The photolithographic process includes the following basic operations:
- forming a layer of photoresist (processing of the substrate, coating, drying);
- the formation of protective relief, including the combination of (exposure, manifestation, drying);
- transfer images onto a substrate (etching, deposition, etc.).
1. Forming a layer of photoresist. The photoresists and their properties
Rezist1Рельеф is formed as a result of the fact that under the action of actinic radiation incident on certain portions of the layer, the latter in the General sense of the word changes the original properties. You receive the differential solubility of the layer: for example, the bright areas are no longer dissolve in the solvent, removing portions of the layer not exposed to light. The method of formation of the relief resists are conveniently divided into two classes: positive and negative. Differences in behavior of photoresists both classes is shown in Fig.1. Negative photoresists under the action of actinic radiation to form a protective terrain. As a result light curing, zadabrivaniya or other process lit areas dissolve and cease to remain on the surface of the substrate. The relief represents a negative image of the elements of the photomask under the opaque areas of the photomask to the photoresist after the solvent is removed under transparent areas remain protecting from further etching. Positive photoresists, in contrast, convey one-to-one pattern of the photomask, i.e. the relief repeats the configuration of the opaque pattern elements. Actinic radiation so changes the properties of the positive photoresist in the processing in the respective solutions of the exposed portions of the layer are destroyed (washed out).
The basis for relief is the use of photochemical reactions, mainly photopresenter and photolysis. Actinic radiation creates in the layer of active centers; they may be molecules that absorbs the energy of the photons undergo energy transitions, are activated. Photochemical activation can be direct and sensitized. In the first case, the absorbed radiation molecules directly react, for example, dissociate into atoms or are combined with other molecules. In the second case, the absorbed radiation of molecules in the reaction are not involved, but pass their energy to other molecules, in the normal state to the reaction is not capable. When a molecule absorbs light, it goes from a state of thermodynamic equilibrium with the environment and therefore must lose energy in one of three following types: (a) fluorescence or phosphorescence, when all the energy or part of it is emitted back in the form of radiant energy through a small period of time; b) chemical energy, i.e. by transforming the original substance into new compounds, and b) thermal energy, expressed in the increase in the temperature of the reaction system, or the loss of energy in the environment.
Applying a photoresist is preceded by treatment of the substrate to increase the adhesion of the resist layer to the substrate (or deteriorate the wetting of the substrate by the Etchant). This can be used simple cleaning (degreasing) surface; physico-chemical treatment that alters the properties of the surface; applying a bonding sublayer.
For the deposition of layers on a substrate you can use the methods of centrifugation, pulverization, bathing in the solution, irrigation, etc.
Centrifugation is widely used in semiconductors because of its comparatively simple equipment to withstand fluctuations in layer thickness within ±10%. Applied to the centrifuge layers often contain characteristic defects in the form of "comets". These defects arise if the substrate surface remained foreign particles - dust, dirt, etc., or the heterogeneous substrate in the oxide film meet microinclusions (for example, areas with cristobalite structure). In any case, at such places, viscous mass photoresist is delayed and under the action of centrifugal forces arise directed from the centre to local thinning or even ruptures. Important role in obtaining a uniform coating plays a filtering of the photoresist. Resists filter at least 2-3 times through the glass filters SCHOTT (No. 2, 3); the high viscosity is applied by vacuum filtration. This treatment allows you to get rid of foreign particles, dust, etc. A more subtle treatment, which eliminates existing type resists polyvinilcelates subprimary school size of about 0.5 μm, is high-speed centrifugation. The speed of rotation of the centrifuge at 20 000 rpm, and the time required for effective removal of subprimary inclusions, up to several tens of hours. In operation the viscosity of the photoresistor increases, thereby increasing the thickness of the coating and may deteriorate the wetting. To control viscosity suitable standard viscometers.
Pulverization of resists is considered highly promising methods is most convenient for semiconductors.
At the same time, the method of comminution requires a special selection of solvents, as the layer should not drain on the substrate; a thorough cleaning of the resist used for the comminution of air or gas; finally, the development of fairly sophisticated equipment.
