Isotropic Etching to Anisotropic Etching and Semiconductor Manufacturing

June 3, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn about the materials involved in the wet etching process

  • Differentiate between isotropic and anisotropic etching

  • Understand current semiconductor silicon manufacturing workflows

Picture of semiconductors layered overtop each other

Knights of old certainly ooze romanticism. After all, knights rescued damsels-in-distress.

But...the word “ooze” takes on a slightly different meaning when we think about the realism of some guy sweating up a storm while weighted down with 40 or so pounds of armor. Since all this occurred before anyone used deodorant, any damsel-in-distress knew that the brave knight was on his way.

For that reason, sophisticated knights had armor that they wore for battle and a lighter, more ornate version that they wore while impressing peasants, other knights, and...of course...damsels. That ornate armor featured etchings of everything from flowers and beasts to people and intricate curly Q designs. 

Historical Etching Processes 

 Artisans used chemical etching to cut those decorative patterns into the armor with a mild acid. The process of etching armor began with covering the surface with acid-resistant wax or paint. Then, the knight’s favorite artist would carefully use a tool to scribe a pattern. The acid permanently etched the decorative pattern into the metal.

Although 16th century armor does not meet the specifications for rigid or rigid-flex PCBs, the etching process follows a similar path. Etching in the PCB world removes non-circuit copper areas from the board. Imaging occurs through the application of etch-resistant materials that protect or mask predetermined copper areas used as conductive paths for the circuit. Semiconductor manufacturers also apply etching processes during the production of semiconductor devices.

Wet Etching and Earlier Semiconductor Manufacturing

Going back in time a bit (but not to the 16th century!), PCB and semiconductor manufacturers relied solely on wet etching processes based on using chemical reactions caused by the application of liquid acids--called reagents--to remove material. The following table shows common chemicals used in the wet etch process.

 Common Chemical Reagents Used for Etching 

Chemical Reagent

Formula

Concentration 

(Percentage)

Etch Application

Hydrofluoric Acid

HF

49

Glass/Silicon

Nitric Acid

HNO

69.5

Silicon

Phosphoric

H3PO4

85

Aluminum

Ammonium Fluoride

NH4F

40

Glass

The concentration percentage shown in the table indicates that the chemical reagent mixes with other solutions to produce the etchant. Achieving the best results with wet etching requires good selectivity--or a significant spread rate difference between the etchant and the masking material.

Those processes immerse a blank, masked PCB or semiconductor wafer in chemical etchant. While the etchant diffuses to the surface both vertically and horizontally, the PCB or semiconductor surface either absorbs, reacts to, or diffuses the etchant. As the process continues, the etchant removes any material not required for conducting paths. Depending on the tasks, wet etching may involve the use of a wet etching station that includes a fume hood for gas removal, a water supply, an acid drain, a vent, and a moving shield that protects workers from etchant splash.

Other wet etching stations use recirculating wet etch tubs use a pump to transfer etching fluids from a tank to the material and then through a filter for reuse. A heater and thermal sensor maintain the temperature of the etchant for optimal use. In most instances, normal room temperatures offer the best conditions for etching. However, some etching solutions require temperature ranges of 40 - 50o centigrade and 150 - 180o C.

Another technique called dry etching relies on a process that uses energy to convert gas into plasma. The dry etching process exposes unneeded copper material to plasma ions that remove material. As with the wet etching method, dry etching relies on a mask to protect desired areas of desired copper from the ion bombardment.

Isotropic Etching to Anisotropic Etching

Within the major categories of wet and dry etching, production facilities use two types of etching methods. Isotropic etching applies uniform amounts of etching in all directions and at the same rate. The term “isotropic” derives from the Greek “iso” for “equal” and “tropos” for “a turn.” While proceeding in all directions, isotropic etching removes more material at the top edge than at the bottom edge.

In contrast to isotropic etching, anisotropic etching applies unequal amounts of etching and responds to specific directions. The addition of the prefix “an” to isotropic indicates that the anisotropic etching method works differently than the isotropic method.

Silicon wafer in die-attach machine in semiconductor manufacturing

Understanding the semiconductor manufacturing process more helps optimize your design for it.

Semiconductor Manufacturing Techniques

While modern production processes may use wet etching for large surfaces, wet etching cannot provide the level of detail needed for the PCBs used in modern devices. The lack of precision seen with wet isotropic etching occurs because the etchant can undercut--or remove--material under the mask and because the etchant removes more material at the top edge than at the bottom edge. The inconsistent removal of material results in an etching profile that features a flat bottom with curved edges.

Semiconductor manufacturers supplement isotropic etching with processes that compensate for the undercut. The processes drive towards a perfect etch stop that provides a more consistent etching profile.

Moving away from the imprecise wet etching but maintaining the advantages of isotropic etching has required innovation. Production facilities have introduced dry isotropic etching that uses plasma to equally spread etchant in all directions across a surface.

Thankfully you can stay ahead of the curve in manufacturing with the suite of design and analysis tools from Cadence. By trusting in a premier layout tool such as Allegro PCB Designer, you can comfortably get your products out the door and into the next phases of your production workflow. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts


 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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