Since this is a very often misunderstood subject, we will explain in some detail our own unique approach to this issue, one that we believe is far superior to spending money on a device that for many users does not do what it is expected to do. In short, we have designed into our system a capability that permits the monitoring of coating thickness to an extent beyond what is normally possible with a quartz crystal thickness monitor.
For a sample undergoing off-gassing, there will never be any significant metal deposition. Putting it another way, no deposition occurs so long as a sample is off-gassing. Hence; a critical point dried biological specimen, if being prepared on a humid day, may contain enough adsorbed moisture that essentially no deposition occurs during the preset coating time. The same can be true for virtually any moisture adsorbable (or absorbable) sample such as paper, textiles, cotton and certain high surface area inorganics, such as aluminas, silicas, zeolites or just about any catalyst or catalyst support systems.
Once the sputter coater is put in the "operate mode", just the radiant heat from the glow of the plasma as low a level as it is enough to greatly accelerate the off-gassing rate of the moisture from the sample. The difficulty in reproducibility of coating thickness for such samples is directly traceable to the sample to sample variations of adsorbed or absorbed moisture. Now if one is using a relatively "hot" cathode (something we would not recommend for heat sensitive samples) such as is often times found on non-SPI coaters, the evaluation of moisture occurs quite quickly but for a "cool" cathode such as is found in the SPI systems, this off-gassing time can be considerable, for example 30-60 seconds. Hence for a pre-set coating time of 10-20 seconds, for example, although the system is running, the plasma is glowing, and all appearances are that coating is occurring, in fact, there could very well be nothing depositing. The reason of course is that moisture is still evolving from the sample.
To solve this problem, the SPI system has a unique test mode.
When the test mode control on the sputter module is pushed (a small black button on the front panel), the plasma comes on. The vacuum gauge is then watched, and if any moisture is present, the vacuum immediately begins to deteriorate. The user immediately releases the test mode control and permits vacuum to fall once again.
The cycle is repeated two, three, maybe four times until there is no longer any significant gauge deflection. At this point, there is no longer any off-gassing moisture and the sample is ready for coating. Then the timer can be set, and coating commenced, with great assurance that all samples handled this way, will in the end receive the same amount of coating thickness.
Using the test mode, users report a level of reproducibility in coating thickness that for many samples, there is actually far greater reproducibility than by using a quartz crystal thickness monitor. After all, the quartz crystal presumably does not off-gas at all, and just because the crystal is being coated is absolutely no guarantee the near by sample, if it is indeed off-gassing is being coated as well.
3. Grain size
The grain size obtained when operating in the rapid deposition mode is about 20% larger than what one would obtain if using a conventional geometry cathode.
However, one would not "see" the negative effect of this larger grain size unless operating above "60,OOOX".
In the event the user is operating at higher magnifications, one can also operate the system at more usual deposition rates (taking 2-5 minutes) with a resulting smaller grain size.
To reduce the grain size even further (for SEM operation approaching "100,OOOX"), one would operate in the slow mode using argon as an inert gas, as in other units.
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