KR100209453B1 - System and method for accurately depositing particles on a surface - Google Patents

Abstract

It is a particle precipitation system having an automated machine 11, a wafer carrier 39, a sheath flow means, a particle counter and a computer controller to precisely deposit the desired particle 13 density on the surface. The sheath flow 35 keeps the article 19 clean while the particle flux in the settling chamber 15 rises from zero to equilibrium. The particle counter measures the particle flux by sampling the atmosphere in the precipitation chamber 15. Computer 47 determines when the rate of change of particle flux is substantially zero and then conveys article 19 into the falling particle 13 fog completely or partially away from the sheath flow 35. The computer 47 also calculates the settling time needed to provide the desired particle density on the surface of the article and returns the article 19 into the sheath flow 35 after the desired density is reached.

Description

[Name of invention]

System and method for precisely depositing particles on a surface

[Technical Field]

FIELD OF THE INVENTION The present invention relates to a system and process for uniformly distributing solid particles on a surface, in particular an emitter that releases dry solid particulate material as a mist of separated particles suspended in a gas stream flowing into a settling chamber on an expanded application area. It relates to a system and a process using an atomizer.

[Background]

Particle precipitation systems are commonly used to deposit polystyrene latex references on bare silicon wafers for use in calibrating wafer scanning devices. Typically such particle precipitation systems include a precipitation chamber into which a wafer can be placed, and an atomizer, also called an atomizer, which deposits on the wafer for the time required to achieve the desired particle density or particle count. Used to release particles into the room. Currently useful systems operate by manually placing the wafer in the deposition chamber under the ejection nozzle of the atomizer and turning on the atomizer. The atomizer then generates particle fog that sinks on the wafer. The atomizer is turned off by the operator after a certain time period. The wafer is removed from the settling chamber and inspected to see if the desired particle density on the wafer has been achieved. The desired particle density is achieved by trial and error while adjusting the settling time until a wafer having that density is produced.

Unfortunately, the particle flux in the settling chamber rises from zero when the atomizer is turned on and changes over time to a maximum determined by the gas and particle flow out of the atomizer's discharge nozzle at some time t = t ₀ , and Afterwards it is partly determined by the particle size. This particle flux rise curve is, in addition to particle size, also primarily a function of atomizer pressure and colloidal suspended density of the particle. The determination of the settling time required for the desired particle density in this curve is not very accurate, because the flux does not normally rise linearly with time, and the flux-time curve is not very well characterized, especially This is because the same is true for particle types other than polystyrene latex spheres. In addition, the actual settling time provided by the manual settling system is not very accurate. Because the rise curve is relatively steep, small differences in actual settling times can result in even larger differences in the resulting particle density on the wafer. Therefore, the operation cannot be repeated exactly. Surface scanners can respond differently to different types of actual contaminant particles and different types of surfaces, so that all sizes and types of particles can be placed on any type of surface up to the desired particle density with sufficient precision to be used to calibrate the surface scanner. It is desirable to be able to precipitate.

It is an object of the present invention to provide a particle precipitation system and method for depositing particles on a surface, which can be controlled and precipitated at a specified density by an operator.

Detailed description of the invention

This object is in addition to an atomizer, a wafer counter and computer of a conventional system, a particle counter to sample air in the settling chamber to provide measurement of particle flux throughout the settling chamber, and a purge gas sheath flow over the article. The article is moved into the deposition zone of the settling chamber until the particle flux provided by the atomizer as measured by the computer and the purge zone under the perforated plate to maintain the article free of particles is reached by equilibrium or steady state. It is satisfied with a particle precipitation system that includes computer control means to a wafer carrier and other elements in the system to delay this.

