Patent Application
20070022623
The manufacture of modem electronic circuits begins with a semiconductor material (e.g., silicon). Because semiconductor materials do not conduct electricity well, the material is usually doped with a secondary material or impurity (e.g., boron) to create tiny areas that function as an electrical conductor. An insulator layer may also be formed on the semiconductor materials through the oxidation of silicon. To create an electronic circuit, an ingot of the semiconductor material is first sliced into thin wafers. The wafers are then sent through a series of processing steps, during which patterns of chemicals are placed on the wafers, creating the aforementioned transistors, conductors, and insulators.
Multiple independent electrical components can be patterned on a single wafer. The patterned wafer is typically diced into dies—or more informally, chips. Dicing enables a manufacturer to discard only the dies that contain flaws, rather than the whole wafer. In general, this process is quantified as the yield of a process, which is defined as the percentage of operational dies from the total number of dies on the wafer. Operational dies (i.e., the ones that do not contain flaws) can be connected to the input/output pins of a package using a process called bonding. A package refers to material added to an operational die to allow it to be handled without damage.
Many sequential processing steps are performed to create an operational die. Typically, photo-sensitive resistance (“photoresist”) patterns are first masked in minuscule detail (e.g., micrometer, nanometer, etc.) onto a wafer's surface. The wafer is then exposed to short-wavelength ultraviolet light. Next, the unexposed areas are etched away and cleaned. Hot chemical vapors are then deposited onto desired zones of the wafer and baked in high heat so the vapors can permeate into the desired zones. In traditional wafer fabrication, the aforementioned processes are often repeated hundreds of times, depending on the complexity of the desired circuits and connections.
Surface drying is a critical process in wafer fabrication and other technology industries. Surface drying is typically used to remove watermarks on the wafer's surface as watermarks that decrease the number of operational dies created from a wafer. Traditionally, watermarks are removed by drying a wafer's surfaces in a number of ways, including, but not limited to, spin drying, isopropyl alcohol (“IPA”) vapor drying, or Marangoni drying. Currently, most wafer foundries use IPA to dry a wafer's surface. Typically, a wet wafer is exposed to the hot vapor of IPA to displace water from the wafer's surface. The IPA is then evaporated by cooling the wafer's surface. Similarly, IPA also used in drying processes for flat panel displays (e.g., plasma displays, liquid crystal displays, etc.) and hard disks.
Using IPA to dry wafer surfaces, flat panels, and hard disks creates a whole host of problems. For instance, IPA drying has a number of safety, health, and environmental concerns. More specifically, large amounts of IPA are consumed and released into the environment, which poses serious environmental concern. IPA is also hazardous to the human body, creating a serious health concern for the operators in wafer foundries. Additionally, IPA is extremely flammable, thus creating a high risk of fire inside of today's clean rooms. Therefore, it would be advantageous to remove watermarks from semiconductor wafers while avoiding the pitfalls associated with IPA and other surface drying techniques.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The description provided herein generally relates to using a laser to dry the surface of a semiconductor wafer. The excimer laser is configured to produce a laser beam that is directed to a liquid layer on the wafer. The fluence (power density) of the laser beam can be varied by a power attenuator. The number of pulses delivered to the surface is controlled by a shutter, which can block the laser beam or allow it to pass. In one embodiment, the power attenuator and shutter are controlled by various electronics and a computing device. The laser beam may be directed onto the surface through a series of mirrors. A measurement device monitors the liquids on the wafer to determine whether the liquid has been completely evaporated.
The present invention is described in detail below with reference to the attached drawing figures, wherein:
The subject matter described herein is presented with specificity to meet statutory requirements. However, the description herein is not intended to limit the scope of this patent. Rather, it is contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, although the terms “step” and/or “block” may be used herein to connote different elements of methods employed, the terms should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.
As previously mentioned, numerous problems stem from using IPA to dry the surfaces of semiconductor wafers, panel displays, and hard disks. In general, the present invention relates to irradiating the surface of a semiconductor wafer with a laser to evaporate water or other liquids. As described herein, a semiconductor wafer is a thin slice of semiconducting material upon which individual dies, or electronic chips, are constructed by doping—for example, diffusion, ion implantation, etching—with materials containing donor impurity atoms. Examples of electronic circuits that may be imprinted on a silicon wafer include, without limitation, microcontrollers, microprocessors, transistors, integrated circuits, etc.
