What is a CLASS 1000 clean room?

The clean room of the Institute of Physics, Academia Sinica has a class number of 1000. The number 1000 indicates that the number of fine dust particles with a size of ≥ 0.5 microns is less than 1000 per cubic meter. Therefore, a smaller number means a higher level of cleanliness. But how small is 0.5 microns? A 0.5 micron particle is one hundred times smaller than the radius of a human hair. This is especially important in the process of making qubit chips, where particles and other contamination sources should be avoided.

The clean room of the Institute of Physics, Academia Sinica is divided into two floors with various instruments and equipments for different purposes. The functions of these instruments and equipments will be introduced down below.

Exposure and development process

Electron beam exposure system: A machine that accelerates electrons with a voltage of 100 kV that allows for drawing designed patterns we on resists. This is a common equipment for the production of superconducting qubits.
Josephson junction production system: In addition to the electron beam evaporation and oxidation systems used for typical production of Josephson junctions, this system also has reactive ion etching and high-temperature annealing systems. It is a set especially made to produce superconducting quantum bit chips.
Scanning electron microscope: For imaging very small structures that exceeds the limits of an optical microscope resolution (<~0.2 microns), a scanning electron microscope is the instrument for this job. The interaction between the electrons and the sample allows for the creation of an image that shows the surface morphology clearly.

Josephson junction production system

For the first step, the sample is put inside a loading chamber, whose purpose is to transport the sample to various parts of the production system. The sample is then moved to the MEMS wet process, where the sample is submerged in liquid solutions of chemical agents. After this step, the start of the dry processes will take place.

The first dry process is the reactive ion etching. In here, oxygen plasma is introduced and used as a cleaning agent for the sample. The next process is annealing. In the annealing chamber, heat conduction is used to increase the sample temperature by 800~1000 degrees, depending on experimental needs. After a period of time at this high temperature, the sample is made to reach room temperature. The purpose of this annealing step is to maintain eliminate defects from the crystal structure. This is crucial for the electron beam evaporation system, which uses a high-energy electron beam to heat the target material to the saturated vapor pressure. The evaporated target material will then condense to the surface of the sample. Furthermore, the oxidation system is a key step to make the Josephson junction, where oxygen is used to oxidize the metal on the surface of the sample.

Through this set of radially distributed transmission system, the robotic arm samples will be transferred between these cavities. All the functions of the whole equipment are controlled through these two cabinets. These are our sample production process. We first transfer the samples from The transfer chamber is transferred to the MEMS wet process system to clean the surface of the sample, remove impurities on the silicon wafer, and then transfer the sample to the annealing system through the radial distribution transfer system for high temperature annealing, and then transfer the sample to the electron beam The evaporation system performs metal deposition, and then returns to this system for photoresist coating. Then the sample is taken out from the conveying chamber for electron beam exposure process. After the exposure, the sample is returned to this system for development process. After the development is over, choose different etching methods according to the metal we deposited at the beginning. The etching will be completed in a reactive ion etching system or an electro-mechanical wet process system. After completion, the sample surface will be cleaned, coated with photoresist, and then taken out The sample is exposed to the electron beam, and then the sample is sent back to the MEMS wet process system for development. After the end, the sample is passed through the electron beam evaporation system and the oxidation system to make a Josephson junction, and finally the sample is sent to the MEMS wet process system to place the sample on the After the photoresist is removed, the qubit chip is completed. We will briefly explain several manufacturing processes later.

Exposure and development process

When taking the negative film to the photo studio for processing, a layer of photosensitive agent is first applied to the surface of the photo, and then the photo is exposed under the negative film, and then the photosensitive part is removed with another chemical agent, and the photo is completed. And our chip production is to use photoresist to cover the chip, and then use the pattern on the photomask as a mask for exposure. At this time, there will be a light pattern on the exposed chip, and then the chip is exposed to the photoresist. Remove, this step is called development, if it is to develop photos, then this is over, but if it is applied to our chip production, the next step is the focus.

