Know All About Diffusion Pump Part-1

Q.1 How does a diffusion pump work?

Ans. A diffusion pump works on the principle of gas diffusion to create vacuum. In a diffusion pump, diffusion pump oil is heated to its boiling temperature. Vapor of this oil is compressed in a vertically tapering hollow cone as it moves up. Simultaneously this oil vapor escapes from jets along the height of hollow cone. Oil vapor escaping at great speed traps air and then collides with water cooled wall of Diffusion Pump and moves down. Trapped air is released at bottom of Diffusion Pump. This creates differential pressure in diffusion pump. Pressure at top is minimum (high Vacuum) and at bottom is maximum (low Vacuum) from bottom of diffusion pump the air is sucked by rotary pump.

Q.2 Is a diffusion pump made only in cylindrical shape?

Ans. Diffusion Pump has been successfully made in rectangular shape also. In fact rectangular shaped Diffusion Pump has an advantage that its inlet connected to Vacuum Chamber can be of larger area and Diffusion Pump itself can be adjusted to occupy less space. This also works with Diffusion Pump Oil as a working medium just like cylindrical Diffusion Pump.

Q.3 I understand that Turbo molecular pump (TMP) is an alternative to diffusion pump. Is it better or worse than diffusion pump?

Turbo Molecular Pump

Ans. Turbo molecular pump works on the principal of positive displacement. In this pump a series of inclined blades are mounted on a shaft which rotates at very high speed. These are called Rotors. Fixed blades or stators are fixed in such a way that Rotors and Stators alternate. Because of rotation of shaft and incline rotors push air down to next row of Stators. Stators too are inclined in same direction and push this air to next row of Rotors and so on to exit, where a Rotary Pump draws away the air.

Though this pump creates good vacuum and can be used in place of Diffusion Pump but its applications are limited to critical applications like semiconductor coatings. This is because of its high cost, more requirement of maintenance and the fact that since this pump cannot be made of large size because of mechanical considerations, a diffusion pump can be replaced only by a couple of Turbo molecular pumps. Diffusion Pump on the other hand has no moving parts and requires almost no maintenance other than Diffusion Pump Oil change, is cheaper in cost and can be built in almost any size.

Q.4 Why is topping up required for Diffusion Pump Oil?

Ans. In a Diffusion Pump, DP oil evaporates to produce diffusion action. Since oil is in vapor state so part of it is lost by leakage, some of this oil vapor is also sucked away by Rotary Pump. It is therefore important that lost diffusion pump oil is leveled up by topping.

Not topping up will result in overheating of the remaining oil and loss of oil as explained above will further speed up, causing a drop in vacuum. In extreme cases Hydrocarbon Diffusion Pump Oil may burn to tar like substance and Silicone diffusion Pump oil may turn into grey colored crystals.

Read about diffusion pump maintenance here.

Q.5 Why can a diffusion pump not work alone and requires a rotary pump as backing pump?

 

Belt Driven Rotary Vacuum Pump

Ans. A diffusion pump merely creates a vacuum gradient where top of diffusion pump has a higher vacuum and bottom of diffusion pump has much lower vacuum. A diffusion pump has no moving parts to force air out, so though lower portion of Diffusion pump has a low vacuum but since it is still better than atmospheric pressure so air will not flow out.

This lower portion of diffusion pump is connected to rotary vacuum pump. Since vacuum in lower portion of Diffusion Pump is lower than what Rotary Vacuum Pump Oil is able to create so Rotary Pump evacuates the diffusion pump and diffusion pump is able to work.

Q.6 How can I get more life from my expensive Diffusion Pump Oil?

Ans. Diffusion pump oil is one of the costly consumables of the metallizer. With a few precautions, its life can be prolonged and money saved.

1. Never open the pump when the oil is still hot. Exposing the hot oil to air changes its viscosity, making it thick gel-like.

