Wafer Processing

Separating in a double package

Damage to wafers incurred during processing causes major disruptions not only to the wafer itself but also to the process flow. Here Axel Ziegler, Siemens, Automation and Drives, Automation Systems discusses, in conjunction with Siltronic’s machining process that reduces damage to the silicon rods.

Pizza wafers are silicon discs owing their name to their size. A manufacturer processes these discs automatically after cutting them from the rod. Two new plants separate, clean and dry the wafers and deposit them in confusion-safe trays for further processing. All movements and processes are controlled by a Simatic CPU 317T-2 DP which allows creating motion control data with tools familiar to PLC programmers. (Fig. 1)

With a diameter of 300 millimetres (12“) the latest generation of wafers has indeed pizza-like dimensions. However, it is not pizza Siltronic is interested in. Siltronic’s concern is how to process and machine silicon wafers without damages and interruptions of the crystal lattice structure and with full traceability. Under clean room conditions, Siltronic produces silicon rods with a length of up to 400 millimetres, cuts them in to wafers and places them in a water bath in so-called coarse cleaning carts. (Fig 2) The following intermediate stage before epitaxy has been reorganised and automated by the manufacturer.

For this purpose, the German companies CRP Automationstechnik from Ampfing and Odevis from Burghausen developed first a prototype and then two production plants for the Siltronic factory Freiberg (near Dresden) which separate the cohering wafers and then clean and dry them and place them into defined trays. (Fig. 3)

The incoming wafers from the clean room are handled in a kind of docking station into which one coarse cleaning cart at a time is inserted, exactly positioned and fixed. Then a two-axis robot dips into a deionized water bath where the wafers are separated and isolated from each other. Vacuum suction cups take up one wafer whose top edge is measured and height-adjusted for transfer. In a fast quarter turn, four sensors check the wafer for cutouts and edge imperfections on the periphery. This is to make sure only flawless wafers reach the edge gripper of a 4-station turntable.

Clocked by the turntable, the wafers successively pass through a pre-cleaning bath, an ultrasonic/tenside bath and a rinsing bath with deionized water. The fourth station is the reference point for the transfer to one of two flip stations moving the wafers from a vertical into a horizontal position. Two flip stations are necessary to deposit the wafers on the correct side, depending on the way they are supplied and the requirements of the following process. Carried on an air cushion, the wafers travel through a compressed-air dryer and are then passed on to the hurdler moving from slot to slot on the vertical axis. Full trays are transported by a bogie to a paternoster magazine. The first tray of each lot (rod) is transferred to a measuring station for process control. Only if the results are correct, the next rod is cut in the clean room.

Everything is under control
Under the control of one single unit - the technology CPU Simatic 317T-2DP by Siemens. It is under the own brand of Siltronic and specified in the company standards. The new technology CPU was selected by Helmut Huber, automation engineer and software developer at Odevis. On the one hand because its functionality and data volume were optimally suited for the task, on the other hand to collect experience for further applications already under consideration.

All five axes of a separator are moved by means of Simodrive 611U drives, the actor and sensor technology is linked to the controller via ET200 I/O and Profibus. The paternoster control was implemented in the technology CPU as well as magazine and data management. On the whole, 450 input and output signals are to be processed.

“We opted for a pure Simatic solution because the 317T 2DP has already all required functionalities for this task so that additional function blocks or positioning controllers in the drives are not needed”, says Helmut Huber from Odevis. “A comparable configuration, for example involving the drives, would have led to significantly more bus traffic and certainly to longer cycle times of the total process. Moreover, this would have required much more programming effort.” With the fast technology CPU all processes are controlled by one device and the total throughput time of 17 seconds from coarse cleaning cart to tray could be kept without any problem. The hardware costs of the possible variants (control or drive based) are more or less identical.

Technology and MC functionality via PLC
The innovative aspect of the Simatic 317T 2DP is the integration of technology and motion control functions in form of PLCopen-compliant function blocks. The S7-Technology software library presently comprises 26 blocks which can be logically combined by the user. This allows to realise technological tasks like gear synchronisation, cam discs, travel to fixed stop, print-mark correction by touch probe, time- or position-dependent cam operation and controlled positioning without additional hardware and thus with maximum cost efficiency. The function blocks are certified to PLCopen and are called from the user program. For programming purposes the usual standard languages FUP (function plan), KOP (contact plan) and AWL (application list) can be used, but also engineering tools such as SCL, S7 Graph or CFC. Having everything integrated in one project facilitates engineering as well as future project maintenance.

“The advantages of a solution consistently programmed in Step 7 are quite obvious“, agrees Helmut Huber, “you are familiar with the programming method, do not have to learn a special motion control language and can immediately use all the new function blocks, which are very well documented in my opinion. Working with virtual axes makes the first steps easier, as, without connected drives, the operating principle becomes clear very soon.”

The automation engineer made himself familiar with the new task on his own and was able to commission the five position-controlled positioning axes of the two disc separators within a very short time. To do so, he made use of the clocked Profibus interface with Profidrive profile and the four fast digital inputs on-board the technology CPU. The fifth fast input could be easily realised by means of distributed I/O. Measuring the wafer top edge after reception from the coarse cleaning cart is just as easy.

Odevis, an internationally operating system house for industrial applications in the domain of automation and information technology, primarily employs Siemens automation technology because of the comprehensive product range that allows harmonised integrated solutions. Furthermore, the products are available worldwide and accepted in virtually all sectors of industry and support is ensured all around the world.

Design engineer Walter Kokott of CRP appreciates the 3D data models made available by the manufacturer as they allow to check with the touch of a button whether a component fits into a certain position or not, which greatly facilitates work. CRP plans, designs and manufactures complex special machines for applications in different sectors of industry, especially products for aerospatial products. “Everything counts that saves us time”, says Walter Kokott.

Distributed HMI concept
There are several instances for operation and monitoring. For local visits to the plant, a Simatic Operator Panel OP 170B for the transfer of coarse cleaning carts has been installed on the clean room site. On the other side, on the plant itself, there is a Simatic Mobile Panel 170 for setup and debugging purposes. The mobile device can be plugged into three connection boxes and carried to any point of the plant with a 10 meter long cable set. The operator panels are connected via the combined DP/MPI interface of the technology CPU. On the front side of the machine where the trays are taken out, an additional Simatic Panel PC 670 running the SCADA System Simatic WinCC has been installed. The main task of this instance is the handling and the transfer of all product data to the subsequent systems. The data are acquired by transponders at the coarse cleaning carts and the trays from the beginning to the end of the process and passed on to WinCC by the technology CPU. This allows assigning each single wafer to its lot at any time.

More complex tasks ahead
After the implementation of this successful solution fully endorsed by Siltronic, Odevis intends to use the Simatic technology CPU also in connection with a wafer saw for the first time. This application will be a much more demanding task for the controller’s motion control capabilities. In addition to the positioning functionality as such, synchronous axes and cam discs will have to be implemented by means of interpolation point tables. However, this would be no exception for the technology CPU, which has already gained widespread acceptance in practical applications.