DON’T Do It Yourself!
In the manufacture and inspection of semiconductor wafers, linear motion systems play a major role. And, because of the stringent requirements of the semiconductor industry for precision and performance, it's by no means unusual to find that off-the-shelf solutions are unsuitable. Engineers who reach this conclusion while designing new equipment are all too often tempted to develop a customised motion control system in house. This may seem a logical step, but there are two important factors, which need to be considered.
The first is how urgently the new equipment is needed. In the semiconductor industry, rapid response to customer requirements is essential. Delays of a few weeks in bringing a machine on line, while the kinks are ironed out of the new motion control system, can translate into a commercial disaster. The second factor is that it is common for linear motion systems developed in house to fall short of meeting the user's requirements. This is no reflection on the skill of the in-house design team; it's simply that producing right-first-time designs for linear motion requires specialised knowledge and experience. Anorad is in a very good position to make this statement, as the company is approached many times each year with requests to sort out problems with linear motion systems that users have developed themselves. Almost without exception, those problematic systems are the products of design teams with excellent track records in other areas.
One frequent problem is that the in-house solution is eating budget. This results from the natural tendency of engineers working in an unfamiliar area to overspecify components. The consequence is escalating costs, but at least it may be possible to make the equipment work.
An even more serious problem is where motion control components are under specified. In this case, the equipment will never perform as required, but it is easily possible to spend a huge amount of time tinkering with it before the real source of the difficulties is identified.
Why, though, is it so difficult for otherwise experienced engineers to develop successful linear motion solutions? The answer is that, while data for the components is readily available, data sheets don't tell the whole story.
For example, iron-cored linear motors, with their high force density and high efficiency, may seem a good choice for a particular application. What the data sheet is not likely to mention, however, is that they are somewhat susceptible to cogging. In semiconductor applications where smooth motion is essential, this could be a problem. The solution is to use a motor with an ironless balanced design but, if the equipment has already been built, making this change will be expensive.
So, what is the best course of action when a precision linear motion system is needed?
The answer is to involve an expert supplier - preferably Anorad! - in the design process. And, involve them early, so that they have maximum freedom to develop an efficient, cost-effective solution. With the right supplier, this solution will not only dependably meet the performance requirements of the application, it will also do so right from the outset, without the endless modifications which systems designed in-house often need. A concern sometimes encountered in the semiconductor industry is that processes are commercially sensitive, which means that designers are reluctant to release details to third parties like suppliers of linear motion systems.
In such circumstances, most companies are happy to enter into a non-disclosure agreement. It is also worth bearing in mind, however, the information the company requires to design a linear motion system is limited to such things as the movement of masses of items, speeds, magnitudes of the motions and required accuracy. In other words, it is often unnecessary to release any confidential information at all.
Linear motion systems underpin many critical processes in semiconductor manufacture, so good design of these systems is vital. The best way to achieve this is to call in an expert, since experience has proved that the DIY route is a path far from smooth.
Philip Wallington is Account Manager and Application Engineer for Anorad, a UK supplier of linear motion control solutions. Philip has a broad engineering background and has been working in the motion control industry for more than five years. In addition to the semiconductor industry, his experience in this field embraces holographic and digital printing systems together with a wide range of general motion control applications.
