WO2013068471A1

WO2013068471A1 - Method and apparatus for ablating a dielectric from a semiconductor substrate

Info

Publication number: WO2013068471A1
Application number: PCT/EP2012/072151
Authority: WO
Inventors: Sean Erik Foss; Jostein THORSTENSEN
Original assignee: Institutt For Energiteknikk
Priority date: 2011-11-09
Filing date: 2012-11-08
Publication date: 2013-05-16

Abstract

Method for ablating a dielectric (12) from a surface of a semiconductor substrate (10), which comprises the step of ablating the dielectric (12) using a pulsed laser (14) with a wavelength in the mid-to far IR having a pulse duration of less than 100ns, and a wavelength selected so that the substrate (10) will be substantially transparent and the dielectric will be substantially absorbing to radiation of that wavelength, whereby the majority of the laser energy will be absorbed by the dielectric (12).

Description

METHOD AND APPARATUS FOR ABLATING A DIELECTRIC

FROM A SEMICONDUCTOR SUBSTRATE

TECHNICAL FIELD

The present invention concerns a method for ablating, i.e. removing and/or destroying especially by cutting, abrading, or evaporating, a dielectric, such as an oxide layer, from a surface of a semiconductor substrate, i.e. a semiconductor substrate or wafer. The present invention also concerns an apparatus adapted to ablate a dielectric from a surface of a semiconductor substrate. The present invention further concerns the use of such a method and apparatus particularly, but not exclusively for ablating a dielectric from a surface of a semiconductor substrate intended for a photovoltaic application, such as a solar cell application. The present invention also concerns a semiconductor substrate from which a dielectric has been ablated and a device comprising at least one such semiconductor substrate.

BACKGROUND OF THE INVENTION

A wafer is a thin slice of highly pure, nearly defect-free single or multi-crystalline semiconductor material, such as a silicon crystal, which is used in the fabrication of integrated circuits and other microdevices. The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many micro-fabrication process steps such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning. Individual microcircuits can then be separated (by dicing) and packaged. Several types of solar cell are also made from such wafers. A solar cell is usually made from the entire wafer. A solar cell may also be made from a thin film of amorphous semiconductor material, such as a-Si or other amorphous semiconductor materials.

Wafers are susceptible to react with oxygen or moisture in the air, whereby dielectric layers, such as oxide layers, which deteriorate the characteristics of the device in which the wafer is used, are formed on the surface of the wafer by spontaneous oxidation. For example, if such a natural oxide layer is formed on the surface of contacts, contact resistance increases. In some applications, such as in solar cell applications, dielectric layers are deposited on the wafer/substrate surface, by means of plasma enhanced chemical vapor deposition (PECVD) or other methods, in order to enhance electrical, optical and/or other properties of the device in which the wafer/substrate will be used. Lasers, such as solid state lasers operating at near infrared (IR), visible or ultraviolet (UV) wavelengths, have been used to ablate such dielectric layers from the surface of wafers. However, these lasers tend to damage the electronic quality (and thereby reduce the electronic lifetime) of the wafer material since dielectrics layers are usually more transparent than the wafer material to the near-UV, visible and near IR radiation to which they are subjected during their ablation. As a result, the laser energy is deposited in the wafer material rather than being absorbed directly in the dielectric layer. The ablation process is therefore an indirect process since it is caused by the delamination and lift-off of the dielectric layer caused by increased vapor pressure from the heated wafer material. As this process relies on energy deposition in the wafer material, the dielectric layer ablation process cannot consequently be truly damage free, and will therefore deteriorate the quality of the bulk wafer material. An attempt to overcome this problem is presented in WO2010122028, where an optical absorber layer consisting of amorphous silicon is deposited between the substrate and the dielectric layer. Using this procedure, the laser light is deposited into the amorphous silicon, thereby protecting the silicon substrate while removing the dielectric. However, it may not always be desirable to insert an amorphous silicon buffer layer, due to design issues. (Concerns may be loss of efficiency due to absorption of sunlight in the amorphous silicon layer, incompatibility of the amorphous silicon layer with subsequent process steps etc.) As a result of this, the above mentioned method cannot always be applied.