2. Take care to top up the oil in the pump in a timely way so as to avoid accidental over heating of the oil.

3. Keep a timely watch on the pressure and heater gauges.

4. Back streaming of vapors from the vacuum chamber and/or Rotary pump letting its vapors in to the Diffusion pump are two reasons for the DP oil to get contaminated. This can be prevented by installing filters above the chevron baffle of the DP and on the line in between the rotary and the diffusion pump. These filters must be cleaned from time to time.

5. Silicone DP oil decomposes on coming in contact with even trace elements of alkali metals e.g., sodium, potassium, cesium etc. Do not allow them and/or their vapors to come in contact with the DP oil.

6. Keep a check on the water flow through the cooling lines for proper cooling of the hot oil inside the diffusion pump.

Read more on this topic here.

Q.7 Do I need to open & clean jets of my diffusion pump every time I change oil in it?

Ans. If you are using a good quality silicone diffusion pump oil then there is no need of cleaning jets every time you change oil cleaning be done once in 4 to 5 years. However if you are using hydrocarbon diffusion pump oil then cleaning is a must every time you change oil.

Q.8 Is DC-702/DC-704/DC-705 Diffusion pump oil still available? If not then what is the alternative?

Ans. DC-702/DC-704/DC-705 - these oils are no longer available in the entire world as the manufacturer Dow corning stopped the production of these oils more than a year back.

Supervac Industries supplies SV-SIGMA, SV-SIGMA+ and SV-SIGMA FAST as alternatives of DC-704, DC-705 and DC-702 respectively. These oils are being used by a large number of customers in more than 20 countries.

Q.9 I observed whitish deposits in my diffusion pump. What could this be? It is serious?

Silicone crystals

Ans. On overheating silicone diffusion pump oil turns into whitish grey silicone crystals.

Yes, this is serious. You need to check cooling for diffusion pump and also make sure that level of diffusion pump oil never falls below minimum level. This requires topping up from time to time. Also a faulty temperature gauge not allowing heater cut-off to take place, can be responsible for the oil over heating.

Q.10 What is back streaming in diffusion pump?

Ans. If the pressure is too high in the diffusion pump then the emerging diffusion pump oil vapor from the jets trying to move down collides with air molecules in the pump. This results in no vapor going down the pump. Contrary to this, the vapor itself starts rising up the pump along with the air and raises pressure instead of reducing it. This undesirable process is called “back streaming”.

Because of back streaming Diffusion Pump Oil can travel to vacuum chamber or can even be removed by Rotary Vacuum pump. If it enters Rotary Pump then it contaminates Rotary Vacuum Pump Oil also.

(In the next part-2 of this series I shall explain how to prevent back streaming.)

Written by: Anshuman Punj.   

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Pin Holes: 5 Ways to Eliminate Them

Pin holes, sometimes also referred as pin windows, are the areas where no metal has been deposited. These can be a big problem for quality Metallizing especially in yarn grade metalizing where a pin hole can cause the thread to break. 

Reasons  for  formation  of  pin  holes  and  how  to  avoid  them-  

1) If there was a particle on film that got removed after metallization, it will leave a pin hole. This can be easily identified by careful examination as this pin hole will also have an area of thinner metallization around it caused by 'shadow' of particle in the path of metal for deposition. 



Pin Holes in Metallizing (Image 1)

Solution: Storing the film roll properly, away from dust goes a long way in solving this problem.  For this, vertical stacking of rolls in custom made wooden pallets with core ends jutting out for handling of roll is the best possible way.

2) Spitting from boats may also cause pin holes; this is easily inferred by heat damage to film around pin hole. 

Solution:  Controlling the wire feed and adjusting current to boats can solve this problem. For details on the subject of correct wire feed, refer to my earlier blog post here.

3) Excessive tension while winding of film will also cause pin holes. This can be inferred by examining the film. Here, pin holes will be elongated in the direction of pull.

Pin Holes in Film Metallizing (Image 2)

Solution:  Avoiding this problem requires adjustment of winding tension to the appropriate value.