The wafer carrier receives the article having the surface on which the particles are to be deposited and transports it to the purge area. A gas sheath flow in the purge zone is provided throughout the surface of the article, thereby preventing precipitation of particles on the surface. The atomizer releases the particulate material into the top of the settling chamber in the form of a separate particle mist, typically dry solid particles suspended in a gas stream, so that the particles descend with a substantially uniform distribution over the expanded application area near the bottom of the settling chamber. Or spread. The particle counter continuously measures the particle flux in the settling chamber while transmitting the measured flux information to the counter input of the system computer. The computer processes this received information while calculating the rate of change of the particle flux measured, and determines when this rate of change is virtually zero. The ship's velocity in the settling chamber will then remain substantially constant until equilibrium is reached and the atomizer is turned off. Once this equilibrium is reached, the computer sends a control signal to the wafer carrier to take the article out of the sheath flow of the purge area and into the falling particle fog of the application area, thus the particles settle on the surface of the article. The article may also move only partly into the mist of particles relative to the partial coverage of that surface and partly remain in the sheath flow. The computer also calculates the settling time required to achieve the desired density from the measured particle flux and the desired particle density previously specified by the operator. Since precipitation occurs only while the flux is in steady state, the calculation is simply a function of particle flux and particle size and can be stored in a computer's read only device (ROM) and divided by the desired density. After the article is in the application area for the required settling time, the computer again sends a control signal to the wafer carrier to return the item into the sheath flow of the purification area. The article can then be removed from the system. Additional articles may settle into the particles while the vessel speed is still in steady state, or the atomizer may be turned off.

The advantage of this system and method is that no rise characteristic of the ship's velocity over time is required. At equilibrium the ship velocities are virtually the same throughout the settling chamber and the calculation of the required settling time is simple.

[Brief Description of Drawings]

1 is a side elevational view of the particle precipitation system of the present invention.

FIG. 2 is an interior plan view of the FIG. 1 system along line 2-2 of FIG.

3 (a)-(c) are top views of the wafer surface after being deposited into particles by the system in FIGS. 1 and 2.

4 is a flowchart of the process steps of the present invention.

5 is a graph of measured particle flux (Q) versus time (t) for the precipitation system in FIGS. 1 and 2 from when the atomizer was turned on.

Best Mode for Carrying Out the Invention

With respect to FIGS. 1 and 2, a system is shown that can provide controlled precipitation of small particles on a surface. The system includes an atomizer 1 for releasing fine mists of the particles 13 into the settling chamber 15. Particles 13 drop or diffuse within settling chamber 15 over a wide range of application areas 17 near the bottom of settling chamber 15, where the article 19 has a surface to be settled into particles 13. This can be located. The atomizer 11 is a standard commercial device widely used in the aerosol industry. This is often called a nebulizer. One commercial manufacturer of Atomizer is TSI Inc. of St. Paul, Minnesota. Typically the particles are solid particles that are transported in suspension in liquid form, such as deionized water or isopropyl alcohol, from feed vessel 12 to aerosol generator 14, which sprays the liquid suspension as very fine droplets into the aerosol drying chamber. Very low particle density is possible if the drying chamber 16 has a large internal surface area. Currently, solid particles dried through evaporation of the liquid carrier medium are electrically controlled by controlling the particles in the controller 18 with a beta emitter such as Kr-85 to prevent the particles from electrostatically attracting and sticking together. Charge is neutral. Beta emitters are also available from TS Eye Corporation. The particles are discharged from the nozzle 21 into the settling chamber 15, and the resulting mist consists of the separated dry solid particles suspended in the gas stream. If possible, the particles will be evenly distributed throughout the application area 17 so that the precipitation will be substantially uniform. The particles can also be liquid droplets of an oily substance such as dioctyl phthalate or a liquid unit or salt solution.

Particles distributed in this manner are typically polystyrene latex spheres and, where possible, meet the NTST specification for reference spheres for calibration wafers. Such phrases are available from Duke Scientific of Palo Alto, California, Japan Synthetic Rubber Co., Ltd., and others. Particle diameters ranging from 0.1 μm to 4.0 μm are commonly used in the semiconductor industry. Polystyrene latex spheres on silicon wafers have good optical response and make them useful for calibrating surface scanning devices used in the semiconductor industry. Instead, the particles 13 may be actual impurity particulates or powders such as SiO ₂ , SiC, Al ₂ O ₃ , Fe ₂ O ₃ and Al tools.