WO20081 19949 describes a method where an absorbing film is removed from a transparent substrate, and states that "any wavelength laser from vacuum UV to far IR could be used so long as the thin film is at least partially absorbing and the substrate is at least partially transmissive." While this description may be suitable for some classes of materials, such as insulators (such as glass, which is the main focus of that document), the description is not suitable for semiconductors. Semiconductors will generally absorb strongly at photon energies above the band-gap energy of the semiconductor, while it is transparent for lower photon energies, such as e.g. light in the mid- and far IR. However, if a semiconductor is heated, the thermal energy will increase the number density of conduction band charge carriers. These charge carriers are interacting strongly with light in the IR through free-carrier absorption. In other words, a heated semiconductor will no longer be transparent for photon energies below the band-gap energy, even though it was transparent when cold. This behavior is characteristic for semiconductors. It is possible, if following the teaching of document WO20081 19949 to find a wavelength range where a semiconductor substrate (such as a silicon wafer) is transparent (when cold), and the dielectric is absorbing, namely a range in the mid- to far IR. However, as a result of heat transfer from the dielectric to the substrate, one would find that the substrate will start to absorb during laser processing, and that substrate damage would occur.

Patent DE19715702 describes the removal of a film from a substrate by choosing a wavelength such that the ablation threshold of the film is lower than that of the substrate. No attention is given to heat transfer, particularly for the case of semiconductor substrates. While the ablation threshold of a cold semiconductor at a given wavelength (photon energy below the band-gap) may be extremely high, and the ablation threshold of a dielectric at the same wavelength may be much lower, the ablation threshold of the substrate when covered with a dielectric layer will be reduced due to heat transfer from the laser heated dielectric film.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved method for ablating a dielectric from a surface of a semiconductor substrate.

This object is achieved by a method comprising the step of ablating the dielectric using a pulsed laser with a wavelength in the mid- to far IR (i.e. with a wavelength of 4-25 μηη or more) having a pulse duration of less than 100ns, and a wavelength selected so that the semiconductor substrate will be substantially transparent and the dielectric will be substantially absorbing to radiation of that wavelength, whereby the majority of the laser energy will be absorbed by the dielectric. The pulse duration is so short that significant heat transfer from the dielectric to the semiconductor substrate is avoided, whereby the semiconductor substrate remains cold and substantially transparent during laser processing. Such a method will ensure that the semiconductor substrate remains transparent during the laser processing, and only in this way can the dielectric be removed without semiconductor substrate damage and without the need for an optical absorber layer, or any intermediate layer between the semiconductor substrate and the dielectric. The expression "without semiconductor substrate damage" as used in this document is intended to mean that sufficient energy is not absorbed in the semiconductor substrate, during the method of ablating a dielectric therefrom, to melt the semiconductor substrate, or to melt the bulk of the semiconductor substrate to an extent that would degrade the performance of a device in which the semiconductor was subsequently used. Melting may be detected using a Transmission Electron Microscope (TEM) or an optical microscope. If the bulk of a semiconductor substrate melts during the ablation of a dielectric therefrom, or if substantial surface damage occurs, this can adversely affect the physical properties of the semiconductor substrate. For example, bulk and/or surface damage caused by the absorption of energy in the semiconductor substrate can result in an undesired reduction in the lifetime of minority charge carriers, which can degrade the performance of a device, such as a solar cell, in which the semiconductor substrate is subsequently used.

