Patent Application
20160079055
The present invention relates to an apparatus and a method for cleaning a substrate-like sample such as a semiconductor wafer and, particularly, to an apparatus and a method for removing fine particles adhering to a top surface of the sample mounted on a sample stage within a processing chamber that is adjusted to have a prescribed pressure value by supplying a gas.
With miniaturization in terms of circuit dimensions of semiconductor devices in recent years, a problem has emerged that when the fine particles adhering to the top surface of a semiconductor wafer, where the circuits of the semiconductor devices are formed by processing thin-film layers on the top surface, are removed by performing cleaning with liquid chemicals (wet cleaning), a wall portion between neighboring grooves in a circuit pattern formed by fabrication of a film structure which is configured of a plurality of thin-film layers on the top surface collapses due to surface tension of the liquid chemicals, and yield in manufacturing of semiconductor devices is impaired. In order to work around the problem, technologies for dry cleaning have been proposed to remove the particles without using the liquid chemicals.
As an example of the above prior art, the technology disclosed in, for example, JP-A-H07-096259 is known. According to the technology disclosed in JP-A-H07-096259, by vibrating a wafer while a shock wave is intermittently radiated to the wafer from a shock wave source arranged on a ceiling surface of a processing chamber opposite to the top surface of the wafer mounted on a wafer stage arranged in the processing chamber and ultrasonic waves of 30 KHz to 2 MHz are generated from an ultrasonic transducer arranged within the wafer stage, particles are separated from the wafer to float in space within the processing chamber, and in this state a gas can be supplied along a plane direction of the top surface of the wafer from a gas supply port arranged in a side wall of the processing chamber so that the particles are carried by a flow of the gas to be expelled from the processing chamber together with the gas.
However, the above-described prior art has problems because the following aspects were not considered sufficiently.
Namely, with the technology disclosed in JP-A-H07-096259, it is likely to destroy fine patterns by the shock waves radiated from the upper part within the processing chamber when a wafer on which fine circuit patterns are formed is cleaned. On the other hand, when the particles are tried to be separated from the top surface of a wafer only by applying ultrasonic waves without radiating a shock wave according to the prior art, a possibility of destroying circuit patterns is reduced, but there occurs a problem that fine particles cannot be removed substantially.
As described above, the prior art hardly satisfies both suppression of breakage of the circuit patterns and performance of removal of the particles when the wafer top surface is cleaned, and the problem that a yield of semiconductor device manufacturing is impaired in either case has not been considered in the prior. An object of the present invention is to provide a sample cleaning apparatus and cleaning method that can prevent impairment of the yield.
The inventors have obtained knowledge through studies that, when a sample is cleaned by applying ultrasonic waves to the sample arranged within a processing chamber and also forming a gas flow within the processing chamber, particles are not fully removed from the top surface of the sample because the ultrasonic waves are not transmitted to the sample. The present invention was achieved on the basis of the above knowledge and has a feature that a wafer is ultrasonically vibrated efficiently by an alternating electric field to separate fine particles from the sample into space within the processing chamber and the separated particles are exhausted out of the processing chamber with a gas flow formed within the processing chamber.
More specifically, the above object is achieved by a sample cleaning apparatus, including a vibrating unit which ultrasonically vibrates a sample while the sample is mounted and held on a sample stage arranged within a processing chamber, the vibrating unit including: a dielectric film which is arranged on the sample stage and above which the sample is mounted; first and second electrodes which are arranged electrically isolated from each other and adjacent to each other in the dielectric film; and a radio frequency power supply which supplies radio frequency power at frequencies in a prescribed range to the first and second electrodes while the sample is held on the sample stage; and a gas supply unit which forms a gas flow above the sample, in a direction along a surface of the sample, within the processing chamber so that particles, which are liberated from the surface of the sample, are expelled using the gas flow
Otherwise, it is achieved by a sample cleaning method, including the steps of: ultrasonically vibrating a sample to be cleaned while the sample is mounted and held on a sample stage arranged within a processing chamber by supplying radio frequency power at frequencies in a prescribed range to first and second electrodes, which are arranged electrically isolated from each other and adjacent to each other in a dielectric film which is arranged on the sample stage and above which the sample is mounted, while the sample is held on the sample stage; and forming a gas flow in a direction along a surface of the sample by supplying a gas into the processing chamber above the sample and by exhausting the processing chamber, to expel particles liberated from the sample surface.
When the sample cleaning apparatus of the present invention is used, the particles can be removed efficiently, and collapse of the circuit patterns and impairment of a yield are prevented, which are caused by wet cleaning of the sample.