While semiconductor wafers might be described herein as silicon wafers, embodiments should not be limited to silicon. Rather, wafers of materials other than silicon may also be considered semiconductor wafers. Examples include, without limitation, germanium, gallium arsenide, and other such materials.
As those skilled in the art will appreciate, numerous processes exist for fabricating semiconductor wafers. In general, most are initiated by slicing an ingot of silicon into wafers. Desired dies can then be fabricated onto the sliced wafers using various well-known methods. In general, most fabrication methods consist of a series of steps to deposit special material layers on silicon wafers in precise amounts and patterns. Typically, the steps fall into four categories: deposition, removal, patterning, and modification of electrical properties.
The deposition step incorporates any process that grows, coats, or otherwise transfers a material onto the wafer. One of skill in the art will appreciate the use of traditional deposition technologies, such as physical vapor deposition (“PVD”), chemical vapor deposition (“CVD”), electrochemical deposition (“ECD”), molecular beam epitaxy (“MBE”), and atomic layer deposition (“ALD”) among others.
The removal steps include steps to remove material from the wafer either in bulk or selective form and consist primarily of etch processes, both wet etching and dry etching, such as reactive ion etch (“RIE”). One of ordinary skill in the art will further appreciate that chemical-mechanical planarization (“CMP”) may also be used as a removal process between levels.
The patterning steps include the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as lithography. In conventional lithography, the silicon wafer is coated with a chemical called a “photoresist.” A photoresist may be any light-sensitive material used to form a patterned coating on a wafer's surface. Those of skill in the art will understand that numerous materials may be used as a either a positive or negative resist. A resist is a thin layer of material—typically a viscous solution—used to transfer a circuit pattern to the semiconductor substrate which it is deposited upon. Furthermore, the photoresist may be exposed by a stepper. A stepper is a machine that focuses, aligns, and moves a mask, thus exposing select portions of the wafer to short wavelength light. In one embodiment, the unexposed regions are washed away by a developer solution. Alternatively, IPA may be applied to a wafer to evaporate water. As previously mentioned, IPA has a number of drawbacks due to its flammability and danger to the human body. After etching or other processing, the remaining photoresist may be removed by plasma ashing.
The modification step includes modifying electrical properties consisting of doping sources and drains originally by diffusion furnaces and later by ion implantation. These doping processes may be followed by furnace anneal or, in advanced devices, by rapid thermal anneal (“RTA”), which serves to activate implanted dopants.
The following example describes the fabrication process for a simple Complementary-symmetry Metal-Oxide Semiconductor (“CMOS”) integrated circuit on a silicon wafer. Initially, a p-type epitaxial layer is grown on the silicon wafer through chemical vapor deposition. Next, a nitride layer is deposited over the epi-layer, leaving behind exposed areas on the epi-layer. The exposed areas are then masked again in specific patterns before being subjected to diffusion or ion implantation to receive dopants (such as boron or phosphorus) forming n-wells. Silicon diodes are then grown to form field oxides that isolate the n-wells from other parts of the circuit. Another masking/oxidation cycle then follows to grow gate oxide layers over the n-wells intended for p-channel metal oxide semiconductor (“MOS”) transistors later on. Another mask and diffusion/implant cycle may then follow to adjust threshold voltages on other parts of the epi, intended for n-channel transistors later on. Next, a polysilicon layer is deposited over the silicon wafer followed by a masking/etching cycle to remove unwanted polysilicon areas and define polysilicon gates over the gate oxide of the p-channel transistors. At the same time, openings for the source and drain drive-ins are made on the n-wells by etching away oxide at the right locations. Next, another round of mask/implant cycle follows, this time driving the dopants into new openings of the n-wells, forming the p-type sources and drains. Next, a mask/implant cycle follows to form the n-type sources and drains of the n-channel transistors in the p-type epi. The wafer is then covered with phospho-silica glass, which is then subjected to reactive ion etching in specific patterns to expose the contact areas for metallization. Aluminum is then be sputtered onto the wafer, after which it is subjected to reactive ion etching to create connections between the various components of the circuit. The wafer is then covered with glassivation as its top protective layer, after which a mask/etch process is used to remove the glass over bond pads. The above example is merely provided for illustrative purposes and should not be construed to limit embodiments to any particular fabrication process or step.
As illustrated in the following formula, a KrF laser absorbs energy from a source and causes krypton gas to react with fluorine gas, producing krypton fluoride, which is an unstable compound.