4) Residue of some solvent or masking oil on film can also cause pin holes. These pin holes are clean and are also called pick off. These are formed by removal of Metal from film because the solvent did not let metal adhere to the film.

Pin Holes in Roll to Roll Metallizing (Image 3)

Solution: Storing the film where solvent vapors are present should be avoided. Also, if in any application masking oil has to be put on the film; care should be taken for its proper removal as well.  

Hence, cleaning and properly storing the film solves this problem.

5) Aluminium wire that is oxidized on surface is another reason of pin holes as Aluminium oxide will not deposit properly. This is evident by a uniform pattern of pin holes.

 

Solution: Use of fresh Aluminium wire or ultrasonic cleaning of wire if wire is old, goes a long way in solving this problem.

Pin holes on metallized film can be studied effectively by placing metallized film on a flat surface below which a strong light source has been placed on a diffuser to spread the light uniformly. A large area of film can be quickly scanned in this way. 

                                                                                         

 

 

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 Written by: Anshuman Punj.                      

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Evaporation Boat: Get Your Money’s Worth Out Of It

In the present time of intense competition, it is natural that engineers are trying hard to optimize the use of every resource to reduce costs. In Metalizing, Evaporation Boats undoubtedly contribute most in variable cost. It is therefore important that maximum life is derived from Evaporation Boats without affecting the quality of Metalizing.

Based on my knowledge of Engineering, experience of nearly 20 years in vacuum line and interaction with actual users, I have compiled a list of measures which if implemented can ‘enhance’ the life of Evaporation boats by about 25% so that you get your money’s worth out of it.

 These have been summarized as under -

1)    Initial heating of the new boats also called “break in” must always be slow to get proper life from Boats. Reason? New boats will always have some absorbed moisture. If the boat is heated quickly in this condition, the moisture, converted into steam having much higher volume will have little time to escape and can cause cracks because of high pressure build up.

2)    At the end of the cycle it must be ensured that all the Aluminum or any other metal being metalized has evaporated completely before heating is switched off. Otherwise residual metal will seep into the pores of boats; now differential cooling rates of boat and metal will crack the boat.

3)    Care must be exercised that before the vacuum is broken, boats must have cooled down. This is because sudden contact with cold air can cause boat breakage. About 5 minutes is enough for boats to cool down.

4)    Worn out copper clamps also are a big reason for boats not performing to their full capacity. Non uniform wear of clamp causes sparking between clamp and boat; this overheats and cracks the boat.

5)    A good quality graphite foil is very important factor in enhancing the life of Boat. To save money, sometimes Metalizers use poor quality plumbing grade graphite foil. This foil contains many impurities and binders and offers resistance to the flow of current. This causes the ends of boat to overheat and break. Supervac supplies 100% pure graphite foil free of binders. This is made with the patented process of Exfoliation. This foil gives you the following outstanding benefits –

A)    Offers no resistance to the flow of current.

B)    Saves costly power.

C)    Enhances the life of Boats.

6)    There is one more major factor inter linking the life of evaporation boat with graphite foil. Good quality graphite foil has good compressibility. This allows a proper electrical contact in boats and clamp before heating begins and expansion of boats in heated condition. Poor quality graphite foil on the other hand will not allow expansion of boats in heated condition putting additional stress and subsequent breakage of boats.

7)    Aluminum of purity 99% or more is required to avoid premature cracking. Impure aluminum will have impurities which will have different boiling temperatures; this will lead to an unstable pool of metal in boat cavity. This in turn results in non uniform heating of Evaporation Boat leading to thermal stress and breakage.

8)    Age of aluminum wire also affects boat life. With age, aluminum oxidizes and Aluminum oxide thus formed causes spitting and excessive heating of Boats. Freshly made or “cleaned” wire works best.

9)  Hardness of Aluminum should be maintained in proper range if required by tempering as a very hard wire is likely to puncture the boats.