An article or object that will have particles deposited on its surface is typically a silicon silicon wafer. However, all gases such as patterned wafers, photomasks, optical discs (coated or uncoated) and magnetic discs (coated or uncoated) can be used. The gas does not need to have a complete flat surface. Patterned wafers, for example, are characterized by optically important surface contours.

The particles 13 deposited on the surface of the article 19 by the atomizer 11 do not usually form a continuous dura coating, such as paint pigments, but possibly remain discrete particles on the surface. The particles are scattered irregularly over the entire area of the surface with a substantially uniform distribution. The system of the present invention seeks to ensure the accuracy of the actual particle density deposited on the surface relative to the desired particle density specified by the operator or user of the system.

The system also includes a laser based airborne particle counter that basically includes a laser source 21 for generating a collimated light beam 23 and a photo detector or detector array 25. In the desired configuration, the particle counter precipitates through an inlet 20 below the gas position using an aiming light source 21, such as a laser, and a photo detector or detector array 25 to measure particles per unit volume per unit time. Continuously sample indoor air. The detector 25 is placed at a position relative to the beam 23 so that the degree of light shielding of the beam 23 by each particle 13 intersecting through the beam path, or possibly dimming particles 13 (position of FIG. 2) Light scattering degree (at 25 ') is detected. In both cases the result is to provide a particle count indicating the flux of particles 13 descending through the settling chamber 15. Such volume sampling particle counters are available from TS Eye, Particle Measuring Systems, Inc., Boulder, Colorado, and others. Typical steady state flux values provided by the atomizer 11 range from 10 particles / 0.1 cfm for large particles of about 4 μm diameter to 500,000 particles / 0.1 for small particles of about 0.1 μm diameter, as measured by a particle counter. over the range of cfm (0.1 cfm = 47.195 cm ³ sec ^-1 1).

The system further includes a manifold having a gas inlet 27, a seal 29 and a perforated plate 31, the perforated plate 31 having many openings 33 and forming the bottom wall of the seal 29. Thus, whenever the article 19 is in position 19b below the perforated plate 31, a purge gas, sheath flow (indicated by the arrow 35) is provided on the surface of the article 19. The purge gas received by the inlet 27 may be dry nitrogen (N ₂ ), air or other inert gas. The gas is typically controlled and tightly filtered through a 0.01 μm filter to remove all suspended particles. The gas flows through the opening 33 and around the gas 19b to cause all particles 13 in the settling chamber 15 to be far from its surface. Typically the surface of the article 19 to be kept clean is located within 1 mm from the opening 33 in the perforated plate 31 such that the gas flow 35 is separated by the small gap between the article and the plate 31. To be confined to adjacent surfaces. If only part of the article 19 is below the perforated plate 31, such as at position 19c, for example, the gas stream 35 causes particles 13 to be placed underneath the perforated plate 31. While the particles 13 are separated from the surface portion of the particles, the particles are precipitated on the exposed area of the surface in the precipitation chamber 15. At the bottom of the settling chamber 36, a particle filter 36, such as a HEPA filter, is provided with excess gas 38 from both the sheath stream 35 and the particle suspended stream of gas forming part of the particle 13 mist. Try to get out.

The article 19 can be transferred by any known wafer transfer device known in the semiconductor art. In FIG. 1 and FIG. 2 the article 19 is conveyed from one position to another on a vacuum chuck 37 located on a belt carrier 39 driven by a servo motor 41. Is displayed. However, many types of wafer carriers using movable arms and other instruments are well known and commercially available. A standard commercial wafer adjuster 43 may be used to place the article 19 through the door 45 over the wafer chuck 37 or other carrier below the porous plate 31 of the system.

