OVPD

An alternative to thermal evaporation which has been developed is organic vapor phase deposition. In OVPD, an evaporated small molecule is entrained into a controlled inert gas jet from which it is deposited onto the substrate. The process gives excellent control over the morphology and characteristics of the deposited film [48]. This deposition can be performed as a blanket operation using a showerhead or using a small nozzle which can be translated and the gas get pulsed to create a defined device pattern with almost any small molecule material [105]. 

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Organic vapor phase deposition (OVPD) was invented by S. Forrest at Princeton University to transfer all the benefits of the gas phase process to applications in organic electronics. OVPD technology has the potential to overcome the limitations of VTE, similar to the replacement of MBE by MOCVD. 

In 1995 Forrest introduced the dry OVPD principle and has since investigated the benefits of gas-phase transport by using a simple flow reactor to overcome many... 

Fig. 9.1. OVPD principle and quartz flow deposition module invented by Forrest.

Using a carrier gas in OVPD enables the deposition of organic materials at a controlled pressure of 10 3-10 torr. Thus the OVPD module does not have to be pumped down to high-vacuum conditions, as in VTE, which consequently increases the uptime of the OVPD deposition system. Also, the continuous purge of carrier gas in OVPD prevents any contamination of parasitic surfaces, which increases the reproducibility of the deposited organic film quality. Therefore Forrest envisaged with the flow deposition module that OVPD is an ideal solution for industrial mass production. 

The inherent advantages of OVPD can be exploited on an industrial scale when combined with close coupled showerhead (CCS) technology. With CCS the carrier... 

Fig. 9.2. Close-coupled showerhead technology for industrial-scale production of multiple layers and OVPD. OLED example deposited in a single OVPD module.

Fig. 9.3. Temperature distribution in the OVPD chamber and of the actively cooled substrate in Gen 2.

An important thermodynamic requirement for the homogeneous distribution of the organic gas phase in the close coupled showerhead is temperature uniformity, which also guarantees no unintended condensation of the organic material. Figure 9.3 depicts the temperature simulation of an Gen 2 OVPD showerhead suitable for substrate sizes of 400 mm x 400 mm. The showerhead is uniformly heated to 300 °C, whereas the chiller actively cools the substrate and the mask to 2 °C and to 3 °C, respectively, across the whole area. Both the mask and the substrate are in close contact and thereby actively cooled down across the whole area. 

Experimental proof of control of the mask temperature with the chiller in a Gen 2 OVPD module under process conditions (showerhead heated to 325 °C) was achieved by in situ temperature measurement, as shown in Fig. 9.4. The experiments were performed at atmospheric pressure and at a deposition pressure of 0.9 mbar typical for OVPD, and for chiller temperatures between 5 and 30 °C. The mask temperature can be linearly controlled by the chiller temperature. The observed AT of 6.5 degrees is in good agreement with modeling prediction of 3 degrees in Fig. 9.3. In addition, measurements during a typical OVPD deposition time of 2 to 6 min confirmed there is no temperature drift under process conditions over time. The data prove that heat conductance and radiation is perfectly compensated by the chiller capacity. 

In conclusion, stable and reproducible temperature conditions for the mask and the substrates, essential requirements for a stable production process, have been demonstrated for OVPD. 

Fig. 9.4. Li near mask temperature dependence in OVPD as function of chiller temperature proves heat flux is controllable in OVPD.

Key requirements for industrial deposition of single films are the rate of deposition achievable, controllability and reproducibility, and film quality, which is important for high uptime and production yield. In contrast to VTE, in which the deposition rate is controlled by the evaporation temperature in the crucible, OVPD is kept under steady-state temperature conditions and the deposition rate is adjusted only... 

The uniformity of such an OVPD film of Alq3 is shown in Fig. 9.6. Analysis by variable angle spectroscopic ellipsometry (VASE) confirmed the surface was smooth across the entire substrate area with thickness deviation of +1.7%, a standard deviation, a, of 1.0% only. Atomic force microscopic analysis of such a typical film revealed RMS values to be 6 A, i.e. thickness differences in the range of a single monolayer only, irrespective of deposition rate [20-22]. 

Figure 9.7 illustrates the controllability of the OVPD process for achieving sharp... 

Besides thickness reproducibility a basic requirement for an industrial production process is process stability. Figure 9.8 shows the long-term source stability of Alq3 tested in an Gen 2 OVPD module. Using identical process conditions on days 1 and 21, more than 500 h continuous operation, we observed a deposition rate of... 

Fig. 9.8. Long-term source stability for Alq3 in OVPD.

These data prove that thin layer multi-heterojunction devices are enabled by OVPD with accurate thickness reproducibility and layer performance, which are essential manufacturing requirements in industrial mass production. The OVPD technology also enables controlled material transport and a high material utilization efficiency of 50-70%, based on condensation on to the substrate only. For state-of-the art VTE manufacturing technology material utilization efficiencies are in the range of 1 to 6% [23],... 

Research on organic semiconductor materials has resulted in remarkable progress and the introduction of material combinations using hosts and single or multiple guests or additional host materials [24-30]. This demand for simultaneous coevaporation or doping is often difficult to achieve in VTE, but easily achieved by OVPD. In the previous section we explained the OVPD process conditions essen-... 

Almost linear OVPD calibration curves of the typical dopant rubrene for a variety of source flows up to 10 seem and up to 50 seem are presented in Fig. 9.9, which shows that the deposition rate can be precisely adjusted from 0.06 to 1.6 A s-1. Both curves are an ideal fit and reveal a linear relationship between deposition rate and source flow they were collected with two mass-flow controllers of different capacity ranges (10 seem and 50 seem). Ellipsometric thickness analysis confirmed for both experiments a deposition rate of 0.3564 and 0.3582 A s-1, which is a relative error of only 0.48% and is identical with our prediction of dopant controllability (Table 9.1). Using a standard OVPD deposition rate of 10 A s-1 for a hosts the doping range of rubrene can be very precisely adjusted in the range of 0-16%. 

As we have described, OVPD technology enables the deposition of layers of pure materials or precise compositions of different materials, for example hosts, cohosts, and dopants. Besides layer composition, layer morphology [34], deposition rate [35, 36] and the substrate temperature [37-40] also affect device performance. Layer morphology is strongly affected by the substrate itself, but also by the deposition process and the deposition conditions. 

Forrest stressed as early as 1998 that two different types of growth regimes should be realized in this unique deposition process by variation of unique OVPD deposition parameters, for example substrate temperature, chamber pressure, and... 

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