Laser processing apparatus utilizing laser beam to irradiate semiconductor layer

Laser processing device for irradiating semiconductor layer with laser beam

[0001] This application is a divisional application of the original application with the application number ο 1104514.0, the filing date of June 26, 1993, and the invention title of "laser processing device for irradiating a semiconductor layer with a laser beam".

Technical field

[0002] The present invention relates to a highly reliable laser annealing method suitable for use in the mass production of semiconductor devices and capable of achieving uniform annealing under high productivity conditions. More specifically, the present invention provides a laser annealing method for a deposited film whose crystallinity is severely deteriorated due to damage in processes such as ion irradiation>ion implantation and ion doping.

Background technique

[0003] Nowadays, methods for lowering the processing temperature in manufacturing semiconductor devices are being extensively studied. The reason why low-temperature processing methods have been so actively studied is partly attributable to the requirements for preparing semiconductor devices on insulating substrates made of glass, for example. Laser annealing technology is considered to be the main promising low-temperature processing method.

[0004] However, the conditions of laser annealing have not yet been determined, because traditional laser annealing methods are each independently performed under different conditions, and these conditions depend on the device and coating conditions independently selected in each method. This makes many people mistakenly believe that laser annealing technology cannot give reliable and consistent results that are sufficient to make the method practical. Therefore, the object of the present invention is to establish the conditions of a laser annealing method that can give highly reproducible results for the first time.

Summary of the invention

[0005] In the method of manufacturing a semiconductor device, the deposited film is extremely damaged by processes such as ion irradiation, ion implantation, and ion doping, and thus is damaged in terms of crystallinity, resulting in far from being all-encompassing. Said the amorphous phase or similar state of the semiconductor. Therefore, in order to use laser annealing to activate such damaged thin films, the inventors conducted an in-depth study on how to optimize the conditions of laser annealing. During this research, it has been discovered that the change in the optimal conditions is not only affected by the energy control of the laser beam, but also by the impurities contained in the film and the number of pulses of the applied laser beam.

[0006] The deposited films activated by the method of the present invention are those containing as the main component IV elements of the periodic table, such as silicon, aluminum, silicon and aluminum alloys, or the deposition of IV element compounds such as silicon carbide film. The thickness of the deposited film is 100 A to 10000 A. Considering light transmission, it has been fully confirmed that the use of a laser beam in the short wavelength range, especially 400 nm or shorter wavelength, can well achieve laser annealing of this film. .

[0007] The method of the present invention includes the following steps:

[0008] A laser pulse having a wavelength of 400 nm or less and a pulse width of 50 nsec or less is irradiated onto a thin film containing group IV elements selected from the group consisting of carbon, silicon, aluminum, tin, and lead, and Impurity ions doped into it.

[0009] Wherein in the path of the laser pulse to the film containing the group IV element, a transparent film with a thickness of 3 to 300 nm is formed on the film containing the group IV element, and each of the laser pulses Energy density E in mj/cm²Is the unit and the number of laser pulses N satisfies the relationship: log₁₀NW -0.02 (E-350).

[0010] From KrF excimer laser, ArF excimer laser, XeCl excimer laser and XeF excimer laser

A laser selected from the group consisting of a device emits laser pulses. The doping of impurity ions is achieved by processes such as ion irradiation, ion implantation and ion doping. A thin film containing group IV elements is formed on an insulating substrate, and the insulating substrate is maintained at a temperature of room temperature to 500° C. during the irradiation step.

[0011] It has been confirmed that the use of a laser beam with a sufficiently high energy density to activate can reduce sheet resistance. In the case of a thin film containing phosphorus as an impurity, this tendency must be observed. However, in a thin film containing boron as an impurity, the thin film is damaged by such high-energy density laser irradiation. In addition, it is taken for granted that the increase in the number of pulse shots reduces the fluctuation in the characteristics of the laser-annealed film. However, this is inaccurate because it has been found that the microscopic fluctuations are enhanced due to the increase in the number of shots, and the structure of the coating is degraded.

