JP2008177329A - Wet etching method - Google Patents

Abstract

A wet etching method having high controllability in consideration of an in-plane distribution of etching characteristics and a composition change of an etching solution is provided.
A wet etching method for etching a wafer from the back side to reduce the thickness includes a step of measuring a thickness before etching of the wafer to be etched, a thickness before etching, and an allowable thickness before etching stored in a database. In comparison, when the thickness before etching is within the allowable range, and when the thickness before etching is within the allowable range, the wafer etching conditions are selected from the etching conditions stored in the database. A step of supplying an etching solution to the wafer according to the etching conditions, a step of measuring the post-etching thickness of the wafer after etching, a post-etching thickness, and an allowable post-etching thickness stored in a database To determine whether the post-etch thickness is within an acceptable range. And a step.
[Selection] Figure 2

Description

  The present invention relates to a wafer wet etching method, and more particularly, to a wet etching method capable of controlling wafer etching with high accuracy.

In a method for manufacturing a semiconductor device, a process of forming a plurality of semiconductor chips on a wafer, polishing the wafer from the back side of the wafer, and then etching to reduce the film thickness of the wafer is used.
Conventionally, in order to control etching with high accuracy, a method of etching while performing in-situ measurement using a wafer thickness measuring apparatus has been employed. Specifically, there has been proposed an etching method in which a thickness measuring device is attached to an etching device, an etching time is calculated from a change in wafer thickness during etching, and supply of an etching solution is stopped after a predetermined time has elapsed (for example, , See Patent Document 1).
In addition, when the thickness of the processed part is set to a certain level or more, a method is also proposed in which the surface to be processed is scanned with a thickness measuring device, the point to be monitored is determined, and then etching is performed while performing in-situ measurement. (For example, refer to Patent Document 2).
JP 2002-176087 A JP-A-11-154663

However, the conventional etching method measures and controls the thickness of the wafer at a specific point in the wafer or a point on the circumference having a constant radius, and does not consider in-plane uniformity of etching characteristics. It was. For this reason, when such a wet etching method is applied, there is a problem that the average value of the thickness in the wafer plane deviates from the planned thickness even if the measurement point has the planned thickness.
In addition, even when the number of etching processes increases and the chemical composition changes, the conventional etching method cannot detect the chemical composition change, so the etching process is deteriorated and the in-plane uniformity of etching decreases. There were also problems such as.

  Therefore, the present invention has been made to solve such problems, and an object thereof is to provide a wet etching method with high controllability in consideration of in-plane distribution of etching characteristics and chemical composition change. To do.

  The present invention relates to a wet etching method for etching a wafer from the back side to reduce the thickness, the step of measuring the thickness before etching of the wafer to be etched, the thickness before etching, and the allowable thickness before etching stored in a database. And the process of determining whether the thickness before etching is within the allowable range, and when the thickness before etching is within the allowable range, the etching conditions of the wafer are determined from the etching conditions stored in the database. The process of selecting, supplying the etchant to the wafer according to the etching conditions, etching, measuring the post-etch thickness of the wafer after etching, post-etch thickness, and post-etch tolerance stored in the database Compare with the thickness to determine whether the post-etch thickness is within the allowable range. A wet etching method which comprises a step of.

  As described above, in the wet etching method according to the present invention, the wafer thickness and the in-plane uniformity can be controlled with high accuracy.

  In the manufacturing process of a semiconductor device, after a plurality of semiconductor chips are formed on the front surface of the wafer, the wafer is mechanically polished from the back surface, and then the wafer is made to a predetermined thickness by a wet etching process. In the wet etching process, the front surface of the wafer is covered with a protective tape and placed on a rotatable stage with the back surface facing up. Subsequently, an etching solution is supplied onto the back surface of the wafer while rotating the stage, and the wafer is etched from the back surface to reduce the thickness of the wafer. When the wafer is silicon, a mixed acid containing hydrofluoric acid and nitric acid is used as the etching solution. In actual etching, for example, a mixed acid containing hydrofluoric acid, nitric acid, sulfuric acid, and phosphoric acid is used in order to flatten the surface and make it mirror-finished.