10)  Aluminum wire should be fed in such a way that it meets boat in the center widthwise and in between 1/3 to ½ lengthwise. This ensures proper pool formation and increases the life of boats. (I have written a blog post entirely on this topic in the past. You can read it here.)

11) Feed of aluminum wire must be regulated to ensure that some aluminum is always available as molten “pool” on the surface of Boat. This makes sure that boat is uniformly heated and increases the life of boat.

12) Power to boat must be so regulated that aluminum melts just before touching the boat , otherwise it will cause an indentation in boat cavity, hastening the failure of boat.

13) It has been observed that Boats made of Titanium Diboride and Boron Nitride (also called 2 components or dimet) work out to be more economical than boats where Aluminum nitride is also used in addition to the 2 ceramic powders mentioned above to reduce the cost. Three component boats, though cheaper in cost, have a markedly shorter life.

14)  Experience shows that normal rectangular boats with cavity work out cheaper then Triangular or elliptical boats or boats without cavity or laser grooves on surface.



15) Good quality boats, manufactured by reputed companies like Supervac, having density of the order of 98% of theoretical density have very low porosity. This reduces aluminum seepage in boats and enhances life of boat. These boats work out much more economical than those supplied by phony “importers”/ unknown companies having no quality standards.

16) Contrary to popular myth, using boats with initial resistivity higher then recommended by manufacturer does not give more life. In fact this wastes power, causes overheating and premature cracking of boats.

17) Experience proves that spring loaded end support for boats works best for optimizing the life of boats as it provides a proper contact and allows expansion of boats on heating.

18) Level of vacuum in system is also very important for getting proper life from boat. Working at vacuum above 1x 10^-4 Torr is detrimental to the health of Boats as they get oxidized and damaged.( For getting high level of vacuum in shortest possible time Supervac Sigma+™ Diffusion Pump oil and Supervac SV-77™ Rotary Vacuum Pump Oil can be used.)

Points mentioned above are quite easy to implement and best of all do not cost an extra rupee. This however can result in a saving of about Rs. 9, 00,000/- per year per metallizer (a typical metallizer uses about 300 boats per month, costing about Rs.1, 000 per boat. Saving is 300 x12 x1000 x25% = Rs.9, 00,000/-).  This is not all; implementation of these actions will also lead to better metalizing quality, better maintained plant and saving of costly power also. 

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                                                                                                  Written by: Anshuman Punj.

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Correct Wire Feed for Evaporation Boat

While running the metalizing plant, if we take a few seemingly small precautions, they can go a long way in ensuring optimum usage of energy, expensive materials and thereby money. And as we know, money saved is money earned.

fig.1: Evaporation Boat

Figure 1 shows the Evaporation Boat. Aluminum wire is fed into the cavity of the hot Evaporation Boat (heated up to 1500⁰C) where -

1.    It melts

2.    Forms a small pool to fill the cavity

3.    Evaporates to form vapor

4.    Vapors stick to the substrate to make a thin coating

Ideally the aluminum wire should touch the Evaporation Boat between the 1/3 part and the 1/2 part from the wire feed end; never before or after this. The following photos demonstrate the right as well as the wrong position of the feed wire.

                

  

Positioning the aluminum wire too close to the edge of the Boat will not make the cavity fill up uniformly with the molten metal. This leads to –

Ø Non-uniform coating of the aluminum on the substrate.

Ø Wastage of the Al wire since this will cause excess molten metal to fall off the boat from one end.

Ø Wastage of electricity.

Ø Shorter life span of the Evaporation Boat since improper feeding of the wire will cause variation of temperature across the Boat.

Ø Faulty metalizing: When a molten pool of the metal forms in one part of the Boat’s cavity while the other part remains relatively cold, SPITTING phenomenon takes place. In SPITTING, the liquid metal overheats and bursts forth or ‘jumps’ out of the cavity in small quantities and either sticks to the vacuum chamber walls or on to the substrate.

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                                                                                                  Written by: Anshuman Punj.

Go to Supervac Industries Website

 

   Click Here