METHOD FOR DEPOSITING AN EPITATICAL LAYER ON A SUBSTRATE WAFER

Description

[0001] The invention relates to a method for depositing an epitaxial layer on a substrate wafer. State of the art / problems

[0002] For demanding applications in the electronics industry, epitaxially coated semiconductor wafers, in particular substrate wafers made of single-crystal silicon, which are epitaxially coated with a layer of silicon, are required.

[0003] A frequently used method for depositing the layer is chemical vapor deposition. The material for the layer is provided by process gas, which is passed over the side of the substrate wafer to be coated at temperatures at which a precursor compound containing the material contained in the process gas chemically cleaves. During the deposition of the layer, the substrate wafer lies in the pocket of a susceptor, surrounded by an annular boundary of the susceptor. The process is usually carried out in a device designed as a single-wafer reactor. Such a single-wafer reactor is described, for example, in EP 0 870 852 A1.

[0004] In order for the epitaxial layer to grow as evenly as possible on the substrate wafer, it is particularly important that the substrate wafer is positioned centrally to the boundary in the pocket.

[0005] In US 2010 0216261 A1 it is proposed to monitor the correct position of the substrate wafer on the susceptor by means of a camera system.

[0006] WO17 135 604 A1 describes measuring a thickness characteristic in the edge region after the epitaxial layer has been deposited and changing certain process parameters for subsequent deposition processes depending on the measurement result.

[0007] WO 14 103 657 A1 and JP2015 201 599 A disclose methods which calculate an eccentricity based on the distribution of thickness parameters in the edge region of the semiconductor wafer with epitaxially deposited layer in order to correct the position of a subsequent substrate wafer on the susceptor as a function of this eccentricity.

[0008] US2009 252 942 A1 and JP2002 043 230 A propose measuring the flatness of the substrate wafer before depositing the epitaxial layer and changing certain process parameters during deposition so that edge drop is reduced as the substrate wafer is coated.

[0009] However, thickness differences in the edge region of the epitaxially coated substrate wafer cannot be avoided with this procedure if the substrate wafer itself already has thickness differences in the edge region.

[0010] The object of the invention is to offer a solution to the problem which makes it possible to adjust the thickness of the epitaxial layer in the edge region differently at different positions of the circumference of the substrate wafer.

[0011] The object of the invention is achieved by a method for depositing an epitaxial layer on a substrate wafer from a gas phase, comprising measuring an edge geometry of the substrate wafer, which assigns a thickness characteristic value to an edge of the substrate wafer as a function of edge positions, wherein the substrate wafer has a thicker edge portion and a thinner edge portion which are opposite one another; placing the substrate wafer in a pocket of a susceptor of a device for depositing the epitaxial layer, wherein the pocket is surrounded by a boundary with a circular circumference; heating the substrate wafer; and passing process gas over the substrate wafer; characterized by placing the substrate wafer in the pocket such that a distance which the substrate wafer has from the boundary of the pocket is smaller at edge positions of the thicker edge section with a larger thickness characteristic than at edge positions of the thinner edge section with a smaller thickness characteristic, wherein the thickness value is either the ZDD of the front side defined according to SEMI standard M68-0720 or the ESFQR defined according to SEMI standard M67-0720.

[0012] The invention is based on the finding that the centered placement of the substrate wafer in the pocket is accompanied by the expectation that the substrate wafer will have a uniform edge geometry. In practice, however, this is rarely the case because processing steps such as grinding and polishing are not capable of creating a substrate wafer with a completely uniform edge thickness. The method according to the invention is capable of standardizing the edge geometry of an epitaxially coated semiconductor wafer compared to an uncoated substrate wafer, in particular when the substrate wafer has a comparatively thin edge thickness in one edge section and a comparatively thick edge thickness in the opposite edge section. Notwithstanding this, homogenization of the edge thickness can also be achieved if, for example, only one edge section with a comparatively thin edge thickness is present.

[0013] First, the edge geometry of the substrate wafer is measured so that it is available in the form of a thickness parameter. Preferably, an outermost edge piece with a radial length of 1 mm or 2 mm is excluded from the measurement as an edge exclusion. In principle, any measured value that allows a statement to be made about the relative thickness at two different sections in the edge region of the substrate wafer can be used as a thickness parameter. According to the invention, the ZDD (Z double derivative) of the front side, which describes the curvature of the edge drop and is defined in the SEMI standard M68-0720, or the ESFQR, which quantifies the flatness of sectors (sites) in the edge region and is defined in the SEMI standard M67-0720, is suitable as a thickness parameter. The following description uses ESFQR as a representative thickness parameter.

