The molecular constituents of organic light emitting devices (OLEDs) are typically soluble in organic solvents. Consequently, OLEDs cannot be exposed to the key components of conventional photolithographic processes. The OLED industry relies instead on thermally evaporated thin organic films that are patterned by metal shadow masks. But shadow masks presently limit OLED displays to smaller substrate sizes than their liquid crystal display (LCD) counterparts.1–3 Serial printing techniques may eventually provide a solution if they can be sufficiently parallelized to reduce takt time,4–9 the cycle time of the process. The ultimate goal for OLED manufacturing, however, is to replicate the widespread success of photoresist lithography. Hence, there is motivation for a renewed examination of variants of this inherently parallel, high speed approach.
Traditional photolithographic techniques have been applied to organics by making use of their orthogonality with highly flourous materials.10–12 They've been patterned by crosslinking and dissolving away undesired regions of the organics themselves akin to photoresists.13 A polymer film has been shown to protect organic materials during standard photolithography steps.14 In addition, super critical carbon dioxide has been employed to dissolve resists without interfering with the active material.15 Despite demonstrating respectable figures of merit, shadow masking remains the industry standard.
As early as 1978, IBM researchers investigated dry lithographic patterning of thin films using sublimation of a phase-change resist.16, 17 The absence of solvents, or liquids of any kind, may make such a process compatible with OLED patterning. More recently, Golovchenko et al. have used frozen H2O resists to pattern features at the nanometer scale.18–20 But water has been shown to cause degradation processes in OLEDs.21, 22 Instead, we consider the application of inert, frozen carbon dioxide (CO2) to the lithography of OLEDs.
A typical process flow for phase-change lithography is shown in Figure 1. The resist gas is first applied to a cryogenically cooled substrate where it freezes. The desired pattern can be formed in the resulting resist film via localized thermal excitation; in this work, we investigate resistive heating and stamping. As explained below, stamping is preferred, and this process of pattern transfer is described in Figure 1c. Following physical vapor deposition of the organic semiconductor or metal, the substrate temperature is raised and the resist sublimes, lifting off unwanted materials, and leaving behind only the intended pattern of organic or metallic thin film. The material that is lifted off can be caught by a shutter below the sample or, in the case of a manufacturing line, the step can be performed in a separate chamber to recover the material for reuse. A video of the frozen CO2 mask lifting off is available in the Supporting Information.
To avoid degradation of our active materials, we employ inert CO2 as the phase-change resist. The phase diagram of CO2 is shown in Figure 2a.23 In a low pressure process like thermal evaporation, the sublimation temperature of CO2 is reduced. For example, at our operating pressure of 10−6 Torr, the sublimation temperature of CO2 is roughly 90 K. Thus, for a stable lift-off mask, the substrate must be cooled to at least 85 K, which is still within the range of relatively inexpensive cooling with liquid nitrogen.
Once the CO2 resist is patterned by selective sublimation, it is important to control the partial pressure of CO2 in the chamber to prevent unwanted re-condensation of CO2 vapor on patterned regions of the substrate. It is also possible to freeze other impurity gases onto the substrate, notably H2O, whose phase diagram is shown in Figure 2b.24 In previous studies of frozen CO2 films at 10−7 Torr, Gerakines et al. measured a water deposition rate of 2 nm h−1.25 At these rates re-deposition must be considered in our experiments, but should ultimately be of little consequence in high throughput manufacturing since the acceptable background pressures of CO2 and H2O increase with reduced takt time.
The ultimate resolution of this lithographic process is determined by the resist thickness, which in turn is limited by the thickness of solid CO2 required to withstand the thermal energy carried by the incident organic or metal film.26 To estimate the minimum resist thickness, tCO2, of an unpatterned film, we consider the balance of the heat capacities cv, enthalpy of sublimation hs, and heats of fusion hf and vaporization hv of the resist and evaporated materials:
where ρ and t are the density and thickness of a material, respectively. The subscripts film and CO2 correspond to the evaporated thin film to be patterned and the resist, respectively. The minimum resist thickness obtained using Equation 1 is approximately 3 μm and 400 nm for 100 nm of deposited silver and standard organic materials, respectively. The density of the resist depends on pressure, temperature,27 and gas flow rate and, as mentioned in reference 28 a more amorphous resist avoids inhomogeneity at the length scales of the crystalline domains and is preferred for greater resolution. For the operating conditions in these experiments, the density is 1.51 ± 0.15 g cm−3; see the Supporting Information for a description of the interferometric technique employed to measure density and film growth rates.
The inherent disadvantage of phase-change lithography complicating the patterning step is the relatively large amount of thermal energy that must be supplied to overcome the heat of sublimation and remove the resist during patterning.18 Traditional optical lithographic exposure methods would require a great deal of power at wavelengths that are not readily available, λ = 2.7 or 4.3 μm,29 to achieve a suitable sublimation dose in a reasonable amount of time. Thus, although a number of selective heat sources are possible, we investigated resistive heating and a stamping technique to selectively sublime regions of the CO2 mask. Both techniques are capable of rapidly injecting a significant amount of heat into the CO2 resist.