[0012] This can be explained as the growth of crystal nuclei in the coating caused by the repeated irradiation of the laser beam applied to the film. As a result, a particle size distribution in the range of 0.1 to lum size appeared in the coating, which was previously composed of crystal grains of uniform size. This phenomenon is especially obvious when using high-energy laser irradiation.

[0013] It has been found that the deposited thin film (ie, semiconductor thin film) must be coated (covered) with a 3 to 300 nm thick light-transmitting coating, and cannot be exposed to the atmosphere. From the viewpoint of light transmission, the light transmission coating is preferably made of silicon oxide or silicon nitride. It is best to use a material mainly containing silicon oxide, because it can also be used as a gate insulating material in general. Needless to say, the light transmission film can be doped with phosphorous or boron to passivate movable ions. If the film containing group IV elements is not coated with this light transmission coating, the uniformity will be destroyed in an accelerated manner.

[0014] It has also been found that under the above conditions and when the following relationship is satisfied:

[0015] logioN WA (EB)

[0016] The pulsed laser beam can be used to obtain a smoother (uniform) coating, where E (mJ/cm²) Is the energy density of each laser pulse irradiated, and N (number of shots) is the number of pulsed laser shots. The values of A and B depend on the impurities incorporated into the coating. When phosphorus is present as an impurity, select A as -0.02 and B as 350. When using boron as an impurity, select A as -0.02 and B as 300.

[0017] Using a transparent substrate instead of a transparent film can achieve similar results. That is to say, the laser processing method according to the present invention includes the following steps:

[0018] doping impurities into the semiconductor thin film formed on the transparent substrate; and

[0019] irradiating a laser pulse having a wavelength of 400 nm or shorter and a pulse width of 50 nsec or shorter through the transparent substrate onto the semiconductor thin film;

[0020] Wherein, the unit is mj/cm²The energy density E of each of the laser pulses and the number N of the laser pulses satisfy the relationship:

[0021] log₁₀NW -0.02 (E-350).

[0022] FIG. 7(A) shows the doping step, and FIG. 7(B) shows the irradiation step. Reference numeral 71 represents a transparent substrate, and 72 represents a semiconductor thin film.

Description of the drawings

[0023] FIG. 1 is a schematic diagram of a laser annealing device used in an embodiment of the present invention;

[0024] FIG. 2 is a diagram showing the difference between the sheet resistance of a silicon thin film (phosphorus-doped, N-type) obtained by laser annealing and the applied laser energy density when changing the number of repetitions of pulse emission according to an embodiment of the present invention relation chart;

[0025] FIG. 3 shows the number of repetitions of pulse emission between the sheet resistance of a silicon film (phosphorus and boron doped, p-type) obtained by laser annealing according to an embodiment of the present invention and the applied laser energy density. Time relationship diagram;

[0026] FIG. 4 is a diagram showing the relationship between the structure of the silicon thin film obtained in an embodiment of the present invention and the laser energy density applied and the number of repetitions of pulse emission;

[0027] FIG. 5 is a schematic diagram of an optical system of a laser annealing device used in an embodiment of the present invention;

[0028] FIG. 6 shows a laser annealing method according to the present invention; and

[0029] FIG. 7 shows another laser annealing method according to the present invention.

Detailed ways

[0030] The present invention is described in more detail below with reference to a non-limiting example, but it should be noted that the present invention cannot be construed as being limited to this example.

[0031] In this example, impurities are doped into a thin film composed of group IV elements so that it has one of N-type and P-type conductivity, and another impurity is doped into a part of the thin film with a mask. , So that the part has the remaining one of N-type and P-type conductivity. In Fig. 1, the laser annealing device used in this embodiment is schematically shown. The laser beam is generated in the generator 2, after passing through the total reflection mirrors 5 and 6, is amplified in the amplifier 3, and is introduced into the optical system 4 after passing through the total reflection mirrors 7 and 8. The initial laser beam has about 3 X 2cm²1to1cm. The rectangular beam area, but with the help of the optical system 4 is processed into a long beam, which is about 10 to 30cm in length and about 0.1 to 1cm in width. The maximum energy density of laser light passing through this optical system is 1000mJ/single shot.

