BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1C are electron micrographs of kerfs cut through finished wafers using a conventional mechanical saw.
FIGS. 2A and 2B are electron micrographs of kerfs scribed in finished wafers using lasers with wavelengths of approximately 1064 nm and 355 nm, respectively.
FIG. 3 is a side view schematic of an exemplary work piece that is scribed according to certain embodiments of the invention.
FIGS. 4A and 4B are side view schematics illustrating the work piece of FIG. 3 processed according to conventional laser scribing techniques.
FIGS. 5A and 5B are side view schematics illustrating the work piece of FIG. 3 scribed with a q-switched CO2 laser according to certain embodiments of the invention.
FIGS. 6A-6C are electron micrographs of kerfs scribed through passivation/encapsulation layers using a q-switched CO2 laser according to certain embodiments of the invention.
FIG. 7 is an electron micrograph of a kerf scribed through passivation/encapsulation layers using a q-switched CO2 laser and a Gaussian picosecond pulse laser beam according to an embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The ability of a material to absorb laser energy determines the depth to which that energy can perform ablation. Ablation depth is determined by the absorption depth of the material and the heat of vaporization of the material. Parameters such as wavelength, pulse width duration, pulse repetition frequency, and beam quality can be controlled to improve cutting speed and the quality of the cut surface or kerf. In one embodiment, one or more of these parameters are selected so as to increase energy absorption in outer passivation and/or encapsulation layers and reduce the amount of fluence (typically measured in J/cm2) required to ablate the passivation/encapsulation layers and/or additional layers(referred to herein as “ablation threshold.”) Thus, the amount of excessive energy deposited into the material is reduced or eliminated. Further, using a lower fluence reduces or eliminates recast oxide layers, heat affected zones, chipping, cracking, and debris. Thus, die break strength is increased and the amount of post-laser cleaning required is decreased.
In one embodiment, laser pulses having a wavelength in a range between approximately 9 μm and approximately 11 μm are used to scribe a finished semiconductor wafer. At these wavelengths, the passivation and encapsulation layers are configured to absorb a large portion of the pulse energy. Thus, the passivation and encapsulation layers are ablated before being cracked and blown off due to ablation of lower layers. Further, silicon substrates absorb very little pulse energy at these wavelengths. Thus, there is very little or no substrate heating that can cause cracking.
The laser pulses have short pulse widths in a range between approximately 130 nanoseconds and approximately 170 nanoseconds. In one embodiment, a q-switched CO2 laser is used to generate the laser pulses. An artisan will recognize that q-switching is a technique used to obtain energetic short pulses from a laser by modulating the quality factor of the laser cavity. Using the q-switched short pulse CO2 laser eliminates or significantly reduces chipping and cracking during wafer scribing and wafer dicing processes.
The short pulse widths are selected to provide higher peak energy than that of continuous wave (CW) pulses or long pulse widths. U.S. Pat. No. 5,656,186 to Mourou et al. teaches that the ablation threshold of a material is a function of laser pulse width. CW pulses or pulses with long pulse widths (e.g., in the millisecond range) generally require a higher ablation threshold as compared to that of shorter pulse widths. Shorter pulses increase peak power and reduce thermal conduction. Thus, scribing finished wafers using the short pulses is more efficient. The result is a faster scribing process.
For convenience, the term cutting may be used generically to include scribing (cutting that does not penetrate the full depth of a target work piece) and throughcutting, which includes slicing (often associated with wafer row separation) or dicing (often associated with part singulation from wafer rows). Slicing and dicing may be used interchangeably in the context of this invention.
Reference is now made to the figures in which like reference numerals refer to like elements. For clarity, the first digit of a reference numeral indicates the figure number in which the corresponding element is first used. In the following description, numerous specific details are provided for a thorough understanding of the embodiments of the invention. However, those skilled in the art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, or materials. Further, in some cases, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the invention. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
FIG. 3 is a side view schematic of an exemplary work piece 300 that is scribed according to certain embodiments of the invention. The work piece 300 includes a first layer 302, a second layer 304, a third layer 306, a fourth layer 308, a fifth layer 310, and a sixth layer 312 formed over a substrate 314. As an artisan will recognize, the layers 302, 304, 306, 308, 310, 312 may include interconnect layers separated by insulation layers, including low-k dielectrics, to form electronic circuitry. In this example, the top two layers 302, 304 form a passivation and encapsulation layer. The first layer 302 may include, for example, silicon dioxide (SiO2) and the second layer 304 may include a silicon-nitride (SiYNX). For example, the second layer 304 may include Si4N3. An artisan will recognize that other materials can be used to form passivation and/or encapsulation layers.
In this example, the third layer 306 comprises a metal (e.g., Cu or Al), the fourth layer 308 comprises a dielectric (e.g., SiN), the fifth layer 310 comprises a metal (e.g., Cu or Al), and the sixth layer 312 comprises a low-k dielectric. Low-k dielectric materials may include, for example, an inorganic material such as SiOF or SiOB or an organic material such as polymide-based or parylene-based polymer. An artisan will recognize that the materials discussed for the layers 306, 308, 310, 312 are for example only and that other types of could also be used. Further, an artisan will recognize that more layers or less layers can be used for particular ICs. As shown, the substrate 314 comprises silicon (Si). However, an artisan will also recognize that other materials useful in IC manufacture can be used for the substrate 314 including, for example, glasses, polymers, metals, composites, and other materials. For example, the substrate 314 may include FR4.