Resistive heating was performed by applying a voltage along an indium tin oxide wire patterned on a glass substrate. At a current density of 625 kA cm−2, the heat dissipated by the resistive load is sufficient to sublime the overlying frozen CO2. An in vacuo photograph of this arrangement is shown in Figure 3a.
A schematic representation of the setup used for pattern transfer by stamping is shown in Figure 3b. The process is performed using two motorized linear stages (Standa Ltd.) to allow for motion control under vacuum: a 150 mm motorized linear stage to traverse the length of the cryogenically cooled sample and another, 30 mm in length, with actuation normal to the substrate to perform the stamping operation and to adjust the focus of a digital microscope. The microscope is used for positioning the stamp relative to the substrate and to observe the sample surface throughout the process.
In a demonstration of patterning after resistive heating, an approximately 100 μm-wide silver line is patterned by this method; see Figure 4a. The silver wire is observed to follow the outline of the underlying indium tin oxide (ITO) wire where it is narrowest, and the heat generation is the largest. Due to the high heat flux and diffusion through the substrate, however, this method yields an insufficiently rapid sublimation process resulting in cruder definition and undesirable debris. For this reason, the method was abandoned in favor of the stamping technique that heats the resist rapidly and directly, rather than through the substrate.
To demonstrate pattern transfer using a stamp, arrays of 20 μm × 50 μm features were patterned from thin films of the common organic compound tris(8-hydroxyquinolinato) aluminum (Alq3). A micrograph of one of these stamps is available in the Supporting Information. This resolution is compatible with commercial OLED display production, and exceeds that which might be used in a mobile display. The CO2 mask and the subsequent patterned Alq3 thin film are shown in Figures 4b and c. Below that Figure 4d is a photoluminescence micrograph of a λ = 405 nm LED-pumped array of Alq3 pixels with 78 μm pitch. For clarity, a longpass wavelength filter was used to remove the pump from the image. Profiles of patterned pixels were acquired with an optical profiler (Veeco Instruments Inc.) and contact surface profilometers (Veeco, Tencor). Figure 4e is a false-color surface topography showing multiple pixels while Figure 4f shows the two-dimensional cross section. The relative heat capacity of a stamp maintained at room temperature is more than sufficient to rapidly remove the frozen resist. To prevent abrasion and dust formation, the surface of the stamp need not make contact with the hard substrate surface if a universal burn-off step is performed to uniformly ‘etch’ the residual resist.19 In this step, all of the resist is uniformly removed a suitable depth such that none remains in the areas where the desired thermally deposited film is to remain.
To examine the impact of cold substrate temperatures on OLEDs, we built and tested OLEDs on substrates cooled to 112 ± 24 K. The external quantum efficiency (EQE) versus current density, J, and electroluminescence spectrum of these devices are shown in Figure 5. The best devices on cold substrates yielded efficiencies comparable to the room temperature grown control devices, suggesting that cooled substrate temperatures can be employed in OLED fabrication without degradation of performance. The performance and yield of the cold OLEDs was highly variable, however, and we observed a visible grey tint in the hole transport layer due to a slight coarsening in the morphology. While morphological changes might occur during low temperature depositions, we attribute the significant variation in device performance to the uncontrolled condensation of water vapor or CO2 on our substrate surface during substrate cooling and the growth of the thin films.21, 22 This can be rectified by reducing the takt time from the ∼1 h process used in our laboratory, and reducing the partial pressures of water and CO2 using cold traps.28
In addition to addressing concerns of the organic layers' growth under cold conditions, the transistor backplane of active-matrix displays must also withstand the low temperatures of the process. To verify this, a small active-matrix OLED display was removed from a digital photo frame, pumped down to high vacuum and cooled with liquid nitrogen, and then replaced in its housing and connected to its driver. There was no noticeable difference in pixel brightness, uniformity or operation aside from the seal of the passivation glass coming loose—passivation being a manufacturing step strictly after full device fabrication.
To conclude, the use of an in situ definable mask has been shown to be a viable alternative to patterning thin films of organic semiconductors and metals at the large scale. The dry resist material frozen directly to the surface of the substrate alleviates many of the issues of scaling up as fine metal masks have proven increasingly cumbersome with area. While pixel density as high as 325 pixels-per-inch has been demonstrated, there is no reason to believe this is a fundamental limit—especially knowing that nanometer-scale patterning has been demonstrated with an electron beam paired with frozen water resist.18–20 Patterning organics at the nanoscale may be possible pairing phase-change resists with nanoimprinting techniques. Employing a micro-featured stamp roller pipelined with the necessary cooling apparatuses, phase-change resist patterning should allow for scaling of parallel patterning beyond what the current technologies offer.