[0014] The method according to the invention provides for measuring the edge geometry of a substrate wafer before it is coated with an epitaxial layer. This produces, for example, a map that assigns an ESFQR value to each sector and thus displays a thickness profile of the substrate wafer along its circumference. Substrate wafers with a wedge-like cross-section or those that are thinner at one edge section than at others are particularly suitable. The term "edge section" refers to an edge region that extends over a distance of up to 50% of the circumference, preferably 7% to 42% of the circumference in the circumferential direction. A substrate wafer with a wedge-like shape has a thicker edge section and a thinner edge section that are opposite one another, i.e., have the greatest possible distance from one another.

[0015] For the deposition chamber used to coat the substrate wafer, a single-wafer reactor, a correlation function is created that assigns a thickness value to a displacement vector. The displacement vector indicates the direction and amount by which the substrate wafer, with the thicker edge section, must be offset from its centered position into the susceptor pocket so that more material is deposited on the opposite, thinner edge section during deposition of the epitaxial layer than on the thicker edge section. Accordingly, the thickness value after deposition in the thinner edge section of the substrate wafer is greater by the assigned thickness value than it would be after deposition of an epitaxial layer during which the substrate wafer lies centered in the susceptor pocket.The displacement vector thus represents the eccentricity of the position of the center of the substrate wafer resting on the susceptor compared to the position the center would have if the substrate wafer were centered in the susceptor pocket. The correlation function is determined experimentally before coating the substrate wafer by determining which eccentricity results in which change in the thickness parameter.

[0016] The placement of the substrate wafer into the susceptor pocket is preferably performed by a robot, which performs this task according to the correlation function. Alternatively or additionally, the robot can be configured as a self-learning system that uses the measured geometry data of the substrate wafers and the resulting epitaxially coated semiconductor wafers to determine and implement the necessary eccentricity for coating subsequent substrate wafers. Furthermore, it is preferable to monitor the placement of the substrate wafer on the susceptor and its position in the susceptor pocket using a camera system.

[0017] The substrate wafer and the epitaxial layer deposited thereon preferably consist essentially of semiconductor material, for example, monocrystalline silicon. The substrate wafer preferably has a diameter of at least 200 mm, particularly preferably at least 300 mm. The epitaxial layer preferably has a thickness of 1 µm to 20 µm.

[0018] The invention is described in more detail below with reference to drawings. Short description of the characters

[0019] Fig. 1 shows qualitatively how the invention works. Fig. 2 shows a substrate wafer placed in the pocket of the susceptor according to the invention. Fig. 3 shows a correlation function that assigns an expected change in a thickness parameter to an eccentricity E. Fig. 4 shows the deviation of the thickness of a substrate wafer from a target thickness along its diameter Fig. 5shows the deviation of the thickness of the epitaxially coated substrate wafer according to Fig.4 from a target thickness along the diameter of the epitaxially coated substrate wafer Fig. 6 shows the deviation of the thickness of the epitaxial layer of the coated substrate wafer according to Fig. 5 of a target thickness along the diameter of the epitaxial layer. Fig. 7 shows the course of the deviation of the thickness parameter from a target value along the circumference of a substrate wafer before and after the deposition of an epitaxial layer. List of reference symbols used

[0020] 1 Substrate disc 2 epitaxial layer 3 Susceptor 4 Bag 5 thick edge section of the substrate disc 6 thin edge section of the substrate wafer 7 outer boundary of the pocket 8 Map E eccentricity ESFQR ESFQR value Δ ESFQRDifference between the ESFQR value and the target value Δt Difference between thickness t and target thickness d diameter WP Marginal position Detailed description of embodiments according to the invention

[0021] Fig.1 shows a sectional view (vertical section) of a substrate wafer 1 deposited in the pocket 4 of a susceptor 3, which has a comparatively thick edge section 5 and an opposite, comparatively thin edge section 6. The substrate wafer 1 is not centered in the pocket 4, but rather eccentrically, such that the thicker edge section 5 is a smaller distance from a boundary 7 of the pocket than the thinner edge section 6. This configuration has the consequence that where the distance is smaller, the rate of material growth during the deposition of the epitaxial layer is lower than where the distance is larger.

[0022] The lower part of Fig.1shows the situation after the deposition of the epitaxial layer 2. The substrate wafer 1 has become a substrate wafer with a deposited epitaxial layer, wherein the epitaxial layer 2 is thinner in the region of the edge section with a smaller distance to the boundary 4 than in the region of the edge section with a greater distance to the boundary 4. If the thickness of the substrate wafer with deposited epitaxial layer 2 is observed and compared with the thickness of the substrate wafer 1, it can be seen that the thickness of the epitaxially coated substrate wafer is more uniform in its edge region than the thickness of the substrate wafer 1. The differences in thickness in the edge region are more pronounced in the case of the substrate wafer 1 than in the case of the epitaxially coated substrate wafer.