Some simple methods of deposition of photoresists - bathing in the solution and watering on a horizontal plane, is borrowed from the printing and photographic techniques. When applying photoresist bathing the substrate is immersed for a few seconds, the liquid photoresist, and then removed and dried in a vertical or inclined position. The excess of the resist flows and permanently removed through ash-free filter. The resulting film is a wedge-shaped thickness: at the bottom of the substrate layer is always thicker. This drawback limits the application of the method of bathing. Some improvement in uniformity in thickness can be achieved by repeated bathing, where the substrate is lowered into the solution to its upper part. Undoubted .the advantages of bathing method are simplicity, good film adhesion. Sometimes a useful role plays the fact that the layer is applied directly on both sides of the substrate. Watering on a horizontal plane provides the best in comparison with the method of bathing the uniformity of the film thickness (although inevitably arise when watering thickening but the edges).
By this method it is possible to obtain thick layers of photoresist to 10-20 μm, which is difficult by other methods. Thick, layers provide protection during deep etching; in Meuse-technology is often used method of irrigation. Fit for irrigation is not complicated: a platform with three adjusting screws or (better) suspension pendulum perpendicular to the ground. The thickness of the layer when watering is controlled by the concentration of the deposited photoresist.
Operation, the final formation of the layer is dry. A General idea about the mechanism of film formation is based on the following provisions: the chain structure of most polymers are amorphous; the molecules of the polymers have klubneobraznogo form due to the fact that the separate parts of the molecule are movable relative to each other; upon evaporation of the solvent, the film-forming agents (polymers) to the glassy state.
The process of evaporation of the solvent plays an important role in the formation of the film. One of the conditions for obtaining high-quality film is a definite low-temperature exposure after application of the polymer solution required for the orientation of macromolecules. Subsequent drying of the film at elevated temperature leads to rapid removal of solvent. Note that when forming a film of a macromolecule strive to enter the steady state, i.e., to adopt a form that would correspond to the minimum value of free energy. This relaxation process takes some time, so too rapid drying can lead to stresses in the film.
Sometimes recommended layer infrared drying; it is effective for the formation of thick layers of about 3-5 microns and above; for ordinary thicknesses (0.3-0.4 µm), the difference in the methods of drying a little much.
Good results when working with thick layers of positive resists (for example, for deep etching of silicon) vacuum drying gives with the slow increase in temperature.
2. The formation of protective relief
One of the most important stages is the combination of images on the template and the substrate
In any photolithographic method, a pin, a projection, and in the method of the scanning beam is a necessary starting point is a certain template, a sample that contains information about the size, location, configuration, etc. of the obtained images.
Most often used for this purpose a plate of optical glass with the photographic or otherwise opaque elements, although for some purposes, may be used, for example, a flat metal plate with through holes.
Characteristics of photomasks
Using modern photoresists and proven technology the quality of photolithography is largely determined by the quality of the photomask, and its production is currently one of the most challenging processes associated with photolithography.
The complexity of manufacturing high-quality photomasks for common currently contact photolithography is determined by their specific characteristics:
1. high resolution. The photomask contains elements of very small size from 0.1 microns
2. a large number of identical images, the number of images reaches 20000 pcs;
3. high contrast images, i.e., the greatest optical density of the opaque areas and transparency in other areas, the optical properties of the photomask is measured in the visible range, and using a photomask, typically in the UV region. The view of the spectral properties of the material of the photomask is a characteristic feature of photolithography PA semiconductors;
4. high precision observance of the sizes of elements and spacing between elements, dimensional accuracy and precision by step are determined by the need for consistent alignment of the photomasks of the kit. In semiconductor technology in 90% of cases there is not a separate photomask, and a set of many (up to 20) of photomasks, the degree of consistency which entirely depends on the specified precision;
5. high quality and uniformity, under homogeneity, it is understood uniformly high quality of all the duplicate images;
6. stability characteristics (geometric, optical, etc.). Photomasks, for example, are produced on photographic film (although there are a lot of varieties of film with very high resolution), since the size of elements and their mutual arrangement should not change with changes in humidity and ambient temperature;
7. high resistance to abrasion, emulsion photomask loses quality after 20 operations contact printing, sintered metal, in particular chromium, photomasks have much better (according to some 3000 times) durability than the emulsion;
8. flatness of working (contact) side of the template.