Other objects, features, and advantages or the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
Embodiments of the present invention are described below with reference to the drawings.
The inventors have studied about conditions necessary for separation of particles from the top surface of a semiconductor wafer only by vibrating the semiconductor wafer made of silicon, which is a substrate-like sample to be cleaned. It is presumed that whether or not the particles are separated from the wafer depends on a balance between a separation force FL generated on the particles and an attraction force FA of the particles to the wafer.
When the water is vibrated, the separation force represented by the following formula is generated to a particle by its inertia:
where FL is an inertia force acting on the particle in N, ρ is the density of the particle in Kg/m3, a is the amplitude of the ultrasonic vibration in m, ƒ is the frequency in Hz, and d is the diameter of the particle in m.
On the other hand, an attraction force due to the van der Waals force is generated on the particles on the wafer, and its magnitude is represented by the following formula:
where FA is the attraction force acing on a particle in N, H0 is the surface roughness of the wafer in m, and A is Hamaker constant in J.
For separation of particles from the wafer, the separation force FL must become larger than the attraction force FA, so that the condition under which the particles are separated from the wafer by vibration is represented by the following formula:
Specifically, when particles have a smaller diameter (particle diameter), the vibrations are required to have a larger amplitude and a higher frequency. In the manufacturing process of semiconductor devices, the particles which cause problems such as wafer contamination, damage to a device, and deterioration of performance are currently considered to have particle diameters of 100 nm or less; therefore, in the cleaning for removal of the particles without depending on the chemical liquid, capability of removing the particles having a particle diameter of at least 100 nm is required. Here, it was presumed that alumina particles are adsorbed on a silicon wafer, and a value af2 was studied as a technical requirement necessary for removal of the particles having a particle diameter of 100 nm.
As a result, the magnitude of af2, which is the inertia (acceleration) necessary for removal of the particles, became 2.36×108 m/s2. The amplitude needed to realize the above value becomes 26 cm for the minimum frequency of 30 KHz in the frequency range of the vibration disclosed in JP-A-H07-096259, and it becomes 60 μm when the highest frequency of 2 MHz is used. On the other hand, the amplitude which can be generated with the ultrasonic transducer is 70 μm at most. Therefore, the highest frequency of 2 MHz in the frequency range of the ultrasonic vibration in JP-A-H07-096259 was used to detect the removal rate by actually performing removal of particles having a fine particle diameter.
For the above detection, a sample cleaning apparatus according to the prior art shown in
In the apparatus shown in the figure, a wafer stage 3 disposed on a lower portion in a processing chamber within a processing vessel has an ultrasonic transducer 4 attached therein, and ultrasonic vibration of an amplitude of 70 μm at a frequency of 2 MHz can be supplied to the top surface of the wafer stage 3 by vibrations of the ultrasonic transducer 4. Further, a wafer 1 is held to adhere on the top surface of the wafer stage 3 by a vacuum chuck mechanism so that it is configured that the wafer 1 does not displace on or drop from the wafer stage 3 even when ultrasonic waves are applied to the wafer stage 3.
In the apparatus, while a nitrogen gas of 20 slm (standard liters per minute) is supplied into the processing chamber from a gas supply port 5 which is arranged in a side wall of the processing chamber and above the top surface of the wafer stage 3 within the processing vessel, the gas in the processing chamber is exhausted through an exhaust port 6 which is arranged in a side wall of the processing chamber on the side opposite to the gas supply port 5 with the wafer stage 3 between them. A steady gas flow 7 is thus generated along the surface of the wafer 1, the particles are separated from the top surface of the wafer by vibration with ultrasonic waves, and the particles being floated in the processing chamber are carried by the steady gas flow 7 and exhausted out of the processing chamber, thereby cleaning the top surface of the wafer 1. Meanwhile, the processing chamber had a pressure of 75 kPa at this time.
The result of evaluation of particle removal on the top surface of the wafer 1 (opposite to the processing chamber inner wall) using the apparatus is described with reference to
It is seen as shown in the figure that a removal rate (a ratio of the number of reduction of particles after the cleaning to the number of particles on the wafer before mounting onto the wafer stage) of particles of 200 nm or less is 5% or less according to the above construction of the apparatus, and the particles cannot be removed substantially. The inventors made studies on the cause that the particles could not be removed, and it was found as a result that when ultrasonic waves are applied to the wafer stage 3 or the wafer 1, the wafer 1 partly floated up from the top surface of the wafer stage 3 and vibrations of the wafer stage 3 by the ultrasonic waves are substantially not transmitted to the wafer 1.