The system also includes a computer 47 to control the precipitation process such that the specific particle density on the surface required by the user of the system is achieved with great accuracy and precision. The computer 47 may include a keyboard 49 or other input device such that the particle size, desired particle density or modulus within the atomizer 11, and the desired coverage (full coverage or half) for particles on the article surface. User-specified information, such as coverage. The computer 47 is also connected to a particle counter such as a processor chip in the detector 25 or a particle counter to receive the measured particle flux information. The computer 47 is also connected to a wafer transport device such as a motor 41 to control the operation of the device and the transport of the article from one location to another. Such process control computers are well known and commercially available. The operation of the computer is determined by the computer software and will be further described below with reference to FIG.

3 (a)-(c) show some of the various possible surface deposits that can be specified by the user. In Figure 3 (a) the wafer 51 has particles 53 distributed substantially uniformly over its entire surface. Such full coverage places the wafer 51 completely inside the extended application area 17 at positions 19d in the first and second degrees so that the wafer is completely out of the sheath flow 35 below the perforated plate 31. Can be given by Once the wafer 51 is removed from the settling chamber 15 it essentially appears to be shown in FIG. 3 (a). In FIG. 3B, the wafer 55 has particles 57 substantially uniformly distributed over about half of its surface, while the other side of the wafer surface is a region 59 that is substantially free of particles. Half of such coverage is placed on the wafer 55, part within the application area 17 and part under the perforated plate 31 at position 19c in FIG. While the particles may be precipitated, the area under the porous plate 31 is kept free of particles by the sheath flow 35 of the purge gas. The boundary between the two regions (indicated by dashed line 61 in FIG. 3 (b)) corresponds to the limit of the sheath flow on the wafer surface. The boundary is relatively clear due to the small gap of 1 mm or less between the porous plate 31 and the wafer. FIG. 3C shows a wafer 63 having a surface that results from two semi-coverage precipitations with the wafers placed in a direction perpendicular to the wafer direction during the first precipitation during the second precipitation. Quadrant area ( ) Was virtually free of particles since it was below the perforated plate 31 during both settling. Quadrant region A has a first density of particles 65, while quadrant region B has a second density of particles 67 or particles of different sizes. In the latter case the size of the atomizer particles is changed while the wafer is rotated 90 °. The quadrant region designated A + B is in the application zone 17 for two precipitations, thus receiving both a first density of particles 65 during the first precipitation and an additional second density of particles 67 during the second precipitation. . Typically both precipitation densities will equally change only particle size, so quadrant A + B will have particles of two sizes deposited thereon. Alternatively or additionally, the particle density may change. Particles of other patterns may be formed in addition to those shown in FIGS. 3A to 3C.

Referring to FIG. 4, the computer 47 of FIG. 2 under the direction of the computer software coordinates and controls the precipitation process performed by the systems shown in FIGS. First, the computer prompts the operator of the system for information about the intended precipitation (step 71). In particular, the operator may be informed about the size (S) of the particles in the feed hopper of the atomizer, information indicating the desired particle density or particle count (P) to be provided on the article surface and for example any of the full or semi-application ranges. Provides information on the desired surface brief range if this is required.

Next, a sheath flow of purge air or other inert gas is started, and the wafer or other article is received from the wafer adjuster at position 19a, and at the position 19b shown in FIGS. 1 and 2 by the wafer carrier. It is carried to the purge zone under the sheath flow (step 73). Computer control of the gas flow is performed by connecting a computer output line to the valve between the gas source and the inlet 27, and the inlet 27 is activated by a control signal received on the computer output line. Computer control of wafer transport can be done with a commercial wafer transport apparatus via a second computer output line to operate the motor 41.

The atomizer is then turned on and the particle counter is turned on equally to continuously measure the particle flux in the settling chamber (step 75). For this purpose, the computer output control lines are connected to the atomizer 11 and the particle counter elements 21 and 25 to start their operation at a suitable time. The atomizer then discharges the particulate matter 13 to the top of the settling chamber 15 in the form of a mist of separated particles suspended in the gas stream. Particles 13 in the fog are lowered or diffused in a substantially uniform distribution throughout the extended application region 17 near the bottom of the precipitation chamber 15. The pre-initiated sheath flow of purge air over the surface of the article 19 then prevents particles 13 from settling on the surface. The measurement of the particle flux Q provided by the particle counter is transmitted to the counter input of the computer 47 via the data line.