Possible, in principle, two methods of alignment, let's call them basic and visual. The baseline method, the templates set are strengthened in special devices - part, provided with the installation micrometer screws. With the help of these screws under the microscope the photomask is placed in such a way that the items were matched for whole set. The framework also provides locating lugs or plane. On the semiconductor plate should be made a special benchmark. Most often from a circular plate cut segment, the resulting base is cut. The plate is pressed against the two points of the base of the slice and one point on the circle to the locking portions of the frame; thereby the position of the plate is accurately determined. When re photolithography plate is mounted in a frame with a second photomask and again exposed locking elements in exactly the same position as on the first frame. Image of the second photomask in the result, combined with the image remaining on the plate after the first photomask (and so on for all template sets).
The underlying method is characterized by simplicity and high performance, however, is inherent fundamental limitation: low accuracy, and the sensitivity to damage of the plate. Small alignment accuracy due to the fact that a reproducible manner to press the plate to the locking portions of the frame; the method allows an accuracy of ± 10 µm. Further restricts the applicability of the method the fact that damage to the underlying surfaces of the plate becomes incorrigible marriage.
Visual method is that the image on the plate directly observed (under the microscope) in non-actinic light and is combined with the image on the photomask by using, for example, a micrometer feed. Actually, the same operation visual alignment is an integral part and basic method when the photomask is placed in the framework.
Exposure of the photoresist can be carried out by contact or projection method. In any case, under the action of actinic radiation layer formed in local areas with the changed properties. The select source suitable for exposing a photoresist, is determined by the spectral distribution of the sensitivity of the resist. For exposing resists suitable carbon arc, mercury-quartz lamps of ultrahigh and high pressure type DAC-2, DAC-4, SWDS-250, SVD-120, etc. Knowing the spectral characteristics of the photoresist and sources, it is possible to count which ones are most effective.
Manifestation and drying of relief. The process of manifestation in any case - for negative or positive photoresists is to remove the unnecessary areas of the layer; as a result, the substrate surface remains protective relief desired configuration. However, if negative photoresists manifestation is simply removing unpolymerized areas (sometimes in the same solvent which was used for the preparation of the resist), the positive manifestation associated with a chemical reaction of indene-carboxylic acid in soluble salts. The positive manifestation layers - critical process that depends on several factors: a) the type of the developer (an alkaline compound, additives, etc.) and its concentration; b) time and temperature for existence; b) additional mechanical removal of dissolved plots.
For the manifestation of positive photoresists based financeasia typically use highly diluted aqueous solutions of caustic soda or trisodium phosphate. Some firms are making to the specialized photoresists developers.
Drying manifested elevation different from the drying of the layer so that it can not be afraid of the heat crosslinking (or failure) of the photoresist. Accordingly, the temperature of the second drying is set higher, which increases the protective properties of the elevation.
3. Etching the substrate with a protective relief and removal of protective relief
Chrome. Etching of the film sputtered with chromium protective relief from the positive photoresist is carried out in dilute hydrochloric acid (1:1). The etching process is initiated by touching the surface of the chromium aluminum or zinc shavings; then begins a violent reaction. Film is removed in seconds; you need to watch carefully for the moment etching, since over-exposure to the Etchant leads to a significant reduction of quality.
Removal of protective relief from the substrate is carried out in three ways - chemical, mechanical and thermal. In semiconductor technology, it is widely distributed chemical method of removal. This is because germanium, silicon and oxidized silicon can be processed into concentrated acids, well Deplete organic film. There are universal machining to remove both negative and positive photoresists. The positive resist readily eliminated cold 10-15% solution of potassium hydroxide, after careful washing, since alkali metal ions are actively adsorbed on the surface of the substrate. In planar technology, where alkali metal ions may cause the instability anisloveu a passivating film, remove the relief should be boiling in pure sulfuric acid.
In those cases, when treatment in sulfuric acid is invalid, for example, deleting a relief with a film of aluminum, used in boiling organic solvents, most often trichloroethylene.
Good results ensures long swell of relief in the chloride methylene chloride (polyvinilcelates) or a mixture of dioxane acetone, dioxane (nattokinaise with novolaks), followed by rubbing.
Kits of reagents for photolithography, produced by a number of companies, typically include compositions for removing photoresist.