2Kr(g)+F2(g)electron energy→2KrF(g)
When the supplied energy is stopped, the compound will decompose and the excess chemical energy stored in the compound will release in the form of strongly synchronized radiation, as described in the following formula:
2KrF(g)→2Kr(g)+F2(g)+energy
The result is an excimer laser that radiates energy in an ultraviolet frequency of 248 nm. Other gases contemplated by
In one embodiment, a KrF laser, generally referenced as numeral 100 in
In one embodiment, the laser 204 includes the components described in reference to
Additionally, the laser beam 206 may be directed to a beam homogenizer (not shown for clarity). In general, a beam homogenizer smoothes out irregularities in a laser beam profile to create a more uniform profile. For example, the beam homogenizer may comprise multifaceted mirrors to reflect the laser beam 206 in different angles—thus creating a square beam with uniform power. Embodiments should not be limited to any particular beam homogenizer since beam homogenizers are well known to those of ordinary skill in the art.
During a laser-drying process, the fluence of the laser beam 206 is vital. Fluence by definition is the energy per pulse divided by surface area. More precisely, fluence is the energy density of one single pulse of the laser beam 206 per size (i.e., energy/size). If the fluence of the laser beam 206 is too strong, the beam may damage the wafer surface 202.
In operation, the laser beam 206 is emitted from the laser 204. Next, the laser beam 206 is then directed to the power attenuator 208. The poer attenuator 208 is optionally configurable to adjust the fluence of the laser beam 206 using methods well known in the art. For example, the power attenuator 208 may include a diffractive grating to deflect the light beam 206 into several diffractive orders, some of which may be blocked by a diaphragm. In this scenario, the resultant diffraction efficiency and fluence of the laser beam 206 depend on the grating parameters in the power attenuator 208. By varying the grating, the fluence can effectively be changed. In another example, the power attenuator 208 may include a grating wheel mounted in an encapsulating housing with entrance and exit tubes. The housing and tubes effectively block parasitic diffractive orders and absorb laser radiation. Other well known methods of attenuating the laser beam 206 are known by those of skill in the art and may be employed within the power attenuator 208.
The laser beam 206 is directed from the power attenuator 208 to the shutter 210. The shutter 210 is configured to control the number of pulses delivered onto the wafer surface 202. In effect, the shutter 210 acts as an on/off switch, either terminating the laser beam 206 or allowing it to pass.
In one embodiment, the power attenuator 208 and the shutter 210 are both controlled by various electronic components (illustrated in
In one embodiment, a configuration of mirrors 216 are positioned to direct the laser beam 206 onto the wafer surface 202.
Once the laser beam 206 irradiates the wafer surface 202, water that is resident on the wafer surface 202 is evaporated. Not only does the laser beam 206 evaporate the water molecules from the wafer surface, the laser beam 206 also breaks any bonding between the water molecules and the silicon of the wafer surface 202. Consequently, the water is completely evaporated from the wafer surface 202.
The measurement device 218 is configured, in one embodiment, to measure the film thickness of a liquid on the wafer surface 202. The measurement device 218 may include an ellipsometer to measure the refractive index and thickness of semi-transparent films. Typically, ellipsometers can measure water-film thicknesses down to 1 nm. Ellipsometers are well known to those of skill in the art and need not be discussed at length herein.
It may be desirable to measure the amount of water on the wafer surface 202 more precisely. Therefore, the measurement device 218 may also include, in an alternative embodiment, a Fourier transform infrared (“FTIR”) spectrometer. An FTIR spectrometer is generally more sensitive than an ellipsometer because the FTIR spectrometer measures vibrations of functional groups and highly polar bonds. FTIR spectrometers record the interaction of infrared radiation with experimental samples, measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorption. Determining such frequencies identifies the samples'chemical makeup, because chemical functional groups absorb light at specific frequencies. FTIR spectrometers are well known to those of skill in the art.
The measurement device 218 may also include an X-ray photoelectronic spectroscopy (“XPS”) to investigate chemical composition and bonding energies. In one embodiment, the XPS determines the composition of silicon as well as the content of Si, SiO, and SiO2 on the wafer surface 202. XPSs are well known to those of skill in the art.
The present invention has been described in relation to particular embodiments, which are intended in all respects to be illustrative rather than restrictive. Alternative embodiments will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope.
From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects set forth above, together with other advantages which are obvious and inherent to the system and method. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.
This application claims the benefit of U.S. Provisional Application No. 60/703,603 filed Jul. 29, 2006 under the same title.
| Number | Date | Country | |
|---|---|---|---|
| 60703603 | Jul 2005 | US |