[0023] Fig. 2shows a plan view of a substrate wafer placed in the pocket of the susceptor according to the invention. The substrate wafer 1 is not centered in the pocket of the susceptor. The center of the substrate wafer is shifted from the center of the pocket according to the eccentricity E, specifically such that the thicker edge section 6 is a smaller distance from the outer boundary 7 of the pocket than the thinner edge section 6. The direction and magnitude of the eccentricity E depend on a measurement of the edge geometry of the substrate wafer, for example, by measuring the flatness in 5° wide edge segments in the form of the ESFQR value. The result of the measurement is qualitatively indicated by the map 8 and shows the thicker edge section 5 highlighted with "+" symbols and the thinner edge section 6 highlighted with "-" symbols.

[0024] The selected eccentricity E is based on a correlation function determined through preliminary tests for the deposition device used. This investigates the change in the thickness parameter that can be expected when the substrate wafer is deposited in the pocket of the susceptor with a specific eccentricity E. In the selected example, the correlation function according to Fig. 3 which difference ΔESFQR of the ESFQR value in the thinner edge section is obtained after deposition if the substrate wafer is not placed centrally but with the corresponding eccentricity E in the pocket of the susceptor before deposition. The correlation function according to Fig. 3This suggests that after deposition of the epitaxial layer, the ESFQR value in the sectors of the thinner edge section will be approximately 2 nm higher, provided that the substrate wafer was not placed centrally in the pocket of the susceptor before deposition, but with an eccentricity of 100 µm.

[0025] Fig. 4, Fig. 5 and Fig. 6 show the course of the deviation of a thickness from a target thickness of a substrate wafer made of single-crystalline silicon ( Fig. 4 ) and after ( Fig. 5 ) the deposition of an epitaxial layer and the deviation of the thickness of the epitaxial layer ( Fig. 6 ) of a target thickness. According to Fig. 4The substrate wafer had significantly different edge thicknesses, with a thinner edge section in the range of 0 mm and a thicker edge section in the range of 300 mm. After deposition of the epitaxial layer according to the present invention, the difference in edge thicknesses in these edge sections was almost equalized ( Fig. 5 ). The thickness deviation of the epitaxial layer according to Fig. 6 shows that, due to the application of the method according to the invention, the epitaxial layer is thicker at the edge position corresponding to the thinner edge section of the substrate wafer and thinner at the edge position corresponding to the thicker edge section of the substrate wafer. The eccentric position of the substrate wafer resulted in different amounts of material being deposited at the edge sections under consideration.

[0026] The inventive procedure improves the edge geometry, even when viewed over the entire circumference. Fig. 7 shows, in the form of smoothed curves, the edge geometry of a substrate wafer before (curve A) and after (curve B) the deposition of an epitaxial layer, in the form of the ESFQR value as a function of the edge position WP. The curves are based on 72 measurement points, each of which describes the ESFQR value of a sector with a width of 5° and a radial length of 35 mm, taking into account an edge exclusion of 1 mm.

[0027] The foregoing description of exemplary embodiments is intended to be exemplary. The disclosure thus made will enable those skilled in the art to understand the present invention and the associated advantages, and will also encompass obvious variations and modifications of the described structures and methods within the skill of the art's understanding.

Claims

1. Method for depositing an epitaxial layer on a substrate wafer from a vapour phase, comprising measuring an edge geometry of the substrate wafer that assigns a thickness characteristic value to an edge of the substrate wafer as a function of edge positions, the substrate wafer having a thicker edge section and a thinner edge section which are opposite each other; placing the substrate wafer in a pocket of a susceptor of a device for depositing the epitaxial layer, the pocket being surrounded by a boundary with a circular circumference; heating the substrate wafer; and passing process gas over the substrate wafer; characterized by placing the substrate wafer in the pocket such that the distance from the substrate wafer to the boundary of the pocket is smaller at edge positions of the thicker edge section with greater thickness characteristic value than at edge positions of the thinner edge section with smaller thickness characteristic value, wherein the thickness value is either the ZDD of the front side as defined according to the SEMI standard M68-0720 or the ESFQR as defined according to the SEMI standard M67-0720.

2. Method according to Claim 1, characterized by placing the substrate wafer by means of a robot.

3. Method according to Claim 1 or 2, characterized by monitoring the position of the substrate wafer in the pocket by means of a camera system.

4. Method according to any one of Claims 1 to 3, characterized by depositing an epitaxial layer of silicon on a single-crystal silicon substrate wafer.