Lasers in Electronics: Thinning Silicon
Today we will talk about an interesting process – silicon thinning.
Laser thinning offers a number of advantages over traditional methods, improving the productivity and quality of electronic devices.
Article outline:
Process description
Review and comparison of methods
Selecting the optimal method
Examples of thinning using a laser system
What will happen next?
Let's start with the base:
Thinning is the process of reducing the thickness of a material. The main task is to obtain the required thickness of the material with the minimum possible roughness for further grinding.
Why and for what purposes is it necessary to refine silicon?
One of the most common materials needed to produce semiconductor devices is silicon. Therefore, thinning silicon wafers is a task that our technologists have worked with many times.
Thinning plays an important role in improving the characteristics of electronic components (integrated circuits, transistors, detectors and sensors, sensors, optoelectronic components, solar cells, etc.): reducing weight and size, increasing flexibility, improving cooling.
Let's consider the main types of thinning:
1. Mechanical grinding.
Removing material using an abrasive tool.
Mechanical thinning has a high risk of damaging the plate surface and is difficult to achieve uniform plate thickness. The advantage of this method is the relatively low cost of the equipment.
2. Chemical etching.
Removal of material by exposure to chemical reagents.
*Novokreshchenova E. P. Federal State Budgetary Educational Institution of Higher Professional Education “Voronezh State Technical University”. – 2012.
This method can produce plates with minimal thickness and roughness. However, chemical reagents for etching are considered to be quite expensive (it is difficult to name the exact price, since etching takes place in several stages and each stage requires its own reagents in different volumes). The thinning rate is no more than 240 µm/min.
3. Ionic thinning
During ion bombardment, the surface of the sample is sputtered due to the knocking out of surface atoms by ions.
*Kotov D. A. et al. Technological processes of deposition and etching in the technology of manufacturing IC and MEMS: a teaching aid. – 2020.
This method can also be used to obtain thin and rough plates. The main difficulties are the extremely low rate of thinning – about 0.1-0.2 µm/min, the high cost of the equipment and the need to create special conditions for the correct operation of the equipment.
For example, aspiration systems can be installed for the laser installation to operate. For the ion thinning system to operate correctly, it is necessary to at least ensure the presence of a vacuum.
4. Laser thinning.
Material removal using short laser pulses.
The laser method allows achieving a high thinning speed – up to 22 mm3/h. It can be done faster, but increasing the processing speed can negatively affect the surface quality.
The advantage of this method is that it does not require the use of consumables.
The limitation of the method is the impossibility of obtaining plates with very low roughness and thickness values.
Selecting the optimal method
The table shows a comparison of the characteristics of the 4 methods.
The parameters of thinned plates obtained using different methods are marked from 1 max to 4 min. The speed of mechanical grinding depends very much on the quality of the surface (which must be obtained), as well as the features of the installation and materials. Therefore, we cannot even indicate the order of this value here. Chemical etching, ion and laser thinning allow you to obtain a thinned plate with a relatively constant width value along its surface. Difficulties with this mainly arise only with mechanical thinning.
Method | Price (1 max – 4 min) | Speed (1 max – 4 min) | “Smoothness” of the resulting surface (1 max – 4 min) | Uniformity of thickness over the surface of the plate (1 max – 4 min) |
Mechanical grinding | 4 | 1 | 4 | 2 |
Chemical etching | 2 | 3 240 µm/min | 2 | 1 |
Ionic thinning | 1 | 4 0.1-0.2 µm/min | 1 | 1 |
Laser thinning | 3 | 2 from 22 mm3/h | 3 | 1 |
In our opinion, the optimal solution is a combination of laser thinning and chemical etching. At the first stage, laser thinning can be used to remove the bulk of the material relatively quickly. As a result, we will get a plate of uniform thickness and roughness value, which is perfect for further etching. At the second stage, chemical etching can be used to achieve the desired thickness and roughness value.
Examples of thinning using the MicroSET laser system:
First example. Thinning of a silicon wafer to 50 microns
Fig. 1 shows a laser-thinned silicon plate. The plate dimensions are 30*20 mm, thickness 500 μm. At the first stage, laser thinning is performed at high speed. Thinning was performed to a thickness of 70 μm at a speed of 22 mm3/h.
Fig. 1. Laser thinning of silicon
After the first stage of processing, we obtain a rough relief of the plate surface. In the second stage, chemical etching can be used to obtain a smooth surface and reduce the plate thickness to 50 microns.
The final result of etching: the surface of the thinned plate is smooth, the thickness is constant and the plate is symmetrical.
Second example: Creation of a 25-micron-thick silicon membrane
In this case, the task is more difficult, because when reducing the final thickness, it is necessary to select the processing modes more carefully so as not to burn through the material. Fig. 3 shows laser-thinned silicon wafers with a thickness of 460 µm, thinning was carried out to a thickness of 25 µm.
Fig. 3. (a) Thinning of a silicon wafer, (b) photograph of a thinned silicon wafer
As a result, we see that the surface of the thinned plate is smooth, the thickness is constant and the plate is symmetrical.
Third example. Thinning the body of the part and creating a hole.
This is a relatively simple task. In this case, the roughness of the resulting surface was not important, so only laser thinning was performed. Fig. 4 shows laser-thinned silicon wafers measuring 2*2 mm and 600 μm thick. The wafer was thinned to 200 μm and a hole with a diameter of 500 μm was cut.
From the final result we can see that even when using only laser thinning, the surface of the plate is very smooth, and the plate and hole are symmetrical and even.
We are pleased to continue sharing our knowledge and experience in the field of laser technology.
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