Patent Application
20150107273
Vacuum systems are widely used in scientific research and industry. Among many important technology fields that need high vacuum system is the semiconductor manufacturing field. Frequently the performance of devices highly depends on the pressure and impurities present in a vacuum system. Residual gases and/or other impurities in the growth environment could be a significant source of contamination of the product.
Ultra high vacuum regime is the vacuum regime characterized by pressure lower than 10−9 Torr, and is not trivial to achieve. Though pumps can continue to remove particles from a vacuum chamber in an attempt to decrease the pressure in the vacuum chamber, gases enter the vacuum chamber by surface desorption from the chamber's walls or permeation through the walls. Especially when pressure is low, the pressure difference between the inside of the chamber and the ambient environment outside the vacuum chamber makes permeation more serious.
Cryogenic pumps are one type of vacuum device that can be used to attempt to achieve ultra-high vacuum conditions by removing gases from a sealed vacuum chamber at low temperature. Cryogenic pumps trap particles by condensing them on a cold surface.
The description herein is made with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to facilitate understanding. It will be appreciated that the details of the figures are not intended to limit the disclosure, but rather are non-limiting embodiments. For example, it may be evident, however, to one of ordinary skill in the art, that one or more aspects described herein may be practiced with a lesser degree of these specific details. In other instances, known structures and devices are shown in block diagram form to facilitate understanding.
In general, the present disclosure is related to an optimized cryogenic pump in order to achieve ultra high vacuum level and longer regeneration cycles. More particularly, the present disclosure is about introducing a nanostructure material with good absorption characteristics to attain more absorption of multiple particles. Further, in some embodiments, the nanostructure material can be part of adsorbents, in some alternative embodiments, the nanostructure material can be mixed with a fixed glue layer so that its large thermal conductivity would help to lower working temperature and further improve condensation.
Some cryogenic pumps have multiple stages at various low temperatures. For example,
The cryogenic pump 100 can be utilized in fields that require a high vacuum level. For example, in semiconductor industry, the cryogenic pump 100 can be utilized in systems such as for Physical Vapor Deposition (PVD), Molecular Beam Epitaxy (MBE) or implanter chambers. The cryogenic pump 100 can also be used in conjunction with a mechanical pump, which may be referred to in some instances as a roughing pump. The roughing pump and cryogenic pump can collectively establish a high vacuum or ultra high vacuum for semiconductor processing tools.
During operation, the first stage 118, second stage 119, and third stage 120 are cooled by compressed helium, liquid nitrogen, or a built-in cryo-cooler. Water molecules and other molecules with higher boiling points are condensed on the inlet array 120, while gas molecules with lower boiling points within the sealed vacuum chamber condense on a surface of the cryogenic blade array 112 and the adsorbent 116 when temperature is low enough. If the surface becomes saturated with condensate, few additional particles will be able to condense on the surface. To regenerate condensation ability of the cryogenic pump, regeneration is applied by heating the blade array 116 to a temperature allowed by the materials of the pump, to thereby outgas the condensed particles and allow condensation to restart. Time needed for such a regeneration cycle is called cryo lifetime.
To provider better condensation and regeneration ability, some embodiments of the present disclosure utilize nanostructures on the surfaces of the blade array 112. For example, in some embodiments, single walled carbon nanotubes or multi-walled carbon nanotubes are formed on the surfaces of the blade array to improve condensation and regeneration. These carbon nanotubes provide high activation energy for adsorption and de-sorption and high thermal conductivity, which fosters efficient condensation and regeneration. In some embodiments, the nanostructures can be formed on blades of only the third stage 120 to help achieve ultra-low vacuum, but in other embodiments the nanostructures can be formed on blades of the first and/or second stages 118, 119 as well.
To bond these nanostructures to surfaces of the blade array 112, a fixed glue layer is applied on the cryogenic blade array to fix the adsorbent layer 116, which absorbs gas molecules. A nanostructure material is then mixed with either the fixed glue layer or the adsorbent layer to improve absorption and extend cryo lifetime. In some embodiments, the adsorbent layer includes porous activated charcoal. Activation energy for adsorption and desorption of gases with the nanostructure material is lower than the activation energy with activated charcoal material alone. The nanostructure material saturates first before the activated charcoal material starts absorbing particles. Further, the nanostructure material provides desorption at lower temperature than the activated charcoal material which makes it quicker and easier to get complete desorption.
