Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/244—Detectors; Associated components or circuits therefor
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/244—Detection characterized by the detecting means
  • H01J2237/2444—Electron Multiplier
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/26—Electron or ion microscopes
  • H01J2237/28—Scanning microscopes

Definitions

• the invention relates to an electron detector according to the generic part of the first independent claim. Electron detectors are used, for example, in scanning electron microscopes (SEM) where they are employed to detect so called secondary electrons (SEs).
• SEM scanning electron microscopes
• SEs secondary electrons
• the inventive electron detector is to be applicable in particular for the new generation of micro electron column based e-beam tools.
• SEs secondary electrons
• SEs that have less than 50 eV are called “real" SE. They originate from multiple electron-electron scattering. SEs having an energy close to that of the primaries are summarized as “back-scattered electrons” (BSEs). As the name implies, these are primary electrons that were reflected by an atom close to the surface. But also high energy Auger-electrons are sometimes referred to as BSEs. Both kinds of SE (real and back scattered) can be used for image generation in a scanning electron microscope (SEM).
• SEM scanning electron microscope
• SEs are e.g. accelerated towards a scintillator-disc where they generate photons and the photons are then measured by means of a photomultiplier.
• BSEs being able to penetrate much deeper into the surface of the detector can be measured directly using a ring electrode or a pn- or Schottky-junction at the surface of a semiconductor detector.
• SEs it is not necessary to convert the SEs first into photons before detecting them. They can also be directly multiplied by means of an open-window electron- multiplier (e.g. a channeltron).
• an open-window electron- multiplier e.g. a channeltron
• These detectors work like the dynodes of a photomultiplier. They are based on work done by Goodrich and Wiley Il] and by Evans 1 " 1 .
• An SE that enters the detector will hit a specially treated surface and on collision with this surface, generate additional SEs, which are again accelerated towards a further surface, and so on.
• An amplification of the incident current can be achieved if the SE yield ⁇ is higher than 1.
• SEs emitted from an insulating material in the described manner have an energy of about 1 eV.
• the transverse motion of such an SE within a detector channel or tube is combined with an axial acceleration parallel to the channel length which leads to a zigzag path inside the channel and to electron multiplication on each collision with the channel wall. Multiplication of up to 10 6 can be achieved with such devices.
• the advantage of this method over electronic amplification is its high bandwidth.
• Both channeltrons and photomultipliers need to be operated at high voltages and in high vacuum.
• channeltrons or photomultipliers are used for detecting SEs.
• the working distance (distance between the sample and the final lens of the lens system) in such devices is usually between 1 mm and about 40 mm and according to this working distance, two locations are used for detecting SEs.
• the detector is positioned laterally displaced from the e-beam, either at the end of the column between the sample and the final lens (bottom detector), or further upstream, inside the column (top detector).
• bottom detector is used if the working distance is large enough to allow it; a top detector is used for a small working distance.
• H.S. Fresser et al [ '" ] propose to use a metal- semiconductor-metal structure.
• E. Kratschmer et al '[lvl propose to use a miniature multi-channel-plate.
• the channels of this multichannel-plate extend in the direction of the column axis and because of the space conditions can only have a very limited length and therefore, a correspondingly limited amplification.
• the object of the invention to create an electron detector particularly applicable for detecting secondary electrons (real and back scattered) in micro-column based e- beam tools.
• the inventive electron detector is to fit easily into the tight space conditions of a micro column. It is not to influence the low energy e-beam of the tool in an unfavorable manner and it is to achieve an amplification which is satisfactory for all applications of the e-beam tool. Furthermore, the inventive electron detector is to be produceable with known methods and without causing undue problems.
• the given object is achieved by the electron detector as defined by the independent claim.
• the inventive electron detector works on the principle of the channeltron. It comprises means for creating an electric field and at least one channel in which electrons are accelerated by the electric field in the direction of the channel length, the at least one channel having a channel entrance and a channel end, the channel end being equipped for catching and detecting electrons and the inside surfaces of the channel being made of a material suitable for electron multiplication.
• the at least one channel of the inventive detector has a length extending in a plane substantially perpendicular to the primary beam of the e-beam tool and the electric field has field lines extending parallel to this plane rotationally symmetrical relative to the primary beam. Even if the detector for fitting into a micro-column has a very flat form with a thickness of less than 100 ⁇ m, the channels can have a considerable length of several millimeters and give a correspondingly high amplification.
• a preferred embodiment of the inventive electron detector is made by micro- machining from a silicon wafer, i.e. by etching channels extending as open trenches parallel to the wafer surface, by coating the channel walls with a material suitable for electron multiplication (high electron yield ⁇ ) and by integrating means for creating the electric field and means (e.g. Faraday cups) for catching the electrons at the channel ends.
