Patent Application
20080206897
1. Field of Invention
This disclosure generally relates to selective depth processing of semiconductor substrates with a focused light beam.
2. Related Art
Focused laser beams have found applications in drilling, scribing, and cutting of semiconductor wafers, such as silicon. Marking and scribing of non-semiconductor materials, such as printed circuit boards and product labels are additional common applications of focused laser beams. Micro-electromechanical systems (MEMS) devices are laser machined to provide channels, pockets, and through features (holes) with laser spot sizes down to 5 μm and positioning resolution of 1 μm. Channels and pockets allow the device to flex. All such processes rely on a significant rise in the temperature of the material in a region highly localized at the laser beam point of focus.
The foregoing applications, however, are all, to some degree, destructive, and relate generally to focused laser beams at power densities intended to ablate material. In silicon and related semiconductor and electronic materials, such applications are generally for mechanical results (e.g., dicing, drilling, marking, etc.).
Thus, there is a need to provide and control light beams to achieve processing effects for electronic and or optical device fabrication on semiconductor wafers. Furthermore, there is a need to control the depth at which such processing takes place.
Methods and systems of semiconductor material and device processing with focused light beams are disclosed. Specifically, in accordance with an embodiment of the disclosure, a method of processing semiconductor materials includes providing a light beam of a selected wavelength and a selected peak power. The laser beam is modulated to provide pulses of a discrete time pulse width. The laser beam is focused at the surface plane of the semiconductor material. The total energy in each laser pulse is controlled to a selected value. By controlling parameters of the light or laser beam, the semiconductor material can be heated or otherwise processed to or at selected depths. The laser beam is scanned over the surface of the semiconductor material in a programmed pattern. Device fabrication is accomplished by altering material electronic and/or optical properties and features at the surface of the semiconductor material.
Like reference symbols in the various drawings indicate like elements.
In
Four examples of light propagation conditions may be considered to illustrate the results of light propagation and processing effects in substrate 160. Case A illustrates the dependence of light beam energy density as a function of propagation depth into substrate 160 when substrate 160 is substantially transparent, i.e., there is substantially no light absorption. The dependence of light density 142 on depth is strictly determined by spatial dispersion of divergent beam 140 due to the focal properties of lens 120 and the index of refraction (being substantially real and positive, i.e., without absorption) of substrate 160, and all layers therein. As the substrate material is transparent and non-absorbing, there is substantially no thermal heating and no optical interaction between the beam and substrate 160 to cause any process effects to occur.
Case B illustrates the dependence of light beam energy density as a function of propagation depth into substrate 160 when the substrate material is highly absorptive. This may occur as a result of a combination of layers of the substrate having a complex index of refraction (i.e., having a real and an imaginary component) at the selected wavelength of light beam 110, such that the wavelength dependent index of refraction is complex, which may also occur for a wavelength that is shorter than for cases described below. Those of ordinary skill in the art will recognize that a larger imaginary component of index of refraction will result in a larger rate of absorption. In this case, the light energy is rapidly absorbed by the substrate in a relatively short depth of penetration. Therefore, light beam density 148 of divergent beam 140 decreases rapidly with penetration depth, and processing effects due to thermal heating resulting from the absorption will occur preferentially in a short range of penetration, substantially near the depth corresponding to the focal point 130.
Case C illustrates the dependence of light beam density 146 as a function of propagation depth into substrate 160 when the substrate material has medium absorption, as a result of wavelength selection, which may be a somewhat longer wavelength than in Case B. In this case, light beam density 146 decreases more gradually with penetration depth, and correspondingly penetrates deeper into substrate 160. Therefore, two effects may occur: (1) since absorption is somewhat less than in Case B, heating effects may occur more slowly, and therefore more processing time may be required; (2) since the light density decreases more slowly, the energy density remains relatively high to a greater depth, so that processing effects may occur deeper into substrate 160.
Case D illustrates the dependence of light beam density 144 as a function of propagation depth into substrate 160 when layers of substrate 160 have relatively low absorption, which may also occur at relatively longer wavelengths than in Cases B and C. In this case, light density 144 decreases more gradually and penetrates more deeply into substrate 160.
