Material suppliers to the semiconductor manufacturing industry must make their chemicals according to extremely strict criteria that are loosely referred to as electronic grade. What does that mean you might wonder and why is it important? This post will attempt to explain both.
Let’s begin with an organization that more than any other has led the semiconductor industry to create quality standards for that industry, SEMI.ORG. This Santa Clara-headquartered consortium has many goals: conferences, industry advocacy, market research and the aforementioned development of industry standards. The key word in the previous list of goals is “development” of industry standards. With a set of manufacturing design targets that change every two years (see chart below) according to Moore’s Law, SEMI and the allied materials manufacturers must make constant improvements in materials quality standards in order to keep up with the manufacturing requirements:
Well then, just what are the materials standards that must be met in order that functioning devices (with their ever-shrinking design targets) can be manufactured? While SEMI has over 800 standards, for this discussion, there are two tests that are not only critical to this discussion, but actually can be used to define the whole standard: metal ion and particle content. There are several other measurements made on most liquid materials used in semiconductor manufacture (assay, water content, color, etc.); but this post will focus on metals and particles.
SEMI has sectioned the standards such that are aligned to the feature size shown above in the graph:
|SEMI STANDARDS||SEMI Grade 1||SEMI GRADE 2||SEMI GRADE 3||SEMI GRADE 4|
Of the two material standards referred to above, the inclusion of the particle content standard could almost be deduced based solely on the chart above. Imagine manufacturing a device with a 20 nm target but using materials that are contaminated with particles of 50 um in size. It is easy to imagine the effect on any semiconductor device component that is covered by a particle twice or more the size of that component. The greater the number of said particles, the greater the chance such a catastrophic interaction could take place. Clearly in the case of particle contamination, the smaller the offending particle and the smaller the number of such particles, the better chance the semiconductor manufacturer has of fabricating a functional device.
Particle specs will vary from supplier to supplier and often the numbers of particles for a given particle size may be negotiated to some degree with the consuming semiconductor manufacturer. The following chart from KMG Corp. gives an idea of how the size of the particle can change according to the category of spec. Thus, here is a way for a consumer to properly gauge their need (based on CD size) to cost. For example, a semiconductor manufacturer that fabricates devices with targets no smaller than 2.0 um (e.g. SEMI Grade 1) would clearly be misspending his money on the far more expensive TB grade of chemicals vs. the more appropriate and less pricey LP grade.
|1.0, 0.5 um||LP|
|1.0, 0.5 um||MB|
|1.0, 0.5, 0.2 um||GB|
|0.2, 0.065 um||TB|
The other critical test, some would say the most critical test done on liquid materials used in semiconductor manufacturing, is the metal ion content. The problem with metal ion contamination is a phenomenon known as electro-migration. Without going into too much detail, this is a condition driven by current density, target size and ion concentration. As the current density goes up, the target size goes down and the ion concentration goes up, the lifetime of the electronic device in question goes down. For example, in the first days of integrated circuit (IC) manufacture, the chips in question lasted only about three weeks – imagine replacing your iPhone of lap top computer every three weeks! Needless to say a solution was needed. That solution was to reduce the ion concentration (especially sodium, potassium, and iron concentrations) to as low a value as possible. Of course, just like with particles, the lower the ion content, the more expensive the material in question was to manufacture by the materials suppliers. And, of course, the smaller the semiconductor target size, the greater the need.
The following chart gives the names of various grades from Avantor (JT Baker) and KMG for their liquid materials. Note the decreasing concentrations:
|Trace Metal Ion Impurity Level||Avantor||KMG|
In summary then, Moore’s law requires ever smaller CDs (or targets) from the semiconductor manufacturers, which requires every fewer particles and lower concentrations of metal ions, which requires extra processing by the materials manufacturers, which leads to higher and higher materials costs. It is thus clear that a careful alignment of the appropriate SEMI grade that a semiconductor manufacturer is operating in to the proper quality standard for the materials they are using is essential for cost containment and successful chip yield.