• a material suitable for electron multiplication high electron yield ⁇
• means for creating the electric field and means e.g. Faraday cups
• Such a preferred embodiment of the inventive detector has a central opening for the primary beam and it has a plurality of trench-shaped channels which are open on one surface of the die and which extend radially from the opening.
• a correspondingly structured metal coating constitutes ring-shaped electrodes extending coaxially around the primary beam opening and being connectable to earth and/or a suitable voltage and means for catching the electrons at the channel ends and being connectable to a suitable circuitry for quantifying the generated electron current.
• Figures la and lb show the spatial conditions in a standard e-beam column being equipped with a known electron detector in a bottom position (Fig. la) or in a top position (Fig. lb);
• Figure 2 shows the spatial conditions in a micro column and two possible locations for the detector according to the invention
• Figures 3 and 4 show a preferred embodiment of the inventive electron detector in a sectioned three dimensional representation (Fig. 3) and in a top view (Fig.4);
• Figure 5 shows the physical principle of electron multiplication and detection in an inventive electron detector;
• Figures 6a and 6b show the energy distribution of real SEs emitted from metals and insulators ( Figure 6a) and the total SE yield ⁇ of electrons emitted from a copper surface as a function of the energy of the electrons colliding with the surface ( Figure 6b). (taken from Ref. [v])
• Figures la and lb show in a very schematic way a standard e-beam column in section.
• the column comprises a primary electron beam 1, means for positioning a sample 2 in the path of the electron beam 1 and a lens system 3 for focussing the electron beam 1 on the sample 2.
• the working distance d (distance between the sample 2 and the final lens of the lens system 3) in such a column is usually between 1 mm and about 40 mm and an electron detector 4 for detecting secondary electrons is positioned laterally displaced from the electron beam either between the sample and the final lens (Fig. la, bottom detector) or within the lens system (Fig. lb: top detector).
• detectors usually applied in such columns comprise photomultipliers or channeltrons.
• FIG. 1 shows again in a very schematic manner the spatial conditions in a micro electron column.
• the column again comprises a primary electron beam 1, means for positioning a sample 2 in the path of the beam 1 and a lens system 3 for focussing the beam on the sample 2.
• the space 5 between the sample 2 and final lens of the lens system 3 extends laterally on all sides of the beam 1 typically by about 5 mm and has a width (working distance d) of 1 mm or less.
• an electron detector 4 may be positioned between the sample 2 and the final lens of the lens system 3 as indicated by broken lines and designated with 4.1 (bottom detector) or within the lens system 3 as indicated with broken lines and designated with 4.2, provided that the detector has an extension parallel to the primary beam 1 which is not more than about 1 OO ⁇ m and provided that it can be worked with an electric field which does not have an unfavorable effect on the primary electron beam 1. Both these conditions can easily be fulfilled by the inventive electron detector.
• the detector according to E. Kratschmer et al. [vl , is applicable also in the sensor positions 4.1 and 4.2 indicated in Figure 2.
• This detector comprises a correspondingly thin multi-channel plate with channels extending through the plate and parallel to the primary beam 1 and therefore, having a length in the order of 100 ⁇ m. It is this very restricted channel length which leads to the shortcomings of this device as discussed further above.
• Figures 3 and 4 show as an example a preferred embodiment of the inventive electron detector in a sectioned three dimensional representation ( Figure 3) and in a top view ( Figure 4).
• This electron detector has a flat detector body 10 with an opening 11 for the passage of the primary electron beam 1. At least the channel and electrode arrangement of the detector is substantially rotationally symmetrical relative to the axis of this opening 11.
• the channels 12 extend radial and substantially parallel to the detector body surface. They are trenches, i.e. open on the device surface, or may also be closed channels.
• the flat body of the inventive electron detector is preferably micro-fabricated from a silicon wafer, e.g. from a silicon on insulator (SOI) wafer comprising a silicon layer 13 and an insulator layer 14.
• SOI silicon on insulator
• the flat detector body 10 with the channels 12 may also be fabricated by in injection molding a suitable thermoplast.
• the electric field for accelerating the electrons along the length of the trench-shaped channels 12 of the detector as shown in Figures 3 and 4 is generated between an inner ring-shaped electrode 15 and an outer ring-shaped electrode 16.
• the electrodes are constituted by metal coatings whereby the coating constituting the outer ring electrode 16 crosses the trench-shaped channels 12 extending without interruption across their walls and bottom, and whereby the inner ring electrode may be similar or may (as illustrated in Figure 3) extend on the surface of the flat detector body 10 radially inside the channels.
• the two ring electrodes 15 and 16 are advantageously each connected to a connecting pad 17 or 18 respectively.
• the radially outer channel ends are designed for catching and detecting the electrons., e.g. as Faraday cups by being coated with a metal coating insulated from the outer ring electrode 16 by e.g. a ring-shaped insulation trench 20.
