Spray etching

An imager connecting pn-junction detectors or MIS detectors to connection pads of a silicon read-out chip via through holes is presented in EP-A-0137988 (Texas Instruments Incorporated, USA, 24.04.85). The through holes are created by first ion-milling small holes and then enlarge the same by spray etching. 

The rate of etching under diffusirMi-controlled kinetics is therefore increased by spraying the etchant onto the substrate as opposed to immersing it in a static bath of etchant. This is the reason that commercial PCM is carried out in a spray etching machine (Fig. 1) where etchant is sprayed from nozzles at typical pressures of 2-4 bar. Due to the size limitations of such machines, typically CHM applied to large parts is carried out in large tanks that can hold as much as 44,0001 of etchant (Dini 1984). 

Etching, Fig. 1 Spray etching machines (Courtesy of PrecisionMicro, Birmingham)... 

Once the DBC metal is attached to the substrate, the copper metallization can be etched with processes similar to those used in the PCB industry. The key difference is that the metal is far thicker than is used by PCB vendors and requires special etching considerations. The principal method used to etch these conductors is spray etching with a caustic solution. Spray etching is necessary because the amount of material that must be removed is significant, and a standard dunk etch process is ineffective because of saturation of the solution at the copper interface. In contrast, the spray process presents... 

There is strong evidence that the etching rate is dependent on the diffusion of Cu(NH3)2 from the copper surface (Eq. 34.1) into the bulk of the active solution, where oxidation per Eq. 34.2 occurs. Etching can be continued with the formation of Cu(NH3)4 oxidizer from air during spray etching and as long as the copper-holding capacity supported by CT ions is not exceeded. 

Properties and Control. Early cupric chloride formations had slow etch rates and low copper capacity and were limited to batch operation. Regenerated continuous operation using modified formulations has brought useful improvements. Etch rates as high as 55 s for 1 oz copper are obtained from cupric chloride-sodium chloride-HCl systems operated at 130°F with conventional spray-etching equipment. Copper capacities are maintained at 20 oz/ gal and above. However, more recently, higher copper contents and adjusted chemistries at 125°F typically etch at 75 to 90 s for 1 oz copper. 

The etchant is a solution of cupric chloride and hydrochloric acid (Table 34.1, no. 1). Etchant flows continuously between spray-etching machines and a plating tank. In the plating machine, two processes take place simultaneously copper is plated at the cathode, and regeneration of the spent etchant occurs at the anodes. pf>er recovery may not return copper value and may be expensive, inconvenient, and cause difficulty with recycling. 

The concept of spray etching adds another dimension to the fluid dynamics. The surface of the panel is never really free of a liquid film. The top side of a panel has the etch liquid held up between dehvery by the sprays and removal by flow off the edges. This flow is influenced by the number, flow, and pattern of the nozzles, the interaction of the flow between the sprays, the motion of the panels, and the effects of rollers and other contacting and shadowing objects. Even the bottom side retains fluid by surface tension and is influenced by the amount of hquid moving under the direction of the nozzles. The end result is complicate flow patterns that are the result of testing and optimization by the machine manufacturers. 

As the name says, in spray etching the etching medimn is sprayed onto the surface of the sample by an array of nozzles. To optimize the uniformity of the process the sample is moved. Normally many samples are placed on a rotating table and processed simultaneously. The etching rate of the spray etch... 

Spray etching requires a special spray etcher. A spray etcher consists of fluid chambers, a pressure system, an array of nozzles and a sample holder. The complete system is encased to avoid the vaporisation of the etching solution. 

In spray etching the samples are positioned on the sample holder while the etching medium is sprayed by the pressure system through the nozzles on the... 

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