Linear Integrated Circuits: Unit I: IC Fabrication

Photolithography in IC fabrication

different techniques with diagram

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To produce extreamly small circuits and to develop patterns on the silicon wafer, the process adopted is called photolithgraphy.

Photolithography

To produce extreamly small circuits and to develop patterns on the silicon wafer, the process adopted is called photolithgraphy. Conventionally using ultraviolet rays it is possible to produce devices with dimensions of 2 µm. Now a days, using electron beam lithography or X-ray lithography, it is quiet possible to achieve dimensions of the devices less than 1 µm. Basically the photolithography process is subdivided into two processes namely,

i) Photomask generation.

ii) Lithography i.e. pattern generation.

iii) Etching.

1. Photomask Generation

During the fabrication process of semiconductor devices, it is necessary to identify the regions of each circuit on the wafer so as to carry out selective doping process. Basically selective doping means in certain regions of the wafer, identical dopant is deposited or interconnections are made and at the same time remaining portion of the wafer is protected against doping. In general, a wafer consists of hundreds of identical circuits and each circuit may consist millions of devices. Thus identical steps are carried out simultaneously which require selected regions are exposed only; while the others are protected. This requirement can be fulfilled by using mask (protective layer pattern). During fabrication process, for one of the processes such as oxidation, diffusion, ion implantation and epitaxial growth, separate mask is used. First entire wafer is covered by a protective layer with certain pattern generated on it and then protective layer is removed from the selected regions.

A generation of photomask means designing a desired pattern for each device. A mask is simply a glass plate consisting a pattern for the complete wafer. The mask is generated using computer controlled system. The pattern to be generated is stored in the digital form. With the proper commands executed by the computer, a pattern generator is activated which uses an electron beam to draw pattern. This is commonly known as photoengraving. This pattern is drawn on high quality glass which is coated with material such as chromium.

After obtaining mask for each layer, individual mask is photographed. Then using step and repeat camera, photorepeating is done. Photorepeating means obtaining successive images in array, on the photographic plate which is moved in equal steps during exposure. After developing the exposed plate, final mask is prepared. This mask is usually called master mask. From this master mask, working copies are made which are called reticles. Such reticles are used for actual pattern transfer on a semiconductor wafer.

In general, reticle is about 5 or 10 times the actual size of the chip. The reticle consists two types of the regions namely transparent region (with no metal) and opaque region (with metal) when the light is passed through reticle placed on a photoresist coated wafer, only the shadow of the reticle is projected onto the surface of the wafer.

The mask for each level of fabrication process consists a multiple pattern with exactly identical boundaries but different patterns as shown in the Fig. 1.8.1.

For larger wafers accomodating number of individual integrated circuits or sites, a wafer stepper is used. A wafer stepper is an apparatus which holds the wafer and controls accurate movement to align optics to each site. This sequence produces large number of identical sites on the wafer shown in the Fig. 1.8.2. The test site locations contain various test structures and circuits. These test sites are very much essential to test wafer electrically during various fabrication processes.

2. Lithography

Lithography is a process in which the pattern generated on the photomask is transferred or imaged on the wafer covered with photoresist. A lithography process can be realized with subprocesses given as,

i) photomask generation and deposition of photoresist on the wafer,

ii) pattern transfering or imaging on the surface of wafer, and

iii) etching of silicon oxide.

The lithographic process actually states with the generation of a photomask. To transfer this pattern present on the photomask, the wafer is coated with light sensitive liquid called photoresist (or simply resist). To have uniform coating of photoresist on the surface of the wafer, the photoresist liquid is sprayed on a spinning wafer held in a place by a chuck.