• the metal coating constituting the Faraday cups may extend as a collecting ring 19 across all channel ends and may be connected to one only connecting pad 21 (signal-out-pad). It is possible also to collect and detect the electrons in each single channel 12 or section-wise by isolating each Faraday cup from the neighboring ones and by supplying a signal-out pad for each channel or for each connected plurality of neighboring channels.
• the device layer (silicon layer) used to form the channels has a thickness of e.g. 20 or 40 ⁇ m. This thickness defines the channel depth and, to a certain degree, also the channel width (see design considerations below).
• the channels are etched into the device layer by means of deep reactive ion etching.
• the radially extending channels have a length of about 1.5 to 2.5 mm, a width of between 20 to 75 ⁇ m and a depth of between 20 to 40 ⁇ m.
• the entrance of the channels is preferably grounded, such that, the detector potentials have the least influence on the performance of the micro-column and the acceleration potential (V c ) for generating the avalanche is applied to the channel portion opposite the entrance (outer ring electrode 16).
• the channel ends beyond the outer ring electrode are completely isolated from the rest of the channel. They form a kind of a Faraday cup and are connected to the signal-out pad 21.
• This pad 21 is on the same or on a slightly higher potential than the outer ring electrode 16 or the corresponding pad 18. This configuration allows to isolate the current IS E generated by the secondary electrons from a current due to the acceleration potential Vc
• the opening 1 1 provided in the flat detector body 10 for the primary beam 1 is preferably slightly off the center, as shown in Figure 4.
• the portions containing the pads 17, 18 and 21 will project from the lens assembly and can be contacted from the back side of the device via through holes.
• a similar design is used for contacting the individual electrodes of the lenses.
• the detector body needs to consist of a suitable material or the inner surfaces of the channels need to be coated with such a material.
• This material needs to be highly resistive such as a semiconductor or an insulator and it needs to show a high yield of secondary electrons.
• the material is easily applicable in micro-fabrication.
• Figure 6a shows the energy distribution of the real SE emitted of metals and insulators.
• Figure 6b shows the total SE yield ⁇ for a copper surface as a function of the energy of the primary electrons colliding with the copper surface.
• the BSE yield is indicated by the curve ⁇ and the real SE yield by the curve ⁇
• Table 1 shows the SE yield for a few materials suitable for the channels of the inventive electron detector together with the necessary primary energy, whereby the meaning of E m pE, (maximum SE-yield) and of E IPE and E H E P is the same as in Figure 6b.
• E m pE, (maximum SE-yield) and of E IPE and E H E P is the same as in Figure 6b.
• Al O 3 , SiO (Quartz) and Pyrex are particularly advantageous for the inventive electron detector as they are commonly used in micro-fabrication.
• PbO glass having a high secondary electron yield is used in commercially available channeltrons and is applicable for the inventive detector also.
• the SE yield does not only depend on the surface material but also on the incident angle of the primary electrons.
• a glancing angle generates, in general, a higher yield because the electrons that are generated inside of the target are closer to the surface and, hence, can easily escape. Therefore, the effective electron yield may be increased by structured channel surfaces, consisting e.g. of a porous silicon or oxidized porous silicon. Such structuring of channel surfaces is e.g. achieved for channels which are micromachined in a silicon wafer by electrochemical etching in hydrogen fluoride
• Trench-shaped channels are particularly suitable for being micro-fabricated.
• the trenches are advantageously as deep as possible or they are covered after micro-fabrication leaving an opening for electron entry.
• a ratio of more than 50: 1 for the channel length to the channel diameter should be used if a gain of 10 is to be reached at potentials Vc smaller than 2 keV.
• Vc potentials
• the channels advantageously have a width of less than 50 ⁇ m.
• Goodrich and Wiley 1 ' 1 found, that for a given potential Vc, a smaller channel diameter generally exhibits a higher gain.
• the electrons In order to have a multiplication effect, the electrons must acquire an energy E PE higher than E IPE before striking the opposite wall.
• E PE higher than E IPE
• m is the mass of the electron, e its charge, Vc the voltage applied to the end of the trench, and 1 the trench length.
• the velocity perpendicular to the trench wall is:
• the total kinetic energy E P E of the electron is:
• n eV c / (Ep E - E ⁇ )
• the detector is to be operated with a commercially available video controller having a maximal output voltage of 3 kV and the amplification is to be 10 3 or higher, the following values are derived (inner channel surfaces: SiO 2 ):

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• 1999
  • 1999-06-22 WO PCT/EP1999/004326 patent/WO2000002230A1/en not_active Application Discontinuation
  • 1999-06-22 EP EP99929302A patent/EP1090410A1/de not_active Withdrawn

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