FIG. 1 is a configuration diagram of an etching control apparatus used for wet etching according to an embodiment of the present invention.
The etching control device includes a wafer thickness measuring device 11. As the wafer thickness measuring device 11, for example, a measuring device using a non-contact type laser triangulation system is used. In this measuring apparatus, light is incident on the back surface of the wafer at a predetermined angle, the reflected light is detected by a CCD, and the thickness (etching amount) of the wafer is measured from the position of the light incident on the CCD.

  In the wafer thickness evaluation unit 12, an allowable wafer thickness is set in advance for the wafer to be etched before the etching is started and after the etching is completed. An acceptable wafer thickness is usually defined with a certain width.

The etching characteristic evaluation unit 13 calculates the etching time based on the etching data from the measurement data storage unit (database) 14. Specifically, the wafer is determined from the data such as the composition and concentration of the etchant stored in the measurement data storage unit 14, the etching temperature, the etching rate, and the thickness of the wafer measured by the wafer thickness measurement device 11 (the thickness before etching). Calculate the etching time required to etch to the desired thickness. The measurement data storage unit 14 stores values of etching rates for each wafer material, etching solution concentration, etching temperature, and the like.
The wafer thickness data measured by the wafer thickness measuring device 11 is also stored in the measurement data storage unit 14.

  The calculation result of the etching time calculated by the etching characteristic evaluation unit 13 is sent to the processing condition setting unit 15 and used for controlling the etching time. Further, the etching characteristic evaluation unit 13 also calculates the etching rate and the in-plane uniformity of etching. Based on the calculated result, the chemical composition adjusting unit 16 adds a chemical (for example, hydrofluoric acid), exchanges chemicals, that is, supplies waste liquid and new liquid.

  FIG. 2 is a process diagram of single-wafer wet etching according to an embodiment of the present invention. Hereinafter, such wet etching will be described with reference to FIG.

  First, as a pre-stage of wet etching, the surface of the wafer is covered with a protective tape as described above and placed on a rotatable stage with the back surface facing up.

Step S21: The wafer thickness measurement apparatus 11 measures the thickness of the wafer using a non-contact type measurement method such as a laser triangulation method. When the contact-type measurement method is used, scratches are generated on the surface to be etched or foreign matter adheres to serve as a mask during etching to form defects on the etched surface. Therefore, a non-contact measurement method is used.
The measurement points are measured at a plurality of points in the plane, for example, 10 points on a straight line passing through the center of the wafer. At the same measurement point, measurement is performed a plurality of times, for example, about 10 times.
Note that the measurement of the wafer thickness is performed before and after the etching in which the chemical solution does not exist on the wafer because the SN ratio is lowered due to the influence of reflected light or scattered light from the chemical solution during the etching.

  Step S23: FIG. 3 is a top view of the wafer to be etched. A plurality of semiconductor chips 32 are formed on the wafer 31. 4 is an enlarged view of the semiconductor chip 32 of FIG. 3, FIG. 4 (a) is a top view, and FIG. 4 (b) is a side view.

  Normally, in the process of reducing the thickness of the wafer (thinning process), mechanical polishing using a grinder or the like is performed from the back surface of the wafer, and the wet etching according to the present embodiment is performed after polishing to the vicinity of the desired thickness. Etch the remaining part. In the wet etching process, for example, etching of about 20 μm is performed.

However, since the protective pattern 33 made of an insulating film is partially formed on the surface of the chip 32 during mechanical polishing, the wafer undergoes elastic deformation during polishing. As a result, immediately below the protective pattern 33, the protective pattern 33 acts like a beam, so that a partial recess 35 is formed.
Also, due to the thinning of the wafer, the stress generated between the wafer such as silicon and the electrode made of aluminum or the like formed on the wafer appears more prominently, so that the warpage of the wafer increases and macroscopic depressions are formed. It is formed.