The expression "a wavelength selected so that the semiconductor substrate will be substantially transparent" is intended to mean that the wavelength is selected so that the semiconductor substrate will have a low attenuation coefficient/absorption coefficient to electromagnetic radiation for that wavelength, i.e. the intensity of laser energy will not be substantially reduced as it passes through the semiconductor substrate. The semiconductor substrate will therefore remain virtually unaffected by the laser energy. The expression "a wavelength selected so that the dielectric will be substantially absorbing" is intended to mean that the wavelength is selected so that the dielectric will have a high attenuation coefficient/absorption coefficient to electromagnetic radiation for that wavelength, i.e. the intensity of laser energy will be substantially reduced as it passes through the dielectric. The wavelength of the laser should be selected so that the semiconductor substrate at least has a lower attenuation/absorption coefficient than the dielectric, preferably at least 10 times lower, more preferably at least 10² times lower or even lower.

Such a method will result in the ablation of dielectric without damaging the underlying semiconductor substrate material since selection of an adequate wavelength will ensure that the laser energy is absorbed in the dielectric and not in the underlying semiconductor substrate material. By using a pulse duration of less than 100ns, ablation of the dielectric can be achieved almost without heat transfer to the semiconductor substrate and consequently the semiconductor substrate will remain transparent during laser processing, ensuring that the semiconductor substrate will not be damaged by absorption of laser energy. The ablation threshold of a semiconductor substrate is not namely strictly given by choosing the correct laser wavelength, but is also strongly affected by the characteristics of the laser, such as the laser pulse duration, and the thermal conduction properties of the materials in question.

Any process which intends to use the transparency that a semiconductor shows in the mid- to far IR and which simultaneously uses the absorption of a dielectric or thin film in the same wavelength range must be concerned about heat transfer. When a semiconductor substrate and thin film stack is irradiated with a laser, the absorption in the thin film will cause a temperature rise in the thin film. This thermal energy will then be transferred to the semiconductor substrate by heat conduction. Given enough time, the semiconductor substrate will be heated to the point where it starts to absorb strongly, whereby the desired transparency of the semiconductor substrate is destroyed and semiconductor substrate damage through laser energy deposition occurs.

Fig 6. shows the simulated temperature at the silicon surface in the case of ablation of silicon oxide from silicon. It can be seen that the surface of the silicon reaches melting temperature using a pulse duration of around 200 ns, while using shorter pulses reduces heat transfer from the dielectric, thereby reducing the temperature reached in the semiconductor substrate. While the absolute values in these simulations are somewhat uncertain, the trend is valid. Using shorter pulses will result in a cooler semiconductor substrate, at some point leading to ablation of the dielectric without semiconductor substrate damage. It should be noted that the terms "attenuation coefficient" and "absorption coefficient" are generally used interchangeably. However, in certain situations they are distinguished, as follows. When a narrow (collimated) beam of light passes through a substance, the beam will lose intensity due to two processes: the light can be absorbed by the substance, or the light can be scattered by the substance (i.e., the photons can change direction). Just looking at the narrow beam itself, the two processes cannot be distinguished. However, if a detector is set up to measure light leaving in different directions, or conversely using a non-narrow beam, one can measure how much of the lost intensity was scattered, and how much was absorbed. In this context, the "absorption coefficient" measures how quickly the beam would lose intensity due to the absorption alone, while "attenuation coefficient" measures the total loss of narrow-beam intensity, including scattering as well. The attenuation coefficient is always larger than the absorption coefficient, although they are equal in the idealized case of no scattering.

According to an embodiment of the invention the laser is a Q-switched laser, whereby light pulses with extremely high (gigawatt) peak power may be produced, much higher than would be produced by the same laser if it were operating in a continuous wave mode.

According to another embodiment of the invention the laser is a C0₂ laser or an optical parametric oscillator (OPO).. Currently available C0₂ lasers produce a beam of infrared light with the principal wavelength bands centering around 9.2 to 10. 8 μηη.

According to a further embodiment of invention the semiconductor substrate comprises at least one of the following: Si, c-Si, mc-Si, a-Si, GaAs, Ge, GaP, InP.