5 shows a typical rise curve for the particle flux Q measured in the settling chamber 15 from the time point t = 0 when the atomizer 11 was first turned on. The first portion 77a of the curve indicates the non-linear rise of ship speed Q from time t = 0 to the equilibrium ship speed level. Rise time (t ₀ ) typically varies from 45 seconds to 5 minutes depending on particle size. It also depends on the density of the material constituting the particles 13 and somewhat depends on the size of the settling chamber and the arrangement of the particle counter. Since the shape is determined only for polystyrene latex spheres and some other particle spheres, the system of the present invention maintains the article surface in the sheath flow 35 under the perforated plate 31 during that time so that inaccurate particle density will not occur. will be. After the flux Q reaches the equilibrium flux level Q ₀ at time t = t ₀ , the precipitation on the surface in the application area 17 depends mainly on the flux level Q ₀ and the particle size S Will actually occur at a certain rate. What the computer 47 observes is for this steady state region 77b of the curve.

Returning to FIG. 4, the computer 47 tilts ( = dQ / dt) or the time ratio of the line speed increase of the measured line speed Q received from the particle counter to determine when the slope becomes virtually zero within the transposition limit (step 79). Once steady state inspection (dQ / dt = 0) is reached, the computer conveys the article out of the sheath flow 35 and into the application area 17 by signaling to the wafer conveying device. The final position 19c or 19d of the article is either half of the sheath flow (step 81a) or completely away from the sheath flow (step 81b), depending on the desired application previously entered by the system operator. The wafer transport time t = t ₀ is stored in the computer 47 by reference.

The particle settling chamber F (Q, S) can be read from a table of values stored in computer memory or ROM, which ROM relates the settling rate to the measured flux value Q ₀ and the particle size S ( Step 83). The table can be constructed by systematic editing of measured particle densities for different equilibrium flux and particle size values on the test wafer, using a surface scanner calibrated according to standard test surfaces made by other methods. Once the particle settling rate (F) is given, the particle density (P (t)) on the surface currently being precipitated into particles is simply P (t) = This becomes P (t) = F · (tt ₀ ) with respect to the stable particle beam thr (Q = Q ₀ ). The precipitation time (T) is equal to t ₁ -t ₀ . Once P (t) is determined (step 85) to reach the desired particle density P previously input by the operator at time t ₁ = t ₀ + (P / F), the article is subjected to a computer control signal. As a result, the wafer transport apparatus is operated again to return to the position 19b purging area under the porous plate (step 87).

It is then removed from the settling chamber by the wafer adjuster 43 and placed 19a.

If there are no other wafers deposited then the system is turned off by stopping the atomizer 11, particle counter and sheath flow (step 89). However, if additional wafers are deposited or if the previously deposited wafers are to be subjected to a second settling (as in FIG. 3 (c)), new wafers are received from the wafer conditioner and cleaned by the system wafer carrier. 17b) (step 91). The desired density or particle count (P) and the desired coverage are then read from the computer memory, which is received from the operator or stores previously entered user information (step 93). The wafer is then conveyed into the application area 17 as in the case of the first wafer (step 81a or 81b). The ship speed Q ₀ is still in equilibrium, so there is no need to wait for the second and subsequent articles to move into the application area 17. The particle size change thus requires the atomizer 11 to turn off and the time for the first size particle 13 fog to subside before the atomizer 11 is turned back on to emit a second size particle fog. To grant permission.