The inventors thought that the wafer 1 floated because a force larger than the attraction force (e.g., 0.1 MPa) of the vacuum chuck was applied to the wafer 1. Then, a force generated in the wafer was estimated with the inertia force described above.
The inertia force generated in the wafer 1 by the vibration of ultrasonic waves is represented by the following formula:
PL=4π2ρsidwαƒαƒ2 (4)
where PL is the inertia force generated in the wafer in Pa, ρsi is the density of silicon (2.33×103 kg/m3), and dwaf is the thickness of the silicon wafer (0.775×10−3 for the wafer of the diameter of φ300 mm).
The magnitude of 2.36×108 m/s2 of the above-described af2 is used and the inertia force calculated from the formula (4) becomes 17 MPa, which is a force 170 times larger than the attraction force of 0.1 MPa of the vacuum chuck. As a result of supply of vibrations of ultrasonic waves from the wafer stage 3, such the inertia force is generated in the wafer 1 and the wafer 1 temporarily floats partly from the wafer stage, so that a gap is generated between the wafer 1 and the wafer stage 3. It was found that the ultrasonic waves are reflected due to the gap and the wafer is not vibrated. As a result of the study, the gap described above was in the order of 10 μm.
In order to solve the problem that creation of the above gap causes to locally reduce the amplitude of the vibration of the wafer 1 and a prescribed inertia force cannot be generated, thereby resulting in insufficient removal of particles, the inventors have devised a construction of vibrating the wafer directly by an electric field.
An embodiment of the present invention is described below with reference to
A sample cleaning apparatus of
Specifically,
The wafer stage 3 of the present embodiment is a member having a cylindrical or disc shape or a shape substantially the same as them and arranged at a lower part of the processing vessel. The upper part of the wafer stage 3 is configured to have a film which is formed by thermal spraying a dielectric material such as ceramics like alumina or yttria, or a sintered member of a small thickness made of it, and a dielectric film 8 is arranged on the top surface of it to place the wafer 1 thereabove. The top surface of the dielectric film 8 having a round shape of the wafer stage 3 or a shape substantially the same as it has a shape conforming to the circular wafer I and constitutes a mounting surface on which the wafer 1 is mounted.
In addition, the dielectric film 8 is supplied therein with electrodes 9 and electrodes 10 of circular or ring shapes which are arranged with gaps between the adjacent ones in a radial direction from a position corresponding to the center of the wafer 3, and two different voltages are applied to them, respectively. The electrodes 9, 10 are arranged electrically isolated from each other and applied with a different polarity or voltages. In the present embodiment, the electrodes 9 are grounded or electrically connected to another member which is set at a ground potential, and an RF power supply 12 is arranged between and connected to the electrodes 9 and the electrodes 10 via a matching box 11. With this configuration, an RF voltage from the RF power supply 12 is applied to these electrodes.
Referring to
The surface of the dielectric film 8 has unevenness representing roughness of the shape. Due to the unevenness, the portions of contacts between the wafer 1 and the dielectric film 8 have gaps 13 between the wafer 1 and the surface of the dielectric film 8 as shown in
where Pa is the attraction force generated by the electric field in Pa, εr is the relative dielectric constant of the dielectric film, ε0 is the dielectric constant of vacuum in F/m, E is the electric field strength in V/m, Va is the amplitude of the applied voltage in V, ω is the angular frequency in Hz, and t is time in s.
Specifically, it is seen from the formula (5) that the attraction force acting on the wafer 1 changes (alternates) its direction at a cycle twice that of the applied RF voltage with time. By this alternating attraction force, the wafer 1 is displaced in a direction increasing the distance (mutually separating) and a direction decreasing the distance (mutually pushing) and vibrated above the top surface of the dielectric film 8. Such vibrations are normally ultrasonic vibrations formed with a frequency which is in a band of ultrasonic waves (so-called ultrasonic range) exceeding the audible range of humans.
Using the construction of the wafer stage provided with the mechanism to generate such the ultrasonic waves, conditions under which particles can be removed were investigated. The construction of the wafer stage is described with reference to
In
It was found from the results of analysis made by the inventors that resonance is caused by the vibration of ultrasonic waves of the wafer 1 at the above frequencies and the amplitude of vibration takes local maxima. On the other hand, when focus is directed to the particle removal rate at the respective local maximum points, their magnitudes are no greater than around 50%.
As a result of the study made on the cause by the inventors, it was found that the vibration of the wafer 1 has generated nodes of ultrasonic vibration concentrically in the radial direction from the center at an interval of 1.7 mm and particles remain on those portions since those portions hardly vibrate. And, it was also found that the distances of the nodes from the center of the wafer 1 vary depending on the frequency of the RF power.
