Basados en MoS 2

Basics of Atomic Layer Deposition

ALD is a thin film deposition method, which belongs to the methods of chemical vapor deposition (CVD). In CVD, inorganic thin films are produced by chemical reactions of partly organic volatile reagents (precursors) with other gases. The reaction usually takes place between an organometallic precursor and an oxidizing reagent. ALD is different from conventional CVD due to self-limiting nature of the reaction, which contribute to the layer formation [160].

In ALD, the chemical reaction does not occur simultaneously between two or several gaseous species, but alternately between only one gaseous species and the solid surface.

4.1 Film production

Figure 4.3: Schematic diagram of sputtering chamber at DAISY-MAT [13, 159].

Once this reaction is over, the layer growth stops automatically. By alternating self terminating reactions, it is possible for layer by layer growth and this is called Atomic layer deposition (ALD). The sum of two alternating steps is called ALD cycle [160].

The temperature window available for the ALD reaction is schematically illustrated

in Fig. 4.4. Initially, at lower temperature, it is limited by the activation enthalpy

required for the two surface reactions and yield either condensation of precursors on substrate surface or incomplete reaction. While, the upper limit is decided by thermal decomposition temperatures of the precursors and the sublimation or melting tempera- tures of the deposited materials (the latter are usually much higher than the former). This temperature window makes it possible to use and combine the precursor materials. Sufficient vapor pressure at moderate temperatures and the self-limiting nature of precursor-surface reactions are further criteria for suitability of precursor material to be used in ALD process [160].

The amount of material adsorbed in gas–solid reactions can depend on time in various ways, as schematically illustrated in Fig. 4.5. Both reversible and irreversible adsorption can be saturated in nature. For the adsorption to be self-terminating as in case of ALD, the adsorbed material should not be desorbed from the surface during the purge or evacuation. Thus, the monolayer of precursor saturated in an irreversible (irreversible in the time scale of the experiment) way by forming strong chemical bonds (chemisorption) as shown in Fig. 4.5(a). In case of reversible saturation only physisorption (weak bonds like van der waals) formed and once the precursor flux is stopped, surface species will be

Table 4.1: Deposition parameters for RF-magnetron sputtered films

Material ITO In2O3 SiO2−x

Target 10 % Sn-doped In2O3 In2O3 Si Target purity % 99.9 99.9 99.9 Supplier - - - Temperature (◦C) RT - 400 200 - 600 RT - 400 RF power (W) 25 25 40 Pressure (× 10−3 mbar) 5 5 5 Process gas ( O₂%) 0 - 0.5 0 - 0.5 0 - 5 Target - substrate (cm) 10 10 7

Deposition rate (nm/min) 4 - 4.6 4 - 4.6 2 Layer thickness (nm) 8 - 200 20 - 200 1 - 10

Figure 4.4: Schematic representation of the ALD process with temperature (Redrawn from

[161]).

desorbed, seeFig. 4.5(b). This kind of saturation does not contribute to ALD growth. Irreversible adsorption could also be continuous and non-saturating. In this case the process is in CVD regime. The more precursors are pulsed, the thicker film will be

4.1 Film production

deposited continuously, as can be seen in Fig. 4.5(c) [160].

Figure 4.5: Schematic representation of amount of materials adsorbed with time. (a) Irre-

versible saturating adsorption-ALD mode, (b) reversible saturating adsorption, (c) Irreversible non saturating adsorption- CVD mode. The vertical green dashed line marks the end of the reactant supply and the beginning of a purge or evacuation (Redrawn from [160]) .

The practical implementation of ALD process is described below on the basis of the data used in this work and synthesis of Al2O3 (which is well known from many years

of research as a model system) from Trimethylaluminium (TMA) precursor and water. Figure 4.6, describes an ALD cycle of the TMA/water process. The substrate whose surface is to be coated, must be first brought to the required processing temperature under vacuum or inert gas conditions. In addition, the substrate surface should ideally be prepared for the TMA/water cycle before deposition as TMA requires hydroxyl groups for reaction in the first step.