According to an embodiment of the invention the dielectric comprises at least one of the following: Si0₂ , SiN_x, AIO_x , SiC, SiO_xN_y.

According to an embodiment of the invention the laser has a wavelength in the range 4-25 μηη, or 7-15 μηη or 9-1 1 μηη, depending on the absorbance of the semiconductor substrate and the dielectric film.

According to another embodiment of the invention the laser has a pulse duration of less than 100 ns, or 1 - 100 ns or 10 - 50 ns or 10-15 ns.

According to a further embodiment of invention the substrate is illuminated from the top side or the back side, simultaneously or alternately for example.

According to an embodiment of the invention the method is used to selectively ablate one or more dielectrics located on the surface of a substrate or from a plurality of dielectrics, such as a stack of dielectric layers, or different layers of material on the surface of the substrate.

The present invention also concerns the use of a method according to any of the embodiments of the invention for ablating a dielectric from a substrate, such as a semiconductor or wafer, intended for a photovoltaic application, such as a solar cell application.

A solar cell is also called photovoltaic cell or photoelectric cell and is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect. Assemblies of solar cells can be used to make solar modules/solar panels that are used to capture energy from sunlight. It should however be noted that the method according to the present invention is not limited to an application in which the light source is sunlight, but it may be used for the ablating a dielectric from a substrate intended for any photovoltaic application. For example, the method according to the present invention may be used for ablating a dielectric from a substrate intended for a sensor application, to detect and/or measure the intensity of light or other electromagnetic radiation.

The present invention also concerns apparatus adapted to ablate a dielectric from a surface of a semiconductor substrate comprising the features recited in the independent apparatus claim. Embodiments of the apparatus are recited in the dependent apparatus claims. The present invention also concerns a semiconductor substrate from which a dielectric has been ablated comprising the features recited in the independent semiconductor substrate claim, and a device comprising at least one such semiconductor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

Figure 1 shows the absorption coefficient of Si0₂ and Si in the UV to near IR region

(Source: Handbook of Optical Constants of Solids, by Edward D. Palik, Academic Press Inc., 1985), Figure 2 shows the absorption coefficient of Si0₂ ,SiC, AIOx, Si and SiNx in the IR region (Source: Handbook of Optical Constants of Solids, by Edward D. Palik, Academic Press Inc., 1985) (authors' measurements),

Figure 3 shows a Fourier-Transform Infrared Spectroscopy measurement of transmission through SiNx film on Si (Source: Wright et al., "Plasma- enhanced chemical vapor-deposited silicon nitride films; The effect of annealing on optical properties and etch rates", Solar Energy Materials & Solar Cells (2008)), Figure 4 shows examples of how a dielectric can be ablated from a substrate according to embodiments of the invention,

Figure 5 simulated temperature profiles in Si0₂ and Si from a 120 ns laser

Figure 6 shows the simulated maximum temperature of the semiconductor substrate when ablating a dielectric, as function of pulse duration. In this simulation, silicon and silicon dioxide are chosen as the semiconductor and dielectric. It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows the attenuation coefficient of Si and Si0₂. A can be seen, the attenuation coefficient for Si is much higher than the attenuation coefficient for Si0₂ for wavelengths of over 150 nm. This means that a Si substrate having a Si0₂ dielectric layer on the surface thereof were to be irradiated with laser having a wavelength greater than 150 nm, the majority of the laser energy would be deposited in the Si substrate rather than in the Si0₂ dielectric layer. The Si0₂ would thereby be ablated indirectly, because of delamination and lift-off of the Si0₂ dielectric layer caused by increased vapor pressure from the heated Si substrate. This indirect ablation process would however damage the Si substrate and thereby deteriorate the quality of the bulk silicon.

