Deposition of High Quality Films Using Inductively Coupled Plasma - Chemical Vapour Deposition (ICP-CVD) by Oxford Instruments Plasma Technology

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Deposition of High Quality Films Using ICP-CVD

A wide range of insulating thin films are used in modern VLSI circuits providing electrical isolation between conducting regions within a device, and as a final capping passivation layer. Silicon dioxide, silicon nitride and oxynitrides are widely used. Various deposition methods are available dependant on deposition temperature.

Atmospheric pressure chemical vapour deposition and low pressure chemical vapour deposition methods typically require high temperatures in the region of >400 °C whereas the use of plasma enhanced chemical vapour deposition PECVD) typically requires deposition temperatures of <400 °C.

Considerable interest has been directed towards the ability to deposit high density dielectric films at even lower temperatures (<150 °C), especially in temperature-sensitive devices such as organic LEDs. By using the ICP-CVD technique, Oxford Instruments have developed a deposition process in which high quality films can be deposited with high density plasma, low deposition pressures and temperatures.

High Density Plasma Sources from Oxford Instruments

Low temperature depositions are typically achieved by using plasma in which the gases react in a glow discharge. This discharge ionizes the gases, creating active species that react at the wafer surface. The most common method is a parallel plate reactor in which the sample sits on a grounded bottom electrode and radio frequency voltage is applied to the top electrode. This creates a glow discharge between the two plates and the gases flow radially through the discharge. Typically the bottom electrode is heated to 100-400°C and this method is usually referred to Plasma enhanced chemical vapour deposition (PECVD). However in order to deposit high density films dielectric films at even lower temperatures (<100°C) OIPT have developed a high-density-plasma (HDP) source in which the plasma electrons are excited in a direction parallel to the chamber boundaries.

The HDP source used is the inductively coupled plasma (ICP) chamber, in which the plasma is driven by a magnetic potential set up by a coil wound outside dielectric walls (typical design see figure 1). The direction of the electron current is opposite to that of the coil currents which are, by design, parallel to the chamber surfaces. When the plasma is excited in this manner the operating pressure can subsequently be lowered. The lower limit of the pressure is typically dictated by the efficiency of the particular source. In most materials processing plasmas the electron heating is primarily resistive, and the impedance of the plasma scales with the density of neutrals available for inelastic collisions. As the impedance (pressure) is lowered so is the ability of the source to drive the plasma.

Figure 1. OIPT ICP-CVD system

Additional System Features for Plasma Deposition

For plasma depositions there are additional system features:-

• The inductively coupled coil is connected to a 13.56 MHz, 3.0kW RF generator via a matching unit.
• The ICP coil power controls the dissociation of the plasma and the density of the incident ions in the chamber.
• The lower electrode is separately powered by another 13.56 MHz 300W generator, which allows independent control of the bias voltage, i.e. the energy of the ions on the sample.
• In order to reduce the plasma-induced damage during deposition processes and the stress level in deposited films, the ICP-CVD system has been operated in a purely "ICP" mode by applying RF power (100 to 2000W) to only the ICP coil, but no RF power on the lower electrode.
• Helium pressure was applied to the back of the wafers to provide good thermal contact between chuck and wafer.
• The system has precise control of substrate temperature from -150°C to +400°C by using electrical heater and liquid nitrogen. This wide temperature range is important for the advanced plasma deposition processes of different substrate materials.
• Pure silane (100% SiH₄) is introduced into the deposition chamber through a gas distribution ring. Other gases such as N₂ and N₂O are introduced into the ICP source chamber
• Automatic pressure controller (APC) is used to control the pressure (2 to 20mTorr).

ICP-CVD Systems from Oxford Instruments

A summary of the ICP-CVD system configurations are shown in table 1 below:

Table 1. ICP-CVD Tools from Oxford Instruments

Deposition of Materials Using ICP-CVD

ICP-CVD can be used to deposit several materials e.g. SiO₂, SiN_x, SiO_x N_y, a-Si and SiC. In this paper we will concentrate mainly on the ability to deposit high quality SiO₂ and SiN films at substrate temperature as low as 20°C. In an ICP-CVD chamber the silicon dioxide films are deposited by reacting silane which is introduced through the gas distribution ring and nitrous oxide which is introduced through the ICP source. Additionally silicon nitride films are deposited using silane which is introduced through the gas distribution ring and nitrogen which is introduced through the source. Alternatively ammonia can also be used to deposit silicon nitride but the use of nitrogen results in a higher quality film which will be explained in more detail later.

Typical process parameters which are discussed here include deposition rate, film thickness uniformity, refractive index, film stress, wet etch rates, and breakdown voltage.

Typical Deposition Rates of ICP-CVD

Traditionally ICP-CVD processes results in lower deposition rates than PECVD films. Typical deposition rates for silicon oxide and silicon nitride are >8nm/min but higher deposition rates are now possible in which results can be seen in the next section. In a similar way to conventional parallel plate deposition methods many process parameters can be adjusted in order to control the process. Figure 2 and 3 below show typical deposition rate trends with different process parameters.

Figure 2. Effect of ICP power, pressure and silane flow on ICP-CVD SiN_x deposition rate

Figure 3. Effect of ICP power, pressure and silane flow on ICP-CVD SiO₂ deposition rate

Refractive Index of ICP-CVD Deposited Films

The refractive index can be controlled by varying the ratio of the Si:N for silicon nitride deposition or Si:O for the silicon oxide deposition. Silicon nitride films have typical refractive index of 2.00 (at 633nm) although this value can be adjusted by varying the silane and nitrogen flows. Silicon dioxide films have typical refractive index of 1.46. The RI value can be adjusted by varying the silane and nitrous oxide flows. In both films a higher refractive index value usually indicates a silicon rich film. Figure 4 and 5 below show the relationships of refractive index with different gas flow ratios.