[0032] The optical path in the optical system 4 is shown in FIG. 5. The laser light incident on the optical system 4 passes through the cylindrical concave lens A, the cylindrical convex lens B, the fly-eye lens C arranged in the horizontal direction, and the fly-eye lens Do arranged in the vertical direction. Become a rectangular distribution. Then, the laser passes through the cylindrical convex lenses E and F, and is reflected on the mirror G (reflector 9 in FIG. 1), and finally is focused on the sample by the cylindrical lens H.

[0033] In this example, the distances Xj and X shown in FIG. 5₂Is fixed, and the distance X3, distance X between the virtual focus 1 (produced by the difference between the curved surfaces of the fly-eye lens) and the mirror G₄And X₅Is changed in order to adjust the magnification M and the focal length F. which is:

[0034] Μ = (Χ3+Χ4) /Χ₅

[0035] 1/F=1/(X3+X4) +1/X₅o

3mο [0036] In this example, the total length of the optical path X6 is about 1. 3mο

[0037] As described above, the initial beam is modified into a long beam to improve its processing performance. More precisely, after leaving the optical system, the rectangular light beam irradiated on the sample 11 through the total reflection mirror 9 has a larger width than the width of the sample. Therefore, the sample only needs to move in one direction. Therefore, the platform for loading the sample and the driving device 10 can be made into a simple structure, so the maintenance work can be easily completed. In addition, the adjustment work when installing the sample can also be greatly simplified.

[0038] On the other hand, if a beam with a square cross-section is used, it will be impossible to cover the entire substrate with a single beam. Therefore, the sample should move in two directions in two dimensions. In this case, the driving device of the platform will become complicated, and the adjustment must be carried out in a two-dimensional manner, which brings many difficulties. Especially when the adjustment is made manually, this process takes a lot of time, which greatly reduces the productivity of the entire process. In addition, these devices must be installed on a stable workbench 1 such as an anti-vibration workbench.

[0039] The detailed product used in this example is a different type of glass substrate (for example, Corning (Corning) #7059 glass substrate), which has a length of 100mm and a width of 100-300mnio. KrF laser with 248nm wavelength and 50nsec or narrower pulse width.

[0040] A plasma-accelerated CVD (chemical vapor deposition) process was used to deposit 100nm thick amorphous silicon on the glass substrate 61

film. The resultant film was annealed at 600° C. for 48 hours to obtain a crystalline film, and the resultant film was patterned to form island-shaped portions 62 and 63 (FIG. 6(A)). Further, a 70 nm thick silicon oxide film (light transmission coating) 64 was deposited thereon by a sputtering method, and the entire surface of the substrate was doped with phosphorus. In this step, the so-called ion doping process (Figure 6(B)) is used, and phosphine (PH₃) As a plasma source and an acceleration voltage of 80KV. Then, a part of the substrate 65 is covered, and boron is implanted by an ion doping process (Figure 6(C)). In this step, ethane (B₂H₆) As a plasma source and accelerate under 65KV voltage. More precisely, through the light-transmitting coating, phosphorus is injected (doped) into the covered part, thereby obtaining a part with N-type conductivity, and phosphorus and boron are injected (doped) into the part through the light-transmitting coating. The unmasked part results in a P-type conductive part.

[0041] Thereafter, while changing the energy density and the number of pulse emission, the laser beam is irradiated onto the island-shaped portion (semiconductor thin film) to achieve laser activation. The sheet resistance was measured accordingly, and the structure of the crystals constituting the coating was observed through an optical microscope. The results obtained are summarized in Figures 2 to 4.