In the first step of an ALD cycle, a controlled quantity of TMA precursor is introduced into the deposition chamber, which then reacts with the surface of substrate. This primarily reaction is schematically represented in Fig. 4.6(A1) and is given by Eq. 4.1. In the equation the surface of substrate is represented by (k) symbol:

k − O − H + Al(CH3)3 → k − O − Al(CH3)2+ CH4 ↑ (4.1)

This is referred as the 1st half reaction of the ALD cycle. In the reaction, the amount of adsorbed TMA molecules depends on the available hydroxyl groups on the surface. Once the adsorption is irreversibly saturated, no further reaction will occur and is called self - terminated reaction. This is followed by the second step of ALD cycle, in which the remaining precursor material as well as methane by-product are removed from the chamber by purging with an inert gas (usually nitrogen) [162] or evacuation of the chamber [38], as shown in Fig. 4.6 (A2).

Figure 4.6: Schematic representation of the ALD process of Al2O3 production. (A) represents

the first half-cycle and (B) the second half cycle. The legends in the figure describes the chemical species involved. The figure is taken from a PhD thesis of J. Deuermeier, from Technische Universit¨at Darmstadt, Germany [13].

In the third step, the second precursor (water) is introduced into the processing chamber, which will then react with oxidizable species on the surface to form hydroxyl groups. The reaction is represented by Eq. 4.2 and shown in step (B1) of Fig. 4.6 (2nd half reaction):

k − O − Al(CH3)2+ H2O → k − O − Al(OH)2 + 2CH4 ↑ (4.2)

Educts and excess gaseous precursors are removed in the fourth step by purging the chamber again, seeFig. 4.6 (B2). After completion of these four steps, ideally the full ALD cycle will result in a monolayer of Al2O3. However, the availability of surface

sites is limited by steric hindrance of the involved molecules. Consequently, during one ALD cycle typically less than one monolayer is deposited [160]. The pulse and purging times should be calibrated carefully. During pulsing, there should be a complete surface coverage without too much excess precursors, while during pumping, all excess precursors should be removed before the subsequent pulse [38].

The degree of substrate coverage achieved after a complete ALD cycle can be obtained using the Growth Per Cycle (GPC). This means the incremental layer thickness added after an ALD cycle. The average GPC of ALD Al2O3 using TMA/water precursors is ≈

4.1 Film production

0.08 - 0.1 nm/ cycle [160, 162, 163].

Experimental conditions of ALD on this work

In this work, ultra thin layers of ALD Al2O3 were used as a dopant material for TCOs

and were coated on undoped and Sn-doped In2O3 substrates. The ALD chamber used

in this work is part of the DAISY-MAT system at TU Darmstadt as shown inFig. 4.2. Since the ALD chamber is part of integrated UHV4 _{system, the excess precursors were} evacuated via a turbo molecular pump. The base pressure of the deposition chamber was kept at 10 −8 mbar. The substrates were heated to 200 o_{C by radiative heating}

prior to deposition.

Table 4.2: Overview on deposition condition of ALD - Al2O3

TMA pulse time(ms) 80

Evacuation (min) 5

H2O pulse time (ms) 150

Base pressure (mbar) 10−8 Pressure between pulses (mbar) 10−6 Substrate temperature (◦C) 200

ALD cycle 1 - 20

The amount of TMA and water was controlled by setting the pulse length of two individual ALD 3 series valves by Swagelok. Electronic grade TMA was purchased from SAFC Hitech. To achieve highest purities of the water precursor, Millipore water was evaporated and condensed several times in alternate arms of a double arm glass vessel. A hot air gun and dry ice was used for this procedure. The pulse length for TMA was set to 80 ms and for water to 150 ms, the evacuation time between pulses was set to 5 min, which reduced the pressure down to 10−6 mbar [38]. The deposition parameters used during deposition are summarized inTab. 4.2.