Atomic layer deposition (ALD)

Figure 2.1. The 8 inch ALD system at instrument technology and research center (ITRC), Hsinchu, Taiwan. This system was mostly used for the deposition of high k oxides in this work

Principles

Atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE) or atomic layer chemical vapor deposition (ALCVD), can be considered as an advanced variant of the well-known chemical vapor deposition (CVD) technique. The ALD was developed in Finland by T. Suntolan and coworkers in 1974 to meet the industrial needs for producing high-quality and long-life thin film electroluminescent (TFEL) displays [1].

The principles of the ALD for thin film growth emphasize the aspects of a self-limiting mechanism. The distinct feature of ALD is that the film is grown through sequential saturated surface reactions that are realized by pulsing the two (or more) precursors into the reactor alternately, one at a time, separated by purging or evacuation steps [2]. In other words, in ALD, the growth of thin films takes place by surface-controlled growth cycles. An ideal growth cycle consists of (1) exposure of the substrate surface to the pulse of the first gaseous precursor and its chemisorption onto the surface, (2) inert gas purge to remove the unreacted precursor, (3) introduction of the second precursor followed by surface reaction between the precursors to produce the desired film material, and finally (4) inert gas purge to expel the excess of precursor and volatile reaction by-products (Fig. 2.2) [3].

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Figure 2.2. Schematic illustration describing an ALD deposition cycle leading to the formation of a binary oxide film consisting of metal (•) and oxygen (◦) atoms. L refers to the precursor ligand [3]

The process of the deposition of Al₂O₃ by ALD uses tri-methyl aluminum (TMA) and H₂O as precursors. The deposition process schematic is shown in Fig. 2.3 [4]. Starting surface always adsorbs H2O vapor in air, forming hydroxyl (OH) groups. After loading into ALD reactor, TMA is pulsed into reaction chamber and reacts with hydroxyl groups (step 1):

-OH + Al(CH3)3  -O-Al-(CH3)2 + CH4 (2.1)

Reactions continue until the surface is passivated, results in forming a layer of –Al-(CH3)2

on the surface. TMA does not react with itself, terminating the reaction to one layer (step 2). The excess TMA and CH4 product is then purged away by inert gas to finish the first half of cycle. In the second half of cycle, the H2O vapor precursor is pulsed into the chamber. H2O reacts with dangling methyl groups on the new surface forming Al-O bridges and hydroxyl surface groups (step 3):

-O-Al-(CH₃)₂ + H₂O  -O-Al-(OH)2 + 2CH₄ (2.2)

H₂O does not react with OH groups causing one perfect hydroxyl terminal layer. The excess H₂O and CH₄ production are purged away by inert gas (step 4). A new cycle is started with (OH) functional groups surface and TMA pulse. By this way Al₂O₃ film is formed. The bottom-left of the Fig. 2.2 shows the finished Al₂O₃ film after 3 ALD cycles.

The overall reaction of TMA and H₂O in the formation of Al₂O₃ oxide is described as [4, 5].

2Al(CH3)3 + 3H2O  Al2O3 + 6CH4 (2.3)

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Figure 2.3. Schematic describing the alternating TMA and H2O pulses in ALD chamber leading to form sequence Al2O3 layers [4].

The ALD growth temperature is dictated by precursor chemistry, to a regime where the surface-limited reactions occur. Reactivity of the metal precursor mainly determines the temperature range where ALD growth occurs. In addition to self-liming growth, a region of temperature with a constant deposition rate, a so-called ALD window, is often observed (Fig. 2.4) [3]. Except some few cases, the ALD windows are often reasonably wide, can extend over hundreds of degrees [2].

Figure 2.4. Scheme of (a) ALD processing window limited by (b) precursor condensation, (c) insufficient reactivity, (d) precursor decomposition and (e) precursor desorption. If deposition rate is dependent on the number of available reactive sites as in (f ), no actual ALD window is observed [3]

29 Advantages and limitations

The surface-controlled growth mode produces some obvious advantages as summarizing in the table 2.1 [2]. Among them the fact that the precursor pulse length, i.e., dose, has no effect on the growth rate provided that the surface is saturated, i.e., all available surface sites are occupied by the precursor molecules (Fig. 2.5) [2, 3]. Furthermore, because the growth is by cycles, the thickness control is facile and is achieved by monitoring the number of ALD cycles. Similarly, uniform doping is easy to accomplish by replacing, at a desired interval, the growth cycle by a doping cycle [2, 3].

Table 2.1. Relationships between characteristics and advantages of ALD [2]

Characteristic feature

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Another advantageous and inherent feature of ALD originates from its surface-controlled nature which allows substrates of various sizes and geometries to be uniformly coated [2, 3]. The preparation of multicomponent and multilayer materials is further facilitated since the ALD windows are often wide which allows binary processes easy to combine [2]. The alternate supply of the precursors in the well-separated pulses ensures that the precursors never meet in the gas phase. This eliminates risks of gas phase reactions with possible detrimental consequences such as particle formation [2].

Figure 2.5. Saturation of surface reactions in ALD processes is experimentally verified by observing that the deposition rate per cycle stabilizes to a constant level with increasing precursor pulse time or dose [2].

The main disadvantage of ALD is the low deposition rate which is a direct consequence of layer by layer film growth. In addition, ALD does not produce a full monolayer of a film in one deposition cycle but only a fraction of thereof. Thus, ALD is a relatively slow technique when a thick film, hundreds of nanometers or more, needs to be deposited. Typically, the deposition rate is in the range of 100-300 nm per hour. However, for the high k gate oxide application which needs very thin film, the deposition rate restrictions are somewhat more relaxed than most other areas.