited thickness

• turning the samples in and out of the evaporation jet

6.2 Experimental implementation 69

For high vacuum capability, any use of alloys including tin or zinc should be avoided, which excludes soldering. Therefore, clamping was used as often as possible. Ceramics should be chosen as insulator materials, since polymers usually do not endure temperatures above 100◦C commonly used for annealing.

The sample stage consisted of three parts, namely the quartz crystals, the upper station part and the lower station part. All these parts were clamped at the manipulator rod, which consisted of an inner and outer rod and was located in the middle of the evaporation chamber (see Figure 6.4).

Figure 6.4: Photograph of the sample stage. Most relevant parts for the in-situ measure- ments are labeled.

First, two quartz crystals were clamped at the outer rod of the manipulator. One was orientated in the same direction as the upper and lower station part, and one was orientated perpendicular to it. This allowed us to measure the evaporation rate, when both, the samples were turned into and out of the evaporation jet.

Second, the upper stage was clamped below the crystals. The upper stage consisted of mainly two important parts. First, a sliding rail was designed in order to enable a precise mounting of the sample holder. Second, two 20W halogen light bulbs were put behind the designated position of the mounted sample holder and electrically connected to a feedthrough. This allowed us to heat the samples up to temperatures of about 200◦C. Third, the lower stage was clamped at the inner rod of the manipulator. The inner rod of

70 6. Electronic thickness dependent in-situ measurements

the manipulator could be moved independently of the outer rod enabling us to adjust and change the distance between the two stage components. The lower stage part was designed to establish electric contact between the samples in the sample holder and feedthroughs of the chamber. Therefore, we used sliding contacts, that were glued by a vacuum suited silver paste onto a ceramic plate. The sliding contact were connected to further feedthroughs of the chamber using the inner conductors of coaxial cables. The outer conductors of the coaxial cables were connected to the chamber. Furthermore, two guide rods were placed on the lower stage, that guaranteed alignment of the upper and lower stage part.

Sample holder

The sample holder is the counter piece to the sample stage and it is depicted in Figure 6.5.

Figure 6.5: Photograph of the sample holder. Most relevant parts for the in-situ measure- ments are labeled.

It is a plate made of a vacuum-capable composite and contains two sockets for the chip carriers, which are electrically connected to contact patches, that were defined lithograph- ically on the plate. The floor basement of the sockets and the plate below the socket itself have been removed to increase the thermal contact between the halogen light bulbs and the samples in the chip carrier. Finally, for temperature measurement a Pt 100 was glued on the plate and connected to the patches.