According to an embodiment of the invention a Q-switched, 9.3 μηη C0₂ laser may be used for the ablation of Si0₂ , SiN_x, AIO_x and/or SiC from a Si substrate. The absorption band of a material is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite particular transitions in a substance. The mechanism for optical absorption in the infrared spectral range is dominated by absorption due to vibrational transitions in the material. These vibrational transitions in different materials are excited by different wavelengths depending on the inherent physical and chemical properties of the material. When the material reaches higher temperatures, contributions from free-carrier absorption will also be present.

For silicon, there are no strong absorption bands in the wavelength range of 4 - 25 μηη, meaning that silicon is transparent at these wavelengths. As a result of this, the laser energy will be deposited in the dielectric, rather than in the silicon, which would be the case if using a solid state laser at a wavelength of 1064, 532 or 355 nm. As the silicon is only transparent to the C0₂ laser when cold, due to free carrier absorption which is rather strong at such long wavelengths, a short pulsed laser (having a pulse duration of less than 100ns) is necessary. Other substrates showing a high degree of transparency in the 4 - 25 μηη wavelength region are a-Si, GaAs and Ge.

Furthermore, a wavelength of 9.3 μηη is close to the absorption peak of Si0₂, where the ratio between the absorption coefficients of Si0₂ and Si might be as high as 10⁴ (see Figure 2). This should give room for a wide process window. Figure 2 also shows the absorption spectra for SiC and AIO_x where absorption peaks in the infrared region are observed.

Figure 3 shows the infrared transmission spectrum for a PECVD-deposited SiN_x film deposited on a silicon substrate. Absorption peaks between 600 and 1200 cm^"1 (corresponding to the wavelength range of 8.3 to 16.7 μηη) can be seen meaning that significant absorption will be observed in the range of 8.3 - 16.7 μηη.

According to an embodiment of the method according to the present invention a substrate 10, such as a silicon wafer, at least partly coated with a dielectric top layer 12 may be illuminated with light from a pulsed laser 14 having a pulse duration of less than 100 ns, such as 10-15 ns for example, and a wavelength selected so that the substrate 10 will be substantially transparent and the dielectric 12 will be substantially absorbing to radiation of that wavelength, whereby the majority of the laser energy 14 will be absorbed by the dielectric and the dielectric 12 will consequently be photo-thermally ablated. A scanning laser or laser deflection means may be used to scan at least part of the surface of the substrate 10 whereby laser energy 14 may be directed to a selected part or parts of the surface of the substrate 10. However, since the substrate is substantially transparent to laser energy 14 of that wavelength (silicon is for example transparent to laser energy 14 having a wavelength of 9.3 μηη) it is also possible to illuminate the substrate 10 from the back side (as shown in figure 4b), or to simultaneously remove dielectrics 12 from both sides of the substrate 10 (as shown in figure 4c).

It is also possible to envision all of the above-mentioned methods with stacks of dielectrics. According to an embodiment of the invention the method comprises the step of removing a stack of dielectric and metal by irradiating through the substrate, in a similar manner to that shown in Figure 4b.

According to another embodiment of the invention a substrate, such as a silicon wafer, is covered by two or more layers of different dielectrics, a so-called stack. The method may be applied to the removal of all of the layers in the stack simultaneously. Another possibility is that, by proper choice of dielectrics, only the dielectric closest to the surface (air), or a certain number of dielectrics closest to the surface may be removed.

It should be noted that a laser 14 need not necessarily illuminate a substrate 10 at right angles to the surface of the substrate 10. It should also be noted that a substrate 10 need not necessarily be of uniform thickness or have a flat surface. Furthermore, a dielectric 12 need not necessarily cover the whole of a surface of a substrate 10 but may cover only a part thereof and/or be of uniform or non-uniform thickness.

Figure 5 shows simulated temperature profiles in Si0₂ and Si from a 120 ns Gaussian laser pulse. The simulations show the possibility of reaching the vaporization temperature of Si0₂ (2500K) without reaching the melting temperature of silicon. These simulations were performed with constant values for heat conductivity and absorption coefficient in Si0₂, while the values in silicon were temperature dependent. A realistic value for the interface thermal resistance was also included, along with melting enthalpy for Si0₂ taken to be the same as that for c-Si0₂ as detailed knowledge of the deposited Si0₂ film would be required to get better values.