  Usually, in the wafer thickness measurement step, for example, as shown in FIG. 5A, it is preferable that the optical path 42 of the measurement light has the same incident angle and reflection angle. However, when the recess 35 as shown in FIG. 4B is formed, the optical path 44 changes as shown in FIG. Since the optical path of the reflected light deviates from the original optical path in this way, the position where the reflected light enters the detector such as a CCD is deviated, and the measured value of the wafer thickness includes an error.

  Therefore, in step S23, an initial value of the assumed thickness of the wafer to be etched is set as a predetermined allowable value (generally a value having a predetermined width) in the wafer thickness evaluation unit 12, and the measured value is If it is outside the allowable value, the process proceeds to step S24, and the etching process is stopped. Thereby, it is possible to prevent the etching time from being set based on an abnormal measurement value (initial value). If the initial value is within the allowable range, the process proceeds to step S25.

Step S25: The wafer is etched according to the etching time calculated in Step S22. For example, when the wafer is silicon, a mixed acid containing hydrofluoric acid and nitric acid is used as an etching solution, and the etching solution is supplied onto the wafer while rotating the wafer for a predetermined etching time. .
After a predetermined etching time has elapsed, the supply of the etching solution is stopped and the wafer is cleaned with pure water or the like.

Step S26: The thickness of the wafer after etching (thickness after etching) is measured again using the wafer thickness measuring device 11. The measurement points are measured, for example, in 10 planes in the same manner as in step S21. For example, 10 measurements are performed on the same measurement point.
If the finished value (thickness after etching) is within the set range, the etching of the wafer is completed, and the chemical solution is evaluated in step S29. On the other hand, if the finished value is out of the set range, the process proceeds to step S28 and the process is stopped.

Step S29: In the etching characteristic evaluation unit 13, the etching rate is calculated from the measured initial value (thickness before etching) and finished value (thickness after etching) of the wafer thickness. Then, it is determined whether or not the etching rate is within a predetermined range. If the etching rate is outside the set range, the process proceeds to step S30, and chemical addition is performed. For example, when a silicon wafer is etched with a mixed solution of hydrofluoric acid and nitric acid, a small amount (for example, about 10 cc) of hydrofluoric acid is added.
On the other hand, if the etching rate is within the set range, the process proceeds to step S31 and the in-plane uniformity of etching is evaluated.

Step S31: It is determined from the finished value of the wafer thickness (thickness after etching) whether or not the etching uniformity is within a predetermined range.
FIG. 6 shows the relationship between the number of wafers processed and the etching in-plane uniformity when a silicon wafer is etched with a mixed acid containing hydrofluoric acid, nitric acid, sulfuric acid, and phosphoric acid. In the etching of the wafer, hydrofluoric acid was replenished each time so that the etching rate did not deviate from the set range (step S30) so that the etching rate was within a predetermined range.
Thus, even if a chemical solution is added and the etching rate is maintained within a predetermined range, the in-plane uniformity of etching decreases with an increase in the number of etching treatments.
The in-plane uniformity of etching is represented by a value obtained by dividing (maximum etching amount−minimum etching amount) by (maximum etching amount + minimum etching amount).

  Therefore, in step S31, the in-plane uniformity of the etching rate is obtained, and it is determined whether or not the in-plane uniformity is within a predetermined setting range. And when it remove | deviates from the predetermined setting range, it progresses to process S32, a process is stopped, and all etching liquid is replaced | exchanged.

  On the other hand, if the in-plane uniformity is within a predetermined setting range, the wafer etching process is completed, and the wafer is moved to the next manufacturing process.