Defects of the nanostructure material can occur in the form of atomic vacancies, disordering, or impurities. The defects can be of pentagons and hexagons for carbon nanotube. There are also some carbon islands consisting of carbon nanotube clusters. These defects and carbon islands act as bonding sites to enhance adsorption of particles in the cryogenic pumps. These particles as an example include H2O, O2, CO2, H2, N2, or He. Presence of the defects helps in forming bonds with molecules through chemisorption, and helps in forming bonds with atomic particles through physisorption, both of which help to achieve lower vacuum levels. Because carbon nanotubes are an allotropic form of graphite, in some embodiments, the carbon nanotubes can have a high defect density, for example Id/Ig >0.2, wherein Id represents an intensity of crystallographic carbon nanotube defects and Ig represents the intensity of crystallographic graphite when the nanostructure material is analyzed using Raman spectroscopy. Thus, Id/Ig represents an amount of defects present in the carbon nanotube material. The inventors have appreciated that higher defect densities improve absorption for cryogenic pumps, thereby promoting lower vacuum levels.
In some instances, it is advantageous to have adsorbent layer 206 arranged on a lower surface of the blade 212 with the glue layer 202 arranged between the blade and adsorbent layer 206. This is because when the glue layer 202 and adsorbent layer 206 are on the lower blade surface 212, the condensation of molecules tends to leave the pores in the adsorbent layer 206 open. In contrast, if the adsorbent layer 206 is on the top side of the blade 212, pores in the adsorbent layer 206 can become more easily blocked by condensation of other gases, and the adsorbent layer 206 is less able to trap gases like H2, He. Nonetheless, in general, the adsorbent layer 206 could be arranged on the top surface or bottom surface of the blade 212, and/or on both the top and bottom surfaces of the blade, depending on the precise implementation.
a) shows an exemplary structural representation of the activated charcoal material and
Thus, it will be appreciated that some embodiments relate to a cryogenic pumping system comprising a canister having a flange to be coupled to a vacuum chamber. A cryogenic blade array is arranged within the canister. A fixed glue layer is disposed on a blade of the cryogenic blade array, and an adsorbent material is disposed on the fixed glue layer. The adsorbent material or the fixed glue layer includes a carbon nanotube material.
Other embodiments relate to a method of achieving ultra high vacuum levels for cryogenic pumps. In this method, a fixed glue layer is applied on a blade of a cryogenic blade array, and a nanostructure material is applied inside pores of an active charcoal material to form an adsorbent material. The adsorbent material is then applied on the fixed glue layer.
Still other embodiments relate to a multi-stage cryogenic pumping system. This cryogenic pumping system includes a canister having a flange to be coupled to a vacuum chamber. A first stage within the canister is in fluid communication with the vacuum chamber, and includes an inlet array to condense gases having boiling points within a first temperature range. A second stage within the canister is also in fluid communication with the vacuum chamber, but is fluidly downstream of the first stage relative to the vacuum chamber. The second stage includes a cold header to cool a cryogenic blade array in the second stage. The cryogenic blade array includes a carbon nanotube material thereon to trap gases having boiling points within a second temperature range, which is less than the first temperature range.
It will be appreciated that equivalent alterations and/or modifications may occur to those skilled in the art based upon a reading and/or understanding of the specification and annexed drawings. The disclosure herein includes all such modifications and alterations and is generally not intended to be limited thereby. For example, although the figures provided herein, are illustrated and described to have a particular working temperature, it will be appreciated that alternative temperatures may be utilized as will be appreciated by one of ordinary skill in the art.
In addition, while a particular feature or aspect may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features and/or aspects of other implementations as may be desired. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, and/or variants thereof are used herein, such terms are intended to be inclusive in meaning—like “comprising”. Also, “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features, layers and/or elements depicted herein are illustrated with particular dimensions and/or orientations relative to one another for purposes of simplicity and ease of understanding, and that the actual dimensions and/or orientations may differ substantially from that illustrated herein.