Because absorption effects are known to typically obey an exponentially decaying dependence with propagation distance, Cases B, C and D are shown with a rate of decreasing light density that is always greater than the decrease due purely to spatial dispersion of the beam due to focal properties in the absence of absorption.
It is well known to those of ordinary skill in the art that an optical system of a given aperture and with a longer focal length will have a larger diffraction limited spot size at the focal point than will an optical system of the same aperture and shorter focal length. This will limit the light beam power and energy density at the focal point to a lower density relative to shorter focal length systems. Thus, a shorter focal length system of the same aperture will have a higher focal point maximum beam power and energy density. In addition, shorter focal point optical systems will also have a more divergent beam, such that the range of depth may be more restricted at which thermally or optically induced processing effects may take place.
Beams 310a and 310b may each be provided by an incoherent light source of selected wavelength and sufficient intensity for a selected application, by lasers of selected intensity and wavelength, or a combination of incoherent light sources and lasers. A greater plurality than is shown in
If the aperture (e.g., diameter) of a light beam, particularly a collimated laser beam, is sufficiently small and the intensity is sufficient for the application, lens 320 may be optionally omitted.
Beams 310a and 310b may have the same wavelength or have different wavelengths. Additionally, beams 310a and 310b may have the same or different apertures (i.e., diameters), which may result in different diffraction limited spot sizes at focal point 330. Beams 310a and 310b may have the same or different total powers. Beams 310a and 310b may be delivered to the substrate by means of mechanical translation of the optical system over substrate 160, galvano-mirror direction of each beam over substrate 160, by translation/rotation of substrate 160 on a processing stage, or a combination of the above.
The range of wavelengths may be from approximately 200 nanometers (i.e., ultraviolet) to approximately 12 micrometers (i.e., long wavelength infrared). Light sources may be sufficiently intense incoherent sources or highly monochromatic lasers. As indicated above, focusing is optional, as the application may require. The optical power obtained from the light sources for selective depth processing may range from approximately 1 milliwatt to 100 kilowatts for continuous (CW) light sources. Alternatively, pulsed light sources may be used, where the per-pulse energy may range from approximately 1 microjoule to approximately 1 joule.
The various combinations of light source, wavelength, focal length and beam combining at or just below the substrate surface provides for a variety of possible applications. Exemplary applications may include local heating or selective depth heating for material processing such as defect engineering or annealing, curing, stress or strain engineering or annealing, local activation, and localized reactions. Multiple light beams of different wavelengths, power levels, focal point depth/location may provide multiple types of processing effects at different depths simultaneously. Note that although the light density is maximum at the desired focal point depth/location, processing can still occur at depths less than and greater than the focal point, but just at less power and over a wider area.
Because ion implantation causes damage to the crystal structure of the target which is often unwanted, ion implantation processing is often followed by a thermal annealing. This can be referred to as damage recovery. Furthermore, because this damage—referred to as end of range (EOR) damage—tends to occur over a range of depth determined by the residual kinetic energy of the implant ion as it slows, such that nuclear collision scattering increases, producing an imbedded layer at a depth below the substrate surface that is damaged or at least partially amorphous. Selective depth optical processing applied for thermal annealing may be a highly effective method of removing such defects. One or more light beams, such as two or more laser beams, may be focused to provide localized thermal annealing effectively at the site depths where such defects predominantly accumulate.
In another application, dopant diffusion may be selectively controlled both as to depth and through controlled spatial scanning of the light beam or beams over the substrate area. In another application, localized activation or chemical reactions may be induced, using the same techniques.
Yet another application may use light sources of the same or different wavelengths, where nonlinear optical effects in the substrate material or layers become significant at sufficiently high light beam intensities. Under these conditions, multiple photon mixing may occur, where two incident photons combine by interacting with the substrate lattice and a photon of sum and/or difference energy is produced, thereby providing photons with depth penetration and/or absorption characteristics not available from the light sources directly.
Also, only those claims which use the word “means” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. Accordingly, other embodiments are within the scope of the following claims.
