Deposition Rate

Deposition Rate and Ion/Neutral Fraction 

Introduction

Thin film deposition processes play a vital role in various industries, such as semiconductor manufacturing, optical coatings, and surface engineering. The deposition rate and ion/neutral fraction are crucial parameters that significantly impact the quality, properties, and performance of deposited films. Accurate measurement and control of deposition rate and ion/neutral fraction are essential for achieving desired film characteristics. In this article, we will delve into the concept of deposition rate and ion/neutral fraction, their measurement techniques, factors influencing their values, and the significance of precise measurement. We will also highlight Impedans Ltd.’s advanced diagnostic solutions in this field. 

Section 1: Introduction to Deposition Rate and Ion/Neutral Fraction

 In a plasma deposition process, ions play a crucial role in film growth. The ions are accelerated across the plasma sheath, a transition region between the plasma and the substrate surface. The plasma sheath is characterized by a strong electric field that accelerates ions towards the substrate. The ions gain kinetic energy from this electric field, and upon reaching the substrate, they can deposit onto its surface, contributing to the growth of the thin film. 

The deposition rate is determined by the flux of ions arriving at the substrate surface and their energy. The flux of ions is the number of ions arriving per unit area of the substrate per unit time (see this article on ion flux for more information). The energy of the ions is related to the electric potential difference between the plasma and the substrate surface, across the plasma sheath. 

The ion acceleration process across the plasma sheath depends on the electric field in the plasma sheath region. This is given by the sheath potential drop, which is determined by the balance between the ion flux to the substrate and the electron current returning to the plasma. The ions experience a force due to the electric field, causing them to accelerate towards the substrate. 

The deposition rate is directly proportional to the ion flux and ion energy. Therefore, by controlling the ion flux and energy through adjusting plasma parameters, such as gas pressure, applied electric fields, and substrate biasing, one can optimize the deposition rate and achieve the desired film thickness and properties. 

The ion-neutral ratio in the plasma also plays a significant role in determining the deposition rate and overall thin film growth. The ion-neutral ratio refers to the relative abundance of ions and neutrals in the plasma. It is influenced by the plasma conditions, gas composition, and ionization processes. 

In a plasma deposition process, a higher ion-neutral ratio generally leads to a higher deposition rate. This is because ions have a greater kinetic energy compared to neutrals due to their accelerated motion in the electric field of the plasma sheath. The increased kinetic energy allows ions to have higher momentum and thus a higher probability of reaching and adhering to the substrate surface. 

The ion-neutral ratio also affects the energy of ions arriving at the substrate. With a higher ion-neutral ratio, there are more ions available in the plasma, leading to a higher probability of ion collisions and energy exchange. Consequently, the ions gain more energy, resulting in a higher ion energy upon reaching the substrate. This higher ion energy enhances the physical and chemical interactions at the substrate surface, influencing the growth and properties of the deposited thin film. 

Measuring both the deposition rate and ion-neutral ratio in real-time during the plasma deposition process is crucial for process optimization and ensuring film uniformity. These measurements provide valuable feedback for researchers and engineers to adjust process parameters and achieve precise control over the film growth, leading to the production of high-quality thin films with tailored properties for various applications. 

Section 2: Measurement Techniques for Deposition Rate

Several techniques have been developed for the accurate measurement of deposition rate and ion/neutral fraction. Common methods include quartz crystal microbalances, laser interferometry, and ion collectors. 

Quartz crystal microbalances (QCMs)  measure the change in the resonance frequency of a quartz crystal due to the mass added or removed from its surface during film deposition. As the film grows on the crystal surface, it becomes heavier, causing a decrease in the resonance frequency, which is proportional to the deposition rate. They provide continuous and real-time monitoring of the deposition rates during a process, with high sensitivity of detecting small changes in mass. The crystal frequency also has a dependence on temperature however so calibration between frequency changes and mass increase can be challenging.

Laser interferometry measures the change in the thickness of the growing film by analysing the interference pattern created by the reflected laser beam from the film surface and the substrate.

It provides high precision measurements of film thickness, and therefore deposition rates, without being in contact with the substrate surface. This technique is limited however to transparent films due to the need for the laser to pass through film-substrate interfaces. 

Ion collectors are placed in the vicinity of the substrate to directly capture ions arriving during film deposition. The collected charge is proportional to the ion flux, which, in turn, determines the deposition rate. It provides real time monitoring on the deposition rate which allows for immediate adjustments during the deposition process. However, it is difficult to directly convert the ion flux to deposition rates, and using ion collectors will only provide researchers a way of measuring the change in deposition rates rather than a calibrated value. It also has the potential to be destructive for some applications; in some cases the presence of the ion collector can interfere with the desired film properties. 

Section 3: Factors Affecting Deposition Rate and Ion/Neutral Fraction

Several factors influence the deposition rate and ion/neutral fraction during thin film deposition processes. Understanding how these parameters affect the deposition rates provides valuable insights into process control and optimization. 

