Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-06-29
2003-04-29
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S306000, C250S3960ML, C250S3960ML, C250S397000, C250S492200, C250S492300, C250S428000
Reexamination Certificate
active
06555815
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a charged particle beam device for the examination of specimens. In particular, this invention relates to the examination of specimens which have the tendency of getting charged while being radiated with a charged particle beam.
BACKGROUND OF THE INVENTION
Negatively or positively charged particles coming from a particle source can be accelerated by a potential of V volts. The direction of travel of such a moving particle is altered by applying either a magnetic or an electric field; for example, a charged particle moving in a magnetic field experiences a force tending to change its direction of motion except when it is travelling parallel to the magnetic lines of force. Suitably shaped magnetic and electric fields can be used to cause charged particles diverging from a source to converge into a beam, guide the beam along a predetermined path and allow it to impinge on the surface of a substrate or a specimen.
The charged particles interact with the atoms of the specimen and cause a number of different effects in the specimen or on its surface. Without limiting the scope of the present invention, the following explanations will primarily concentrate on the use of electrons as charged particles. The impinging electrons, generally called primary electrons (PE), are deflected by collisions with specimen atoms. These collisions may be elastic when the electron is deflected (even up to 180 degrees), but no energy interchange occurs. They may also be inelastic when the primary electron interacts with the atom and supplies energy for a further process to occur. Such a process could result in the emission of an electron, called secondary electron (SE) and/or electromagnetic radiation. Thereby, the primary electron generally experiences only a small deviation of track. After the collision or collisions, the primary electron may re-emerge as backscattered electron (BSE) or as transmitted electron, or may lose all its energy and come to rest in the specimen. There, the primary electron contributes to the specimen's heating or to the specimen's absorbed charge.
The above indicated physical effects can provide much analytical information about the specimen. In the following, the creation of secondary electrons and their informational content of the specimen will be considered in greater detail. An inelastic collision of an incident primary electron having a kinetic energy of e.g. 1 keV, can result in electrons being detached from the specimen atoms. This leaves behind an ionized atom with a positive charge. The dislocated electrons have a low kinetic energy, typically less than 50 eV, and are easily captured by nearby atoms. Some electrons which are created closer to the surface can be emitted from the specimen and can be collected with specific detectors. Consequently, only a small portion of the secondary electrons formed are available for collection. Since the emitted SE originate from a small region very close to the surface of the specimen, they carry corresponding surface information.
In particular, a surface of the specimen which is tilted relative to the incident beam reacts differently than a surface perpendicular to the incident beam. Compared to a flat surface, the electrons having entered a tilted surface of the specimen propagate a longer distance close to the specimen's surface. This results in a greater proportion of secondary electrons that are to escape, and so the electron emission from the surface will increase. The intensity of the secondary electron emission is therefore an indicator of the surface slope and topography. Therefore, the intensity signals collected by secondary electron detectors can be used for high-resolution surface imaging. Instruments visualizing these surface effects have become increasingly important for the development of e.g. microelectronic components. They are used to identify deviations from predetermined patterns or to evaluate topographical parameters such as height, width or angle of inclination of the structure under examination. An example of a widely used system is the scanning electron microscope (SEM). A critical dimension SEM (CD-SEM) is routinely used to measure dimensions of elements on a semiconductor wafer to a nanometer resolution.
It should be appreciated that in order to obtain an image using the SEM, the number of electrons in the primary beam must be different from the number of electrons emitted from the specimen (i.e., the yield must be different from 1). This is especially true for insulators and semiconductors where charge is easily accumulated. The charge can result in a strong electrical field prevailing at the surface of the specimen and substantially altering the image of its surface by, for example, altering the path of PE's and SE's. In a semiconductor device, for example, electric insulators such as SiO2 are often deposited on conductors such as Al or semiconductors such as silicon. When a PE beam is directed onto the device, the surface of the insulator is charged. The resulting electric field alters the direction of PE's and SE's and results in inaccurate measurement of the features. This problem is even more severe when several lines to be measured are closely positioned, so that many interactions with charge fields occur to cause deviation from the actual measurement. Additionally, such a field at the surface can prevent SE created at the bottom of a contact hole and vias from reaching the detector.
In the past, a variety of methods have been tried to solve these problems. The approaches included adaptation of the acceleration voltage and the current of the electron beam. Others have altered the scanning speed of the primary electron beam or modulated the primary electron beam before impinging on the specimen. However, these methods have not been satisfactory. In some cases the intensity of the emitted secondary electrons is too low, in other cases the results obtained by comparative measurements are unreliable.
In an alternative approach, Environmental Scanning Electron Microscopes (ESEM) have been used. Originally these instruments were developed for the examination of specimens which are sensitive to dehydration caused by the vacuum in the specimen chamber. The use of a low pressure environment in the chamber prevented the dehydration. As a secondary effect, the presence of ions in the irradiated gas impeded charging of the specimen. These ESEM systems, however, cause widening of the beam of charged particles due to the scattering of the primary electrons due to the absence of vacuum. Also, the high gas concentration in the electrical fields between the detectors and the specimen can result in arcing. Therefore, the conventional ESEM systems did not lead to satisfactory results either and cannot be used for semiconductor applications since such applications require high vacuum environment.
Various proposed methods to avoid charging in SEM examination of semiconductors are presented in U.S. Pat. No. 5,869,833. While the focus of that patent is various scanning schematics to prevent charge or cause discharge, there is also a mention of introducing gas into the vacuum chamber. However, the discussion relating to the introduction of gas is basically limited to a single paragraph and lacks many details needed to suggest a working system.
SUMMARY OF THE INVENTION
Accordingly, the present inventors undertook an in-depth study of the subject of semiconductor charging and, in particular, the use of gas discharge in a vacuum chamber. Thus, the present inventors uncovered many of the difficulties needed to be overcome in order to make such a system workable in practice. The various problems and solutions worked out by the present inventors are detailed herein.
The present invention intends to provide an improved apparatus and method for examining a specimen with a charged particle beam. Specifically, the various embodiments of the present invention utilize injection of gas into the vacuum chamber to assist in discharge of the specimen.
Feuerbaum Hans-Peter
Winkler Dieter
Anderson Bruce
Applied Materials Inc.
Sughrue Mion Zinn MacPeak & Seas LLP.
Wells Nikita
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