Thin Film Deposition is a vacuum process that involves the application of coatings of pure materials over the surface of many different objects. The coatings or films typically have a thickness range of microns and angstroms and can be of single material or multiple materials in a layered structure.
This article covers the basic principles involved in controlling the thickness and rate of Thin Film Deposition using quartz crystal monitoring.
Evaporation is a key class of deposition methods involving heating of a solid material within a high vacuum chamber to a temperature at which some vapor pressure is produced. Inside the vacuum chamber, even a relatively low vapor pressure is adequate to raise a vapor cloud, which is then condensed over surfaces in the chamber as a coating or film. This process, including the common type of chamber designs generally used, is an ideal candidate for successfully controlling the thickness and rate by means of quartz crystals.
The key concept of this type of measurement and control involves mounting of an oscillator crystal within a vacuum chamber in order to receive deposition in real time and the crystal is affected by the deposition in a measurable way. In particular, there will be a drop in the oscillation frequency when there is an increase in the mass of the crystal caused by the material being deposited onto it.
The changes in the oscillation frequency is continuously recorded by an electronic instrument, which then converts the frequency data into Thin Film Deposition thickness data by performing appropriate mathematical functions. The data includes both instantaneous rate and cumulated thickness.
Such sensors and devices are readily available in the market, including in an integrated package which not only observes and displays the thickness and rate data, but also gives outputs for other deposition system elements.
This system will feature an analog drive signal for driving the source power supply in a closed-loop system on the basis of the rate data, thus enabling the system to maintain a preset rate in the event of deposition. Moreover, it will feature other outputs to interface with functions like a source shutter triggered to close after achieving the preset final thickness.
Frequency is the actual physical measurement and the remaining is mathematical interpretation on the basis of certain assumptions. One such assumption is that all frequency changes are solely caused by film thickness on the crystal face.
However, frequency changes can also be caused by transient temperature changes of the crystal and be mistakenly read as film thickness. Such transients are typically of finite duration and have trivial overall impact on the measured deposition in most cases. Hence, if there is a surprise transient, then it could be due to this temperature problem instead of actual deposition.
There can be several physical configurations of evaporation process chambers, of which the most common will have the heat source and evaporant material source at the chamber bottom, with vapor stream rising above this with deposits from the bottom onto a rotating substrate holder.
This system will have a movable physical shutter above the source between it and the substrates. When the shutter opens, the substrates and the crystal monitor are exposed to deposition. When the shutter closes, the substrates must be protected from the deposition. The crystal is also usually shielded from deposition as the shutter closes.
Quartz crystal rate controllers generally allow programming of a desired heat profile to pre-condition the material before opening the source shutter to begin actual deposition. This profile is experimentally concluded by the user for each source/material and is usually formatted in the controller to have two separate sequential ramps of power to some 'soak' level and be maintained there for some time.
The first ramp up is typically slower and ends at some point just below vaporization and holds it to reach an adequate level of equilibrium prior to making the second ramp up to vaporization. The second ramp up must preferably be the actual power level to achieve the desired deposition rate.
After the completion of the final soak, the shutter will be opened by the controller, which will then change to a closed loop power control to maintain the preset deposition rate until achieving the programmed final film thickness.
The closed loop control is typically a PID controller with user control of the parameters that include the option of not utilizing all three. There can be many different options available in such controllers, such as setting up closed loop control at some deposition rate in angstroms per second, and continuing under such control, and then ramping the rate up or down to a new rate level and maintaining that rate until achieving the programmed final thickness with potentially many different controlled rate segments all through the way.
A crystal has details to address, such as calibrating readings on thickness of the substrate. Nevertheless, it can monitor only the Thin Film Deposition that lands directly on itself. However, this is not actually what a user wants to understand or to control. What is on the substrates is the subject of interest to users. Hence, it is necessary to perform physical measurements to compare to the crystal reading and to calculate and program a calibration factor (generally called ‘tooling’ factor) into the instrument for subsequent verification.
Crystal failure is another key point. The initial operating frequency (typically 6.0 MHz) of a new crystal will decrease due to material deposition on this material. To determine angstroms of deposition, the oscillation needs to be very good and stable. Hence, it is recommended to change the crystal prior to the point of failure to avoid the risk of scrapping a load of valuable substrates because the crystal may fail earlier due to noise problems or other reasons.
These quartz crystal rate monitors/controllers are employed on almost all evaporators, either e-beam type or filament/boat type. A major reason is the difference in deposition rate vs source drive power in most such instruments. These controllers allow for better control over rate and thickness regardless of any such variation.
Although this crystal monitoring and control technology can be used in a sputtering system, it is not widely used for several reasons. One reason is the adequate stability of the deposition rate vs cathode drive power for process control in sputtering. Moreover, positioning crystals is also difficult in typical sputter chambers and there is a possibility for plasma interference with them.
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Our high-performance production or R&D vacuum sputtering and thin film evaporation systems provide coatings on a variety of materials including plastic films, glass, ceramics, metals and hybrid substrates.
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This information has been sourced, reviewed and adapted from materials provided by Semicore Equipment.
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