Image sensor and method of fabrication

Abstract

The application provides an image sensor and a forming method. The method comprises the following steps: forming a first epitaxial layer on a substrate; and forming an initial first gettering layer in the first epitaxial layer by using a carbon ion implantation process. The carbon ion can effectively absorb metal ions. By setting an annealing temperature and an annealing process, the carbon ion can rapidly react with oxygen ions and generate carbon-oxygen clusters, so as to effectively improve the gettering rate of the metal ions. Meanwhile, the annealing process can also repair the damage caused by the carbon ion implantation. The application can also capture the oxygen ions and the metal ions escaping from the substrate by step by step through the setting of the multi-layer initial first gettering layer.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and more specifically to an image sensor and its fabrication method. Background Technology

[0002] Compared to semiconductor silicon wafers, photovoltaic silicon wafers have lower requirements in terms of purity and warpage. Regarding purity, photovoltaic monocrystalline silicon wafers require a silicon content of 4N to 6N (99.99%-99.9999%), while semiconductor monocrystalline silicon wafers require around 9N (99.9999999%) to 11N (99.999999999%) – the purity requirement is at least 1000 times higher for photovoltaic monocrystalline silicon wafers. In terms of appearance, semiconductor silicon wafers have higher requirements for surface flatness, smoothness, and cleanliness than photovoltaic silicon wafers.

[0003] With the development of image sensing technology, CMOS image sensors are increasingly replacing CCD image sensors in various electronic products. Due to the influence of metal particles, even when no light shines on the pixel unit, the pixel itself will generate an electric charge. As this charge increases and accumulates, it forms a dark current. For a single pixel unit, when the dark current value exceeds the photocurrent generated by capturing photons, the pixel will be defaulted to a white pixel by the control circuit. Therefore, CMOS image sensors are highly sensitive to metal contamination.

[0004] There is an urgent need for a method to form an image sensor that applies photovoltaic silicon wafers to semiconductor silicon wafers, reducing manufacturing costs while improving the performance of the image sensor. Summary of the Invention

[0005] The purpose of this invention is to provide an image sensor and a method for forming it, so as to avoid the influence of impurity ions contained in the substrate on the performance of the image sensor.

[0006] Based on the above considerations, the present invention provides an image sensor, comprising: step S01: providing a substrate; step S02: forming a first epitaxial layer on the substrate; step S03: forming an initial first getter layer in the first epitaxial layer using a carbon ion implantation process.

[0007] Preferably, the initial first getter layer is adjacent to the top surface of the first epitaxial layer.

[0008] Preferably, it further includes: step S04: annealing the initial first getter layer to form a first getter layer.

[0009] Preferably, the first getter layer comprises carbon-oxygen clusters.

[0010] Preferably, before step S04, steps S02 and S03 are executed several times in a loop to form a first epitaxial layer with the initial first getter layer in a corresponding manner on the substrate.

[0011] Preferably, before step S04, the method further includes: forming a second epitaxial layer on the first epitaxial layer.

[0012] Preferably, the method further includes: step S05: forming a plurality of photosensitive units in the second epitaxial layer.

[0013] Preferably, before step S05, an initial second getter layer is formed in the second epitaxial layer using a carbon ion implantation process.

[0014] Preferably, the annealing process further treats the initial second getter layer to form a second getter layer.

[0015] Preferably, each photosensitive unit has a pinning layer adjacent to the second epitaxial layer, the pinning layer being formed under the initial second getter layer.

[0016] Preferably, the method further includes: step S06: removing the substrate, the first getter layer, and the first epitaxial layer.

[0017] Preferably, the carbon ion implantation process uses an implantation energy of 4 keV to 80 keV and an implantation dose of 1E14 atom / cm². 2 ~5E15atom / cm 2 .

[0018] Preferably, the annealing process includes a low-temperature treatment followed by a high-temperature treatment.

[0019] Preferably, the low-temperature treatment temperature includes 600℃ to 850℃, and the high-temperature treatment temperature includes 850℃ to 1200℃.

[0020] Preferably, the annealing process further includes a pre-annealing treatment before the low-temperature treatment.

[0021] Preferably, the temperature of the pre-annealing treatment includes 1000℃ to 1200℃.

[0022] Another aspect of the present invention provides an image sensor formed using the above-described forming method.

[0023] This invention involves forming a first epitaxial layer on a substrate and then using carbon ion implantation to form an initial first getter layer within that epitaxial layer. Carbon ions can effectively adsorb metal ions. Furthermore, by setting the annealing temperature and process, the carbon ions can rapidly react with oxygen ions to generate carbon-oxygen clusters, effectively increasing the gettering rate for metal ions. Simultaneously, the annealing process also repairs damage caused during carbon ion implantation.

[0024] Furthermore, the present invention employs a multi-layered initial first getter layer to progressively capture oxygen ions and metal ions escaping from the substrate. Attached Figure Description

[0025] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings.

[0026] Figures 1-4 A cross-sectional schematic diagram showing a method for forming an image sensor according to Embodiment 1 of the present invention is shown;

[0027] Figures 5-7 A cross-sectional schematic diagram of a method for forming an image sensor according to Embodiment 2 of the present invention is shown.

[0028] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0029] To make the content of this invention clearer and easier to understand, the following description, in conjunction with the accompanying drawings, further illustrates the invention. Of course, this invention is not limited to this specific embodiment, and common substitutions well-known to those skilled in the art are also covered within the scope