Figure 4. Variation of refractive Index with SiH₄:N₂ gas ratio

Figure 5. Variation of refractive index with SiH₄:N₂O gas ratio

ICP-CVD and Film Stress

In some applications such as MEMS the ability to control film stress is very important. Film stress is usually calculated by measuring the curvature change pre- and post-deposition of the film. This difference in curvature as a result of film deposition is used to calculate stress by way of Stoney’s equation, which relates the biaxial modulus of the substrate, thickness of the film and substrate, and the radius of curvatures of pre- and post-process.

In ICP-CVD silicon nitride and silicon oxide depositions the film stress can be controlled by changing various parameters. Process pressure has the biggest influence on the silicon nitride film stress and is shown in figure 6a below. By increasing the process pressure the film stress can be controlled from compressive to tensile. Figure 6a also shows that very low stress can be obtained by fine tuning the process pressure.

ICP-CVD silicon oxide films typically show compressive stress. The film stress can be adjusted by changing a combination of parameters including SiH₄:N₂ ratio, temperature and RF power. Figures 6b and 6c below shows the effect of SiH₄:N₂O gas ratio and temperature with film stress. Low compressive film stress can be obtained by increasing the SiH₄:N₂O gas ratio and decreasing the deposition temperature.

Figure 6a. Variation of SiN_x film stress with process pressure

Figure 6b. Variation of SiO₂ film stress with temperature

Figure 6c. Variation of SiO2 film stress with SiH₄:N₂O gas ratio

ICP-CVD and Film Quality

Quality of the film is most readily shown by wet etching, normally carried out with buffered oxide etchants (BOE) which are typically blends of 49% hydrofluoric acid (HF) and 40% ammonium fluoride (NH₄F) in various predetermined ratios. Typically BOE buffered oxide etchants are used to etch window openings in silicon dioxide layers. The primary application is the etching of thermal oxide layers in IC production. The etch rate of the film by aqueous NH4F/HF solutions, with or without surfactant additives, depends on three primary factors: NH₄F range, etching temperature, and specific HF content. Standard BOE etchants (40% NH₄F/ 49% HF blends) contain over 30% NH₄F, a range where the HF content has primary influence on etch rate.

When testing wet etch rates of the film its usually good practice to measure the etching rate based on a thermal oxide layer as a reference. A low etching rate film usually indicates a high density film. Figures 7 and 8 shows wet etch rate data of SiN_x and SiO₂ deposited using both ICP-CVD and conventional PECVD. The data shows that films deposited at low temperature using ICP-CVD gives comparable film process performance with films deposited using high temperature conventional parallel plate PECVD at 300 °C.

Figure 7. Variation of SiN_x wet Etch rate with electrode temperature

Figure 8. Variation of SiO₂ wet etch rate with electrode temperature

What is Breakdown Voltage?

The breakdown voltage is usually measured by applying a ramped voltage across the dielectric film. The film is normally deposited on a conductive bottom layer (either a doped Si wafer, or a metal layer) together with a metal layer deposited on top of the deposited film. The metal layer is usually patterned either through a shadow mask or by lift-off to form small test pads (typically <<1x1mm). To contact to such small pads a wafer probe station is usually required. Al/Si metal layers are common but other metals could be used. It is important that the interfaces are flat and smooth, i.e. no hillocks or bumps on the underlying metal, and no particles on the surface or in the film, otherwise the breakdown voltage will be significantly reduced (the metal deposition process may need some optimisation if the customer does not have this set-up as a standard test already). This is one reason for having as small a test pad diameter since it is possible to minimise the chances of having a particle within your measurement area. The voltage is then ramped up until a high current peak is observed (i.e. breakdown of the film). The voltage required depends on the film thickness (e.g. 6MV/cm = 120Volts across a 2000Å thick film).

Increased Breakdown Voltage of ICP-CVD Deposited Films

In ICP-CVD film depositions the electrical characteristics of SiN_x deposited at low temperatures (~RT) have shown breakdown electrical fields of more than 3x10⁶ Vcm^-1 with low leakage currents [1,2]. Table 2 below shows the effect of temperature on the breakdown voltage of ICP-CVD SiN_x deposited films.

Table 2. ICP-CVD SiN_x typical breakdown voltage values

Figure 9. Variation of current density with electric field for ICP-CVD SiO₂ film deposited 120°C. The results show breakdown voltage ~>8MV/cm.

Step Coverage of ICP-CVD Deposited Films

In addition ICP-CVD SiO₂ also shows high breakdown voltage when deposited at low temperatures. Figure 9 shows the breakdown electrical fields of >8MV/cm when the SiO₂ film was deposited at 150°C. In comparison a typical SiO₂ film deposited by PECVD at 300°C results in an electrical breakdown electrical fields in the range of >5-6MV/cm.

The step coverage is the ratio of film thickness along the walls of a step to the thickness of film at the bottom of the step. This is referred to S/T and/or S/B in the figure (10) below. For conformal coverage the ratio of S/T and/or S/B is 1. Typically good step coverage is achieved by using high temperatures (>300°C) however it is possible to achieve excellent step coverage at low temperature using ICP-CVD. Figure (10) below shows ICP-CVD SiN_x film coverage when deposited at 20°C. In addition the step coverage also depends on the step height and width.

Figure 10a. Definition of step coverage

Figure 10b. SEM images of cross section of 50 nm ICP–CVD SiN deposited at 22°C on 150 nm metal with good step coverage.

Source: "Inductively coupled plasma chemical vapour deposition (ICP-CVD)" by Oxford Instruments Plasma Technology.

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