Pioneer in the Ultra LSI Age
Semiconductor electronic circuits, called LSI (Large Scale Integration), are incorporated into the electron devices that we use daily, such as mobile phones, personal computers, game devices, digital cameras, audio devices, TVs, etc., The latest LSI contains more than one billion transistor units in a single device. The electron beam lithography system (hereafter called EB lithography system) is a device which is playing an important role in the production, research and development of these semiconductor components.
LSI-the magic wand of electronics
In 1945, the world’s first electronic computer “ENIAC” was developed. It used 18000 vacuum tubes, which would cover an area of more than 160 m2 if all of them were placed on the floor side by side. Its weight was 30 tons. But at present, due to the appearance of LSI, computers tens of thousands times more powerful than “ENIAC” are now small enough to easily carry. This “magic wand” of LSI typically looks like a black block of plastic about the size of a caramel, with many feet, like a centipede. Inside the plastic is a thin plate of about 1 to 2 cm2 (called a chip, and this contains the true magic of LSI.) On the chip, there is a very fine circuit pattern for the specific LSI device drawn very precisely. These fine circuit patterns are drawn using the EB Lithography System.
How is an LSI chip made?
An LSI pattern is designed according to the purpose of the chip. The pattern is transferred to a photomask (which is like the film negative in photography). The photomask is made by applying a coat of resist onto a quartz substrate with a chrome film (the usual size is 152mm x 152mm) and exposing it to the EB to form a pattern in the resist coating that appears during the developing process. Etching is then performed using this resist pattern as a protective film to create a light-shielding layer of chrome. At present, most photomasks are drawn at a size that is 4 or 5 times larger than the pattern that is drawn on the LSI chip. Normally, these enlarged masks are called reticles.
The pattern in the reticle is transferred onto the wafer using a photo transcription device called a stepper and scanner. This is the same principle as developing photos from film. The reticle is equivalent to the film, and the wafer is like the paper. The big difference between a reticle and film is that the drawn circuit pattern on the reticle is much finer than the image on the film. The latest LSI, such as DRAM (memory element) and MPU (processor element), etc. have very fine patterns, with circuit traces only 45nm wide on the wafer. The accuracy required for a mask to draw lines of 45nm, means that a pattern with lines of 0.2μm width (4 times 50 nm) on the 152mm x 152mm square material must be drawn with a dimension error and position error of no more than 2 to 5nm. After pattern transcription, etching using resist as a protection film, diffusion treatment, metal film forming, etc. are performed on the wafer to create the circuits.
This process from pattern transcription to etching, etc., is repeated in several times, to form the layers that make up the objective LSI chip. The LSI chips prepared in this way are cut into individual chips from the wafer using a laser beam, and built into packages. Wafers are a single crystal silicon of 200mm or 300mm in diameter, From this one wafer, several hundreds of LSI chips will be born.
On the other hand, some semiconductors for high frequency to be used for amplifiers for mobile phones, and compound semiconductors such as photo semiconductors which control light source laser for photo communication have very narrow circuit width of 0.1μm or less, and for this process only, the EB lithography system is used to directly draw the pattern on the wafer with the electron beam, without using a reticle.
Freely Movable Electron Beam Pen
Structure of EB Lithography System
The figure shows the structure of a typical spot beam type. Electron beams are irradiated from the electron source, and tightly focused by electron lens onto the material to form an extremely small spot. The pattern data for lithography is stored in the hard disc of the computer used to control the system. Subsequently, this data is transferred from the hard disc to the blanking control system and beam deflector control system through a high speed data processing system. The beam is switched on and off to draw the pattern by blanking circuit, and deflected to the designated position by operation of the deflector circuit. The desired pattern is drawn using a combination of this electron beam movement and the movement of the stage. To precisely control the stage movement, laser interference measurement system is used. This type of spot beam device is used for research and development of next-generation semiconductor components, as well as for production of high frequency semiconductor devices, photo semiconductor devices, etc. For production of a reticle, a device using a variable shape beam method is used. These devices have allowed a breakthrough in high speed lithography, by transforming the concept of the beam shape from a point (spot) to an area (rectangle). In this method the lithography pattern is divided into rectangles, and the lithography is performed by “stamping” the material with a rectangular electron beam whose size can be varied. This type of device is used at production site daily, as a major device for reticle lithography.