Claims (14)

Atomizer means for releasing particulate matter into the top of the settling chamber suspended in a gas stream and in the form of a mist of separated particles falling into a substantially uniform distribution over the expanded application area near the bottom of the settling chamber, and a particle flux within the settling chamber. Particle counter means for measuring, conveying means for receiving an article having a surface to be precipitated with the particles and for transferring the article between the purge zone of the settling chamber and the expanded application zone, when the article is in the purge zone; Means for providing a purge gas sheath flow across the surface of the article to prevent particle deposition on the surface, a user input for receiving user specified information including specified particle density on the surface, and from the particle counter means When operating the counter input and the conveying means for receiving the measured particle flux Processing control means having a conveying output for increasing, said processing control means calculating a rate of time change of said measured particle flux, determining a time t ₀ when said rate of change of time is substantially zero, and measuring said Processor means for calculating the settling time T from the determined particle flux and the specified particle density, wherein the vehicle is operated at time t ₀ to convey the article to the application area and t ₀ + T) operating the conveying means to return the article to the purge zone. 2. The particle precipitation system of claim 1, wherein the particle counter means comprises a laser providing a beam and a detector disposed relative to the laser to indicate a passage of particles through the beam. 3. The particle precipitation system of claim 2, wherein the particle counter means samples the air in the precipitation chamber. The purge of claim 1, wherein the means for providing a sheath flow comprises a purge gas source, a purge air chamber having a gas inlet for receiving the purge gas, and located directly above the purge region, through which the purge gas can pass. And a porous plate at the bottom of the purge chamber. 5. The particle precipitation system of claim 4, wherein the conveying means conveys the article to the purge zone such that the surface of the article is less than 1 millimeter below the porous plate. The method of claim 1 wherein the user specified information includes a specified coverage of the surface and the conveying means is operable by a control signal from the process control means such that if the specified coverage is full coverage. Conveying the article completely out of the sheath flow and completely into the particle fog, the conveying means also being operable by another control signal from the process control means so that if the specified coverage is some coverage, And partially convey the article into the particle fog only partially out of the sheath flow. The settling time of claim 1, wherein the user information comprises the size of the particles, and the processor means determines the settling rate from the measured particle flux and the particle size, and divides the settling rate by the specified particle density. Particle precipitation system, characterized by calculating (T). 2. The particle precipitation system of claim 1, further comprising filter means for filtering excess gas from the settling chamber at the settling chamber bottom outlet. The particle precipitation system of claim 1, wherein the particles in the gas stream of the precipitation chamber are dry solid particles. (a) receiving user-specified information including at least a specified particle density (P) for the particles to be deposited on the surface; and (b) receiving an article having a surface to be precipitated with the particles into the purification area of the settling chamber. Conveying the article, (c) providing a purge gas sheath flow on the surface of the article, and (d) particulate material into the top of the settling chamber in the form of a mist of separated particles suspended into the gas stream. Wherein the particles in the fog descend into a substantially uniform distribution throughout the expanded application area near the bottom of the settling chamber, the particles on the surface of the article in the purge zone of the settling chamber. (e) continuously measuring the particle flux in the form of fog in the precipitation chamber with a particle counter; and (f) calculating the rate of change of the particle flux from the measured particle flux. Determining a time t ₀ when the rate of change of time is substantially zero, and (g) leaving the article out of the sheath flow of the purge zone at the time t ₀ , wherein the falling particle fog of the application zone And conveying into the surface, whereby the fog particles are deposited on the surface, and (h) the specified particle density from the measured particle flux (Q ₀ ) and the specified particle density specified by the user. Calculating the settling time (T) necessary to achieve, and (i) moving the article out of the falling particle mist of the application zone at time (t ₁ = t ₀ + T) and into the sheath flow of the purification zone. The method of claim 1, comprising the step of returning to the article surface. The method of claim 10, wherein the particle flux is measured by sampling air in the settling chamber, wherein the sampling generates particle counts per volume per unit time indicative of the particle flux. 11. The method of claim 10, wherein the user-specified information includes a designated coverage for the surface with the particles, wherein in the conveying step, if some coverage is specified, the article flows out of the sheath flow into the particle fog. And conveying only a portion and, if specified in its entirety, out of the sheath flow and completely into the fog. The particle flux of claim 10, wherein the user-specified information includes the particle size (S) and the required settling time (T) is calculated from the settling rates (F (Q, S)) and stored in system memory. (Q) and T = P / F (Q ₀ , S) as a known function of particle size (S) and the desired particle density (P). 12. The method of claim 10, further comprising repeating steps (g)-(i) for the additional article.