Etching process

The developed wafer will have two different process steps. First, we introduce the etching process. The etching process uses physical or chemical methods to remove metal from areas that are not covered by photoresist.When the photoresist is removed, there will be no metal coverage in the areas that were not covered by the photoresist at the beginning, and the other process after development is the deposition process.

Deposition process

This process is to deposit the metal on the developed wafer. When the photoresist is removed, there will be metal on the surface of the areas uncovered by the photoresist at the start.

Quantum chip fabrication

Here is a comprehensive introduction to how the various processes mentioned above work when actually making superconducting qubit chips. First, we first define the pattern on the photomask, and then use the photomask as a mask to put the photomask on The part of the pattern copied on the wafer is the exposure and development process. Before exposure, polymer materials are attached to the surface of the wafer. During exposure, the links between the polymers will be split or polymerized. In actual production, Exposure is divided into two types, one is photomask exposure. The process time for photomask exposure is relatively short but the exposed structure is relatively thick, because the wavelength of the light source for exposure is limited to the micron level. The other is electron beam exposure. Because the wavelength of the electron material wave is short, it can expose nano-level structures. Our laboratory uses electron beam exposure instead of mask exposure when making samples. After the exposure is completed, the development is carried out. Here, the wafer is put into the developer solution. The photoresist exposed during our development process will be dissolved by the developer solution, so the exposed pattern will remain on the wafer, and then follow Different structures are used for etching or metal deposition processes. The etching process is divided into wet and dry etching. In wet etching, the wafer is usually immersed in acid or alkali solution to produce the body, and the material is removed by the method of chemical reaction; dry etching is to use gas ions on the wafer The material is physically bombarded or chemically reacted to remove the material. The deposition process is divided into physical vapor deposition and chemical vapor deposition. In the metal deposition process, physical vapor deposition is more commonly used. There are many methods of physical vapor deposition. Electron beam evaporation is usually used in our laboratory. For plating, high-energy electron beams directly bombard the metal to heat the metal to near the melting point, and use the saturated vapor pressure of the metal to be deposited for deposition. The thickness of the vapor deposition is about 100 nanometers. In this case, a quartz oscillator is used. To detect the thickness, when the metal is deposited on the surface of the wafer, there will also be metal deposition on the surface of the quartz oscillator, and this layer of film will change the vibration frequency of the quartz, so as to calculate the thickness deposited on the surface of the quartz oscillator to estimate the The thickness of the wafer surface.

Scanning electron microscope images for each process

Here, images for each process are taken using the scanning electron microscope. First, after the development process is over, here we use two layers of photoresist with different sensitivity, which can be made. This special structure with narrow upper and lower width, using this special photoresist structure with our electron beam evaporation machine, first we plate the first layer of superconducting metal, here we use aluminum as the superconducting metal After the first layer is plated, it is then oxidized. After the oxidation is completed, the sample is turned to ninety degrees and then plated with the second layer. Finally, the photoresist is peeled off to create a Josephson junction structure. The structure of the Josephson junction is super. The guiding qubit is a very important structure. Through the special inductance characteristics it provides, we can realize superconducting qubits.

Introduction to Qubit Chips

Here is a brief introduction to some of the components and basic principles of the qubit chip. Microwave frequency signals are used to manipulate and read qubits. Microwave signals enter the area of the chip called the electrode. These electrodes are connected to an external circuit board through wire bonding. The circuit board is then connected to the external microwave equipment for the supply of microwave signal needed. The snake-like winding structure on the chip is called a coplanar waveguide. The microwave signal travels through the central signal line of the coplanar waveguide. The cross structure in the middle of the chip is the qubit. The state of the qubit can be set by coupling with the two control gates. Moreover, when the cavity is coupled with qubit, the cavity resonance frequency shifts. This shift changes with the state of the qubit effectively enabling reading the state of the qubit.

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