As discussed above, the layers 302, 304, 306, 308, 310, 312 form electronic circuitry. Individual circuits are separated from each other by a scribing lane or street 316 (shown in FIG. 3 as two vertical dashed lines). To create individual ICs, the work piece 300 is scribed, throughout, or both, along the street 316. In certain embodiments, the work piece 300 is scribed by ablating one or more of the layers 302, 304, 306, 308, 310, 312 with a beam of laser pulses. Advantageously, the laser scribing process discussed herein creates a clean kerf with substantially uniform side walls in the region of the street 316 with little or no cracking or chipping in regions outside the street 316 that are common with typical laser scribing processes.
FIGS. 4A and 4B, for example, are side view schematics illustrating the work piece 300 of FIG. 3 processed according to conventional laser scribing techniques. FIG. 4A shows laser pulse energy 402 (e.g., at wavelengths ranging from approximately 1064 nm to approximately 266 nm) passing through the passivation/encapsulation layers 302, 304 with little or no absorption. Rather, the laser pulse energy 402 is absorbed in a region 406 of the third layer 306 which causes the region 406 to heat up. Eventually, the heat causes the region 406 to ablate or explode. Thus, portions of the layers 302, 304 are blown off. FIG. 4B schematically illustrates a kerf 408 produced by the explosion. The kerf 408 does not have uniform sidewalls and extends (in chips) outside of the street area 316, which may damage the ICs. As discussed above, FIGS. 2A and 2B illustrate such chipping.
FIGS. 5A and 5B are side view schematics illustrating the work piece 300 of FIG. 3 scribed with a q-switched CO2 laser according to certain embodiments of the invention. The CO2 laser provides a laser beam comprising a series of laser pulses having a wavelength in a range between approximately 9 μm and approximately 11 μm, and a pulse width duration in a range between approximately 130 nanoseconds and approximately 170 nanoseconds.
The passivation/encapsulation layers 302, 304 are configured to absorb the energy of the pulses produced by the CO2 laser. Further, the short pulses have high peak energies that quickly and efficiently ablate the passivation/encapsulation layers 302, 304 to produce clean kerfs with substantially uniform sidewalls. In addition, the silicon substrate 314 is substantially transparent to the wavelengths of the pulses produced by the CO2 laser. Thus, the substrate 314 absorbs little or none of the energy of the pulses produced by the CO2 laser and experiences very little or no heating.
As shown in FIG. 5A, in one embodiment, the CO2 laser is used to scribe the work piece 300 by ablating the passivation/encapsulation layers 302, 304 to create a kerf 502 in the area of the street 316. The kerf 502 has substantially uniform sidewalls and a substantially flat bottom. In some embodiments, the wavelengths produced by the CO2 laser are not as efficient at ablating metal (e.g., the layers 306, 310) as it is at ablating the passivation/encapsulation layers 302, 304. Thus, as shown in the embodiment of FIG. 5A, the CO2 laser is only used to ablate the passivation/encapsulation layers 302, 304.
The remaining layers 306, 308, 310, 312 may be scribed using conventional sawing or laser scribing techniques. For example, the layers 306, 308, 310, 312 may be scribed using near infrared pulses in the picosecond range. The substrate 314 may also be diced using conventional sawing or laser ablation techniques. For example, a laser having a wavelength of approximately 266 nm can be used to efficiently and cleanly dice the substrate 314.
As shown in FIG. 5B, in another embodiment, the CO2 laser is used to scribe the work piece 300 by ablating the layers 302, 304, 306, 308, 310, 312 to create a kerf 504 in the area of the street 316. Again, the kerf 504 has substantially uniform sidewalls and a substantially flat bottom. While wavelengths ranging from approximately 9 μm to approximately 11 μm are less efficient at ablating metals, they can still ablate metals after sufficient heating. Thus, in the embodiment shown in FIG. 5B, the CO2 laser discussed herein can be used as a single process to create the kerf 504 extending from the top surface of the first layer 302 to the top surface of the substrate 314. As discussed above, the silicon substrate is substantially transparent to the wavelengths in the range between approximately 9 μm to approximately 11 μm. Thus, it is very inefficient to dice the substrate 314 with the CO2 laser. Therefore, after scribing, the substrate 314 can be diced using conventional sawing or laser ablation techniques.
FIGS. 6A-6C are electron micrographs of kerfs 610, 612, 614 scribed through passivation/encapsulation layers using a q-switched CO2 laser according to certain embodiments of the invention. As discussed above, the CO2 laser produced laser pulses having a wavelength in a range between approximately 9 μm and approximately 11 μm, and a pulse width duration in a range between approximately 130 nanoseconds and approximately 170 nanoseconds. In FIGS. 6A-6C it can be observed that there is little or no chipping, cracking or contamination. Thus, higher die break strengths and overall process yields are achieved.
FIG. 7 is an electron micrograph of a finished semiconductor wafer 708 scribed with a q-switched CO2 laser and a Gaussian picosecond pulse laser beam according to an embodiment of the invention. As shown in FIG. 7, a q-switched laser scribes a first kerf 710 in passivation/encapsulation layers of the finished wafer 708. Then, a Gaussian picosecond pulse laser beam scribes a second kerf 712 through additional layers of the finished wafer 708. For illustrative purposes, the second kerf 712 also extends beyond the first kerf 710 in an area 714. Where the finished wafer 708 is first scribed with the q-switched CO2 laser, the kerfs 710, 712 have smooth edges and produce little or no cracking. However, in the area 714 where the q-switched CO2 laser was not used, the Gaussian picosecond pulse laser produced cracking in the passivation/encapsulation layers.
It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.
Patent Application
20070272666