[0042] FIG. 2 shows the relationship between the sheet resistance of a silicon film doped with phosphorous ions and the energy density of the laser beam when the number of repetitions of pulse emission is changed. In 2X10¹⁵cm-²Phosphorus is doped into the silicon thin film at a dosage of 100%. Used at 200mJ/cm²Lasers operating at energy densities or smaller require a large number of shots to activate the thin layer, and will produce the undesirable result of a high sheet resistance of about 10k Ω / sq ·. However, using 200mJ/cm²A laser beam with a higher energy density or higher can be fully activated under the operating conditions of the laser with 1 to 10 shots.

[0043] Figure 3 shows the 4X 10¹⁵cm-²The result of laser activation of silicon thin films doped with boron ions at different doses. In this case, use 200mJ/cm²Energy density or smaller can only perform insufficient activation, and for sufficient activation, a large number of pulses is required. Adopt operation in 200 to 300mJ/cm²A laser beam with a high energy density can obtain a sufficiently low sheet resistance through 1 to 10 shots. However, on the other hand, it is used to run at 300mJ/cm²Or higher energy density lasers, the sheet resistance will increase instead. Especially, with the use of 200mJ/cm²Or lower laser beam energy density is activated on the contrary, as the number of repetitions of pulse emission increases, the sheet resistance will increase. This phenomenon can be explained as being caused by the growth of grain boundaries due to the destruction of the uniformity of the film, and the destruction of the uniformity of the film is caused by the use of laser irradiation with too many shots.

[0044] In the actual process, laser annealing is used for both the P-type and N-type regions, as shown in FIG. 6(D). This means that at 350mJ/cm²The laser beam irradiated with a high energy density fully activates the N-type region, but at the same time damages the performance of the P-type region. Therefore, in the process according to this example, the laser preferably works at 200 to 300mJ/cm²Within the range of energy density, preferably in the range of 250 to 300 machi/cm?. The number of pulse repetitions is preferably in the range of 1 to 100 pulses.

[0045] As described above, the structure of the deposited film is significantly affected by laser annealing. In fact, as shown in Figure 4, the number of pulses emitted can be related to the energy density of the laser beam and the structure of the film. In Figure 4, the term "annealing pulse" refers to the number of laser beam pulses emitted. The solid circles in the figure represent the points where the surface structure changes are observed on phosphorus-doped silicon, while the open circles represent the same situation on boron-doped silicon. The upper area on the right of the figure corresponds to the appearance of a bad structure (rough surface) on the surface, and the lower area on the left of the figure corresponds to the appearance of a good structure (smooth surface) on the surface. It can be seen from this result that phosphorus-doped silicon has strong resistance to laser irradiation. Therefore, the condition of laser annealing without damaging the surface structure can be understood as satisfying the relationship:

[0046] logioN WA (EB)

[0047] Here, E (mJ/cm²) Is the energy density of the irradiated laser beam, and N (number of shots) is the number of shots of the pulsed laser. When phosphorus is doped as an impurity, the values of A and B are A = -0.02 and B = 350, and when boron is doped as an impurity, A = -0.02 and B = 300.

[0048] When the structure of the deposited film suffers considerable damage, this characteristic value exhibits large scattering due to the severe local degradation of silicon performance. In fact, on a silicon thin film with a defective structure (rough surface), the dispersion of sheet resistance is observed to be as high as 20% or more. By satisfying the above conditions and setting the laser energy density to an appropriate value, this dispersion can be eliminated.

[0049] For example, when the laser energy density is set to 250 mJ/cm²At this time, the pulsed laser beam is emitted at a frequency of 10 times or less. If the energy density is as high as 280mJ/cm², The laser beam is best emitted at a frequency of 1 to 3 times. By laser annealing under this condition, the change in sheet resistance can be controlled within 10% or less.

[0050] According to the present invention, by setting the optimal conditions for laser annealing as described above, a highly reliable semiconductor thin film with low performance changes is obtained. Therefore, it can be seen that the processing method according to the present invention is beneficial to the semiconductor industry.

[0051] Although the present invention has been described in detail with reference to specific embodiments, it is obvious to those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the present invention.