7 and 8 show distributions of wafer thicknesses (pre-etching thickness and post-etching thickness) before and after wet etching (single-wafer spin wet etching) according to the present embodiment.
As shown in FIG. 7, with respect to the wafer 61, the measurement is performed with respect to a measurement point 62 composed of 20 points (points 1 to 20) on a straight line passing through the center of the wafer and a measurement point 63 on a predetermined radius. went.

FIG. 8 shows measurement results of wafer thickness before and after etching for the measurement points (points 1 to 20) shown in FIG. As is apparent from FIG. 8, the wafer thickness has a distribution in the radial direction of the wafer. Therefore, in this wafer, for example, in order to control the average thickness of the wafer by a measurement method using one measurement point, it is necessary to measure the wafer thickness on the measurement point 71 or the measurement point 72. However, in this way, the measurement point indicating the average thickness is not necessarily located at the position indicated by 71 or 72 depending on the conditions of the mechanical polishing process performed before the wet etching. It is difficult.
In contrast, in the measurement method according to the present embodiment, the thickness is measured at a plurality of points in the radial direction within the wafer surface, so that the etching characteristics of the wafer can be controlled with high accuracy.

  In the present embodiment, for example, a measuring device using a non-contact type laser triangulation method is used as the wafer thickness measuring device 11, but instead of this, for example, Japanese Patent Laid-Open No. 2002-176087. A measurement apparatus using interference of measurement light as described in 1) may be used. In such a measuring apparatus, the measurement light is reflected on the back surface of the wafer and on the front surface of the wafer through the wafer, and the thickness of the wafer is detected from the interference of the reflected light. In this case, the thickness of the portion excluding the electrode material such as aluminum formed on the wafer surface can be controlled.

It is a block diagram of the etching control apparatus concerning embodiment of this invention. It is process drawing of the wet etching by embodiment of this invention. 1 is a wafer to be etched according to an embodiment of the present invention. It is an enlarged view of the semiconductor chip contained in a wafer. It is an optical path of measurement light used for measuring the thickness of a wafer. This is the relationship between the number of processed wafers and the uniformity within the etched surface. This is the thickness measurement point on the wafer. The wafer thickness measured at each measurement point on the wafer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 Wafer thickness measuring apparatus, 12 Wafer thickness evaluation part, 13 Etching characteristic evaluation part, 14 Measurement data storage part, 15 Process condition setting part, 16 Chemical composition adjustment part

Claims (6)

A wet etching method for etching a wafer from the back surface to reduce the thickness,
Measuring the thickness before etching of the wafer to be etched;
Comparing the thickness before etching and the allowable thickness before etching stored in the database to determine whether the thickness before etching is within an allowable range;
A step of selecting etching conditions for the wafer from etching conditions stored in a database when the pre-etching thickness is within an allowable range;
Etching the wafer by supplying an etchant according to the etching conditions;
Measuring the post-etch thickness of the wafer after etching;
Comparing the post-etching thickness with an allowable post-etching thickness stored in a database and determining whether the post-etching thickness is within an allowable range or not. .   2. The method according to claim 1, further comprising: calculating an etching rate from the thickness before etching and the thickness after etching, and replenishing the etching solution with a chemical when the etching rate is smaller than an allowable range. Wet etching method.   The wafer is made of silicon, the etchant is made of a mixed acid containing hydrofluoric acid and nitric acid, and hydrofluoric acid is replenished as the chemical solution when the etching rate is smaller than an allowable range. The wet etching method according to claim 2.   The in-plane distribution of the etching rate is calculated from the thickness before etching and the thickness after etching, and the etching solution is replaced when the in-plane distribution is larger than an allowable range. Wet etching method.   2. The wet etching method according to claim 1, wherein the thickness before etching and the thickness after etching are measured at a plurality of measurement points on a straight line in the radial direction of the wafer.   The wet etching method according to claim 1, wherein the thickness before etching and the thickness after etching are each measured a plurality of times for the same measurement point.

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