Therefore, to solve the problem that the particles remain on the node portions, the particles were removed while sweeping the frequency of the RF power from 9.9 MHz to 10.1 MHz. As a result, it was found that the particle removal rate was improved up to 95%.
On the basis of the above knowledge, the frequency of the RF power generated by the RF power supply in the present embodiment is varied within a prescribed range in time while it is supplied to the electrodes 9, 10. Thus, the particles are prevented from locally remaining on the top surface of the wafer 1.
As another structure, two sets of the matching box and the power supply were used to remove particles as shown in
By providing the wafer stage 3 having the above-described structure within the processing chamber, the sample cleaning apparatus of the embodiment prevents the particles from remaining on the wafer top surface as adhering locally and not separating, and the particles on the top surface of the wafer 1 can be reduced on the entirety of the top surface efficiently. Moreover, the above-described embodiments cover an example that the frequency of the RF power was made variable and an example that two sets of the matching box and the power supply were used and two kinds of RF voltages were superposed, but the same effects can be obtained by superposing two or more kinds of frequencies or by applying a RF voltage including two or more kinds of frequencies with an arbitrary waveform generator and an RF amplifier.
The inventors additionally studied on effects of the surface roughness onto the particle removal rate. In the study, grooves having a V shape viewed in a longitudinal cross section as shown in
In the above study, RF powers of resonance frequencies near 2.5 MHz, 5 MHz, and 10 MHz from the RF power supply 12 were applied to the electrodes 9 or 10, and the particle removal rates for particles having a particle diameter of 100 nm were measured. The results are shown in
It is also seen that when the frequency of the RF power is increased to around 5 MHz (the precise resonance frequency of 5.014 MHz) and further to around 10 MHz (the precise resonance frequency of 10.013 MHz), a threshold value of the groove depth decreases inversely proportional to the square of the frequency. Furthermore, since the groove depth which becomes the above threshold value is inversely proportional to the square of the particle diameter of the object particles, it is considered that the necessary groove depth increases furthermore when the object particle diameter decreases along with miniaturization of semiconductor devices.
On the other hand, it was found that this method has a problem that when the groove depth is increased to greater than 10 μm, abnormal discharges occur on the back surface of the wafer. Therefore, the groove depth is desirably 10 μm or less. And, it is also seen that the RF power supplied to the electrodes 9, 10 must have a frequency of 2.5 MHz or more for removal of particles with the groove depth of 10 μm or less. Incidentally, although the example of
In addition, the inventors have made studies on a requirement that a circuit pattern formed on the wafer does not collapse.
First, it was found that the width w of the pattern hardly affects the pattern collapse. In addition, it was found that the height h of the pattern which collapses and the frequency of the RF power are in an inverse proportion relation to each other as shown in
As described above, to prevent abnormal discharge on the back surface of the wafer, it is necessary that the RF power has a frequency of 2.5 MHz or more; therefore, in the semiconductor device production process, the particles can be removed effectively by using the RF power of the frequency in a range of 2.5 MHz or more and 100 MHz or less supplied from the RF power supply 12 in the sample cleaning apparatus of the present embodiment.
Referring to
In the figure, a difference from the embodiment shown in
The end of the matching box 11 is also connected to the electrode 10 via a transformer 17, and a voltmeter 18 for detecting a voltage formed across the transformer is connected to the secondary side of the transformer 17. Incidentally, the inductor 15 is connected between the transformer 17 and one end of the matching box 11. When the magnitude of the voltage supplied from the DC power supply 16 is amply larger than a Peak-to-Peak value of the voltage of the RF power from the RF power supply 12, the amplitude of the voltage generated on the secondary side of the transformer 17, which is detected by the voltmeter 18, is represented by the following formula:
where S0 is the area of the wafer in m2, V0 is the magnitude of the DC voltage in V, and L12 is the mutual inductance of the transformer in H.
This value is proportional to the particle removing capability described with reference to the formula (3), so that the particle removing capability can be increased by adjusting the frequency to increase the amplitude. Based on the above fact, the frequency at which the particle removing capability becomes maximum or desirable is detected and selected, and by using the RF power of that frequency in the sample cleaning apparatus of
Also, as another method, by arranging a laser type vibration measuring device on a ceiling surface, which is opposite to the top surface of the wafer 1, of the processing chamber of the sample cleaning apparatus shown in
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-185849 | Sep 2014 | JP | national |