Further modifications of the invention within the scope of the claims would be apparent to a skilled person.

Claims

1 . Method for ablating a dielectric (12) from a surface of a semiconductor substrate (10), characterized in that it comprises the step of ablating the dielectric (12) using a pulsed laser (14) with a wavelength in the mid- to far IR having a pulse duration of less than 100ns, and a wavelength selected so that the semiconductor substrate (10) will be substantially transparent and the dielectric will be substantially absorbing to radiation of that wavelength, whereby the majority of the laser energy will be absorbed by the dielectric (12).

2. Method according to claim 1 , characterized in that said laser (14) is a Q-switched laser.

3. Method according to claim 1 or 2, characterized in that said laser (14) is a C0₂ laser or an optical parametric oscillator (OPO).

4. Method according to any of the preceding claims, characterized in that said substrate (10) comprises at least one of the following: Si, c-Si, mc-Si, a-Si, GaAs, Ge, GaP, InP.

5. Method according to any of the preceding claims, characterized in that said dielectric (12) comprises at least one of the following: Si0₂ , SiN_x, AIO_x , SiC, SiO_xN_y.

6. Method according to any of the preceding claims, characterized in that said laser (14) has a wavelength in the range 4-25 μηη.

7. Method according to any of the preceding claims, characterized in that said laser (14) has a pulse duration of 1 - 100 ns.

8. Method according to any of the preceding claims, characterized in that said substrate (10) is illuminated from the top side or the back side.

9. Use of a method according to any of the preceding claims for ablating a dielectric (12) from a substrate (10) intended for a photovoltaic application, such as a solar cell application.

10. Apparatus adapted to ablate a dielectric (12) from a surface of a semiconductor substrate (10), characterized in that it comprises a pulsed laser (14) adapted to have a wavelength in the mid- to far IR and a pulse duration of less than 100ns, and a wavelength selected so that the semiconductor substrate (10) will be substantially

5 transparent and the dielectric will be substantially absorbing to radiation of that wavelength, whereby the majority of the laser energy will be absorbed by the dielectric (12) and the dielectric (12) will be removed without semiconductor substrate damage.

1 1 . Apparatus according to claim 10, characterized in that said laser (14) is a Q- 10 switched laser.

12. Apparatus according to claim 10 or 1 1 , characterized in that said laser (14) is a C0₂ laser or an optical parametric oscillator (OPO).

15 13. Apparatus according to any of claims 10-12, characterized in that said substrate (10) comprises at least one of the following: Si, c-Si, mc-Si, a-Si, GaAs, Ge, GaP, InP.

14. Apparatus according to any of claims 10-13, characterized in that said dielectric (12) comprises at least one of the following: Si0₂ , SiN_x, AIO_x , SiC, SiO_xN_y.

20

15. Apparatus according to any of claims 10-14, characterized in that said laser (14) has a wavelength in the range 4-25 μηη.

16. Apparatus according to any of claims 10-15, characterized in that said laser (14) 25 has a pulse duration of 1 - 100 ns.

17. Apparatus according to any of claims 10-16, characterized in that said laser (14) is configured to illuminate said substrate (10) from the top side or the back side.

30 18. Apparatus according to any of claims 10-17, characterized in that said substrate (10) is illuminated from the top side and the back side.

19. Use of the apparatus according to any of claims 10-18 for ablating a dielectric (12) from a substrate (10) intended for a photovoltaic application, such as a solar cell 35 application.

20. Semiconductor substrate (10) from which a dielectric (12) has been ablated, characterized in that said dielectric (12) has been ablated using a method according to any of claims 1 -8 or an apparatus according to any of claims 10-18.

21 . Device, characterized in that it comprises at least one semiconductor substrate (10) according to claim 20.