Chemistry of the Cupric Chloride etch
Replenishment of the Cupric Chloride Etchant
The Cupric Chloride etchant generates large quantities of Cuprous ion as the Copper is being etched. It is important that this be removed essentially immediately, to maintain etch speed. The Cuprous ion is "gotten rid of" by converting it to Cupric ion, which becomes the etch feedstock for the next set of Copper to be etched. There are a number of systems for doing this. We will discuss three of them.
Chlorine as a Replenisher
The simplest replenisher, from a chemical point of view, and arguably one of the most difficult to implement in the real world, is Chlorine. The use of Chlorine as a replenisher is elegant in many ways, because the etchant then becomes, in theory, steady state. The overall reaction is this:
There is only one reagent that is consumed, in theory, and this can be monitored by an Oxidation Reduction Potential (ORP) meter. Some Hydrochloric Acid (HCl) will be lost due to evaporation, and dilution, and this will have to be monitored. However, since the HCl is not consumed by the etch/replenishment reaction, it will vary in concentration much more slowly. Further with the use of Chlorine as a replenisher, it is possible to achieve the widest possible operating window on the etching parameters. It is possible to etch with, and maintain etch solution Baumes as high as 40+, and thus achieve very high etch rates, with very low etch factors (see chapter 12). And it turns out, that the economics of the use of Chlorine are also the most attractive.
However, the practical difficulties of using Chlorine for replenishment, not just the actual safety issues, but more importantly, the perceived problems, in the heavily legislated, risk-averse, environment of today, make the use of Chlorine not very common, even in the largest facilities.
It should be noted here, that much of the perceived safety of alternate replenishers is just that, perceived. Any replenishing system in use today, if the controls do not function correctly, can generate significant quantities of Chlorine gas. Chlorine gas can, of course, be very dangerous to any form of life, even to chemical process engineers. In the ideal system, the ORP of the etchant is continuously monitored, and Chlorine is injected to raise the ORP to the point where there is just the barest trace of free Chlorine. The HCl can be controlled either by an automatic titrator, conductivity monitor or by manual titration. Water is added automatically by a device that continuously measures the specific gravity of the solution, ("Baume Controller") and adds it to keep it below a set point. This maintains the Cupric Chloride content at a point below saturation, so that the etcher remains sludge free. Excess etchant overflows the etch sump, and is pumped to storage where it is later sold. The Cupric Chloride produced this way is very pure, and is more readily sold, and can get a better price than etchant contaminated with Sodium Chloride.
The Sodium Chlorate/colorimetric Replenisher
Perhaps the most popular method of replenishment of the Cupric Chloride etchant today in the US, uses Sodium Chlorate (NaClO3) . This method has some inherent safety features, but these features come at a price. The overall etch/replenishment reaction, using this method becomes:
There are two problems that come attached to this type of system, the obvious one is that the spent Cupric Chloride etchant is contaminated with Sodium Chloride, and thus becomes more difficult to get rid of. In fact, most facilities using this technology in the United States are forced to dispose of spent etchant to hazardous waste disposal/recycling companies, who actually charge to remove it from the facilities.
Further the fact that the etchant contains Sodium means that the Baume (which means Cupric Chloride concentration) must be kept fairly low (<22 Be), so as not to precipitate Sodium Chloride from the etchant solution. This makes the etch chemistry slower than it would be at a higher Baume (concentration), and further increases the volume of spent etchant, thus increasing disposal costs.
Hydrogen Peroxide as a Replenisher
The Hydrogen Peroxide (H2O2) system of replenishing has been popular in the Photochemical Milling industry. Using this system requires a reliable control system, for both components required to replenish the system, Hydrogen Peroxide, and Hydrochloric Acid.. The overall etch/replenishment reaction for Hydrogen Peroxide is:
The use of Hydrogen Peroxide also produces a purer Cupric Chloride, without Sodium Chloride (NaCl) impurities, but the Hydrochloric Acid level must be carefully monitored, as it is consumed in the regeneration part of the reaction. This replenishment method also suffers from the fact that over-replenishing with the Hydrogen Peroxide can generate free Chlorine gas.
The fact that the replenisher ingredients are supplied in water solutions, as well as the fact that the replenishment reaction produces water, limits the maximum Cupric Chloride content of the etch, but it is still possible to get to as high as 40° Be’, depending on the concentration of the Peroxide and HCl replenishers.
Etch conditions and Operating Parameters
The convention in the photochemical milling industry today is to have any conveyorized process essentially complete by the time that the part is half way through the spray chamber in which it is being processed. This is to insure that any spurious, slow-to-process details are completely taken care of by the time the part emerges from the process. The point in the spray chamber that the process is essentially complete is called the "break point". Thus the break point in the etch process is the point that the Copper is etched completely through. This implies that a spray etcher must actually be about twice as long as is actually required to complete the majority of the etching.
Table 11.2 Operating Conditions for Cupric Chloride etchant
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Cupric Etch*
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|---|---|
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Copper Content
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24 ounces/gallon
(180 grams/liter) |
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Temperature
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120°-130° F (50°-55° C)
|
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Effective Speed
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0.6 mil (15 microns) Copper/min.
|
Chemical Operating Parameters:
* The operating parameters of the Cupric Chloride etch actually vary over a huge range, The issue of etch speed is difficult to discuss, as it is very much a function of the details of the etch chemistry. The speeds cited in Table 11.2 are reasonably common to the industry. However, etch rates can be more than doubled by small changes in the etchant chemistry. It is further important to realize that speed of etching and etch “undercut” go hand in hand, the faster the etch rate, the greater the undercut.
In the case of the Cupric etch, the etch speed is a sensitive function of the free acid. The Cupric Chloride etchant is controlled and replenished in a number of different ways.
When Hydrogen Peroxide (H2O2) and Hydrochloric Acid (HCl) are used to replenish the Cupric Chloride etchant, the Hydrogen Peroxide additions are controlled by an Oxidation/Reduction Potential (ORP) meter, which functions very similarly to a pH meter. The Hydrochloric Acid additions are controlled by a very sensitive conductivity meter. The excess etch solution generated is overflowed into a sump and pumped to barrels or a holding tank for shipping out.
The speed and quality of the etching is very dependent on maintenance of the chemical operating parameters. The speed of etching is unusually sensitive to the HCl content in the Cupric Etch. The essential reason is the free HCl, is the etch reaction rate-limiting-reagent in the dissolution of the Cuprous salts formed during etching, and thus is the real etch rate controlling reagent. Further to all of this, it is important to realize that it is not possible to have both Cuprous ion, and free Hydrogen Peroxide present in the same solution, so the ORP meter actually functions as both a monitor of Hydrogen Peroxide and Cuprous ion.