Then the radiations from the radiating source are directed to the wafer through the photomask. The radiations travel through the clear or transparent portions of the pattern only. While the opaque portions block the radiations. To remove the material from the surface of the wafer which is not masked, the etching process is used. The schematic representation of the lithographic process is as shown in the Fig. 1.8.3.

a. Optical lithography

The lithographic process carried out using ultraviolet light as radiating source is called optical lithography. As explained earlier, a light sensitive liquid called a photoresist. The photoresist in a liquid form is sprayed over the surface of the wafer. The wafer is spinned at high speed to achieve uniform thin coating of the photoresist over the surface of the wafer. To remove excess solvent, the wafer is baked in an oven. This is called prebaking. Then the wafer is cooled at room temperature. Then the wafer is exposed to U.V. light through the mask and then it is developed using developer solution (e.g. trichloroethylene) which dissolves the unexposed regions on the photoresist. After developing, the wafer is baked again, commonly called postbaking.

In general, a photoresist can be classified as positive photoresist and negative photoresist. With the positive photoresist, the window is opened wherever the ultraviolet light is passed through the transparent portion of the photomask. The common examples of positive photoresist are MP-2400, HPR-206 etc. With the negative photoresist, the window is opened only under the opaque parts of the photomask. The common example of negative photoresist is kodac microneg 747. The negative photoresist on exposure to the light becomes less soluble in a developer solution, while positive photoresist becomes more soluble.

In the optical lithographic process, we have studied that the photomask is placed in contact with the wafer through a photoresist. At the surface of crystal, there is a possibility of irregular particles along with the dust particles. During process these particles stick to the mask. When such a mask is used for further operations, it causes defects in the surface. To overcome this, a proximity printing process is used. In this process, the distance of 10 to 20 µm is maintained between mask and the wafer.

b. Electron Beam Lithography (EBL)

When the feature size required is less than 1pm, electron beam lithography is prefered as against optical lithography. The EBL offers higher resolution as compared with optical lithography. In this system, the photomask is generated using either electron beam pattern generator or electron beam projection system. The exposure time is longer in EBL as compared with optical lithography. Also the equipment is very costly. Hence this method is used only when very small device dimensions are demanded.

c. X-Ray Lithography

All optical materials become opaque, at a reduced wavelength, because of fundamental absorption but the transmission increases in the X-ray region. The X-ray wavelength is small, hence the diffraction effects are reduced which gives high quality image. Using proximity printing technique, a pattern is generated on the wafer. The feature size of 0.1 pm is also possible with X-ray lithography. But similar to the EBL, the proces is too slow and the cost of the equipment is very high.

3. Etching

Etching is a process in which the material which is not masked by the lithographic process is removed. The etching technique is useful in removing unmasked material uniformly or selectively. The most widely used etching a techniques as wet etching, reactive plama etching and ion etching.

In wet etching, the unmasked regions on the wafer are etched using wet etchants (i.e. chemicals) such as nitric acid, hydrofluoric acid. The main advantage of wet etching is high throughput. That means large number of wafers can be immersed in the etchants simultaneously to etch off unmasked regions, the etching rate in wet etching process is dependent on temperature. The selectivity is higher in wet etching process. This process minimises lateral removal of resist and overetching of the substrate material.

The reactive plasma etching process is dry etching process. The wafer under consideration is kept in a reaction chamber. The reaction chamber is filled with reactive gas. This chamber is excited by high value R.F. field producing a plasma discharge in the reaction chamber. The plasma is a collection of electrons, positive ions, negative ions and molecules generated due to the interaction of gases introduced in the chamber. These ions react with the wafer and remove the unmasked material. The reactive plasma etching is preferred over wet etching because it is less sensitive to temperature changes and the process can be controlled easily. The pattern obtained after reactive plasma etching is superior than obtained using wet etching. The commonly used gas for etching silicides, silicon, silicon dioxide is CF₄ and its compounds. For SiO₂ (silicon dioxide) SiCl₄ is most commonly used gas while for silicides and silicon etching NF₃ is also preferred.

In the reactive ion etching process, the wafers are kept in a reaction chamber between two electrodes, anode and cathode. Using electrodes, the plasma is produced between these different ions, which partly react with wafer and partly remove the material by ion bombardment. This process electrode is advantageous as it provides highest selectivity and uniformity.

The plasma reactor used in the reactive plasma etching process is as shown in the Fig. 1.8.4.