1. Precursor Flow Rate: The precursor flow rate directly affects the deposition rate by controlling the number of precursor molecules available for film growth. Higher precursor flow rates typically result in increased deposition rates due to a larger supply of precursor species. By adjusting the precursor flow rate, researchers can modulate the amount of material being delivered to the substrate and thereby control the deposition rate.

2. Gas Composition: The gas composition in the deposition chamber influences the ionization process and the ion/neutral fraction within the plasma. Different precursor gases or gas mixtures can lead to varying degrees of ionization. Higher ionization levels result in higher ion/neutral fractions, which can increase the deposition rate due to enhanced ion bombardment on the substrate surface.

• Substrate Temperature: The substrate temperature plays a significant role in thin film deposition. Higher substrate temperatures increase the mobility of adsorbed species on the substrate surface, leading to more effective surface diffusion and film growth. Consequently, elevated temperatures generally result in higher deposition rates. However, there is an optimum temperature range for each specific deposition process, as excessively high temperatures can cause unwanted reactions or affect film quality.   
• By carefully controlling these factors, researchers can tailor the deposition rate to achieve desired film thicknesses and optimize the overall thin film growth process.Power Input and Applied Electric Fields/Substrate Biasing Techniques: Power input and applied electric fields or substrate biasing techniques impact the ion energy and flux within the plasma. Higher power input or stronger electric fields can accelerate ions, increasing their energy and flux towards the substrate. This increased ion bombardment can enhance the deposition rate. Substrate biasing techniques, such as applying a negative bias voltage, can further control the ion energy and flux, influencing both the deposition rate and ion/neutral fraction.

Section 4: Importance of Deposition Rate and Ion/Neutral Fraction in Thin Film Applications

The deposition rate and ion/neutral ratio play crucial roles in thin film applications as they significantly influence the film properties and quality.  

The deposition rate directly determines how quickly the film grows on the substrate. Precise control of the deposition rate is vital to achieving the desired film thickness. In most applications, such as semiconductor manufacturing, precise control of film thickness at the nanometre level is critical for device performance. By adjusting the deposition rate, researchers and engineers can control the film’s final thickness and ensure uniformity across the entire substrate. A direct consequence of this is that the deposition rate will affect how long a deposition will take, ie if the desired thickness is 500nm, it will take twice as long if you half the deposition rate.  

However, the effect of deposition rate on film morphology and microstructure must be considered. A higher deposition rate may lead to larger grain sizes or columnar structures, while a lower deposition rate can result in a more compact and fine-grained film. In some cases, controlling the deposition rate is crucial for achieving specific film microstructures, which can impact properties like mechanical strength, adhesion, and electrical conductivity. 

Similarly, the ion neutral ratio has direct effects on film properties. A high ion/neutral ratio can lead to significant ion bombardment of the growing film surface. Ion bombardment can influence the film’s microstructure, crystallinity, and defect density. Controlled ion bombardment can be beneficial for improving film density and adhesion, but excessive bombardment may cause damage or lead to undesirable film properties. 

The ion/neutral ratio affects the number of ion-assisted chemical reactions at the surface. A higher ratio can increase the probability of ion-induced reactions, altering the film composition and stoichiometry. Additionally, ion bombardment can enhance adatom mobility on the surface, affecting film growth mechanisms. 

In summary, accurate measurement and control of the deposition rate and ion/neutral ratio are critical for achieving high-quality thin films with reproducible properties. By optimizing these parameters, researchers and engineers can tailor thin films for various applications, leading to improved device performance, enhanced material properties, and overall technological.

Section 5: Impedans Ltd’s Solutions for Deposition Rate and Ion/Neutral Fraction Measurement

Impedans Ltd offers an advanced Quantum RFEA system with a QCM crystal embedded in the sensors for precise deposition rate and ion/neutral fraction measurement. By switching on and off retarding voltages, the deposition rate for both the ions and neutrals can be measured. From these deposition rates, the ion neutral ratio can be calculated. 

This system is compatible with many different deposition systems, with the analysis and calibrations done by free Impedans software to provide direct measurement of deposition rates and ion neutral ratios. To overcome the dependence of the QCM on heating effects, a controlled calibration crystal is used. An example of a measurement of the deposition rates is shown in the plot below, with the values given in the table on the right, and the graph representing the thickness deposited on the QCM over time with the retarding voltage switched on to block ions (green and red lines) and switched off (blue and purple). 

This solutions enable researchers and industry professionals to achieve accurate measurements during thin film deposition processes.

Conclusion

Deposition rate and ion/neutral fraction are critical parameters in thin film deposition processes, impacting film thickness, composition, and properties. Accurate measurement and control of these parameters are essential for achieving desired film characteristics and optimizing process performance. 

Impedans Ltd, with its expertise in diagnostic solutions, provides reliable and advanced tools for precise deposition rate and ion/neutral fraction measurement. By utilizing Impedans Ltd’s state-of-the-art technologies, researchers and industry professionals can gain deeper insights into thin film growth, improve process control, and advance their respective fields. 

References and further reading: 

1. Ohring, M. (2012). Materials Science of Thin Films: Deposition and Structure (2nd ed.). Academic Press.
2. Vepřek, S. (2019).