Formation of the contact-metallization systems with improved technological parameters
Considered are the technological approaches that provide improved metallization adhesion, reduced structural stresses in the film during deposition and reduced electrical migration at the formation of contact-metallization systems when producing integrated electronics elements.
Formation of contact-metallization systems when fabricating integrated electronic elements presents a complicated process task as the surface relief is formed by steps in the boundaries of contact windows and by a significant growth of profile non-uniformity in the multilevel interconnections due to the increasing number of contact windows.
Formation of the contact-metallization systems
In forming multilevel interconnections  there are steps situated at the metallization track intersections of different levels. The step profile formed by a bottom level of interconnections depends on the etching profile of lower metallization level and on how the interlayer dielectric repeats the steps shape. Thickness of the interlayer dielectric is the lowest in the top area of a metalized film but the inner stresses here are the highest – it leads to cracking of the dielectric and electrical shorting of the metalized levels or interruption of the top-level metalized films. These cracks can be caused by the "self-shadowing" effect occurring when the film is being formed because of the differences in its grow rates on smooth and rough surfaces . The uniformity in the interlayer dielectric thickness and the absence of shorts and interruptions on steps may be obtained by smoothing of the substrate surface staircase relief.
- increasing of the substrate surface temperature and higher film application rate in order to enhance the adsorbed atoms mobility in a film;
- use of various accessories to apply metal at an angle to the substrate surface thereby providing for a fill coefficient of the complicated profiles that is close to unity, etc.
Interfacial connections in the "film-substrate" contact area are essential because the substrate surface roughness produces a strong interfacial macroscopic connection even at weak interatomic binding. Roughness of the substrate surface at vacuum deposition can lead to emergence of various structural defects like hollows and vacancies that cause adhesion weakness .
Low level of metallization adhesion can be explained by a non-conformity of the contacting material properties or improper choice of the methods or processes during preparation of films, or poor metallization process regimes and insufficient quality of the substrate surface which leads to the formation of barrier layers at the boundaries and limited interaction of the contact materials which can be eliminated at the interoperational inspection stage.
Deterioration of the metallization adhesion at thermal treatment may be caused by high internal stresses in metal films due to the arising compressing stresses caused by increasing of grain sizes. Thus, the total mechanical stresses may be increased or decreased depending on the sign of the intrinsic stresses in the films. When the critical value which is dependent on the boundary phase contact properties is exceeded, detachment of the film from a substrate can be observed.
High inner stresses result in the detachment of the metal films and disruptions of the film in the areas with the maximal stress values. Diagnostics of these types of failures is usually complicated because of the long destruction period and the increasing static fatigue of the materials. Semiconductor substrates are fragile and commonly destruct by detachment without being preliminary deformed.
Mechanical stresses arising due to different film properties, and "film-substrate" boundaries and the substrate itself may be significantly reduced at a certain film thickness. In addition, micropores and other defects on the boundaries which may concentrate inner stresses must be absent. The inner stress value in the thin metal films depends on technological parameters of its deposition and followed treatments.
The structural stresses in a dielectric film arise at film deposition and are dependent on the conditions of fabrication (substrate temperature, deposition rate, etc.). Thermal treatment is necessary to decrease structural stresses for increasing film density and decrease of the point defects density. This effect becomes more pronounced as the treatment temperatures grows up to the definite point depending on the nature of the dielectric film material.
Structural stresses arise in the dielectric films due to the following factors:
- surface-tension in the films;
- nature of crystalline defects;
- uniformity of the crystalline structure across the film thickness and others.
The important role to ensure good quality of contact-metallization systems is played by electrical migration. The electrical migration phenomena connected with a transfer of the mass of the interconnection materials under the influence of the external electrical field has an important role in the multilayer interconnections systems together with a current which is produced by this field .
In order to raise the electrical migration resistance, metals are doped with different impurities (beryllium, copper, magnesium) which present effective barriers for mass transfer. These impurities have a relatively high bound energy of the vacancies. Use of the protective dielectric coatings as a mechanical barrier is another method to decrease electrical migration.
At that, a greater thickness of the dielectric coating increases a probability of little hills formation across the entire surface to be metalized, not of the individual high peaks only. Usually, uniformity of the dielectric layer is improved by the use of an aluminum oxide film Al2О5 obtained by anodizing of the aluminum conductor surface and ensures reliable protection against electrical migration. To reduce electrical migration, high-melting metals are used: molybdenum, tungsten, chromium, titanium, and others, which, in addition to a high melting point in recrystallization as compared to aluminum and other more fusible systems, have a lower diffusion coefficient and a higher value of the self-diffusion activation energy, which indicates lack of exposure to electrical migration.
The action of high internal stresses in metal films depends on the technological parameters of the process of their deposition and subsequent processing and causes deterioration of adhesion during thermal treatment, which leads to detachment of metallization or its rupture. To improve adhesion of metallization, consistency of the contacting materials properties is to be ensured, the technological mode of producing films and preparation of the surface of the substrate are to be correctly selected. Reduction of the structural stresses in the dielectric film during the deposition process is achieved by thermal treatment, which reduces point defects and increases the film density. High-melting metals are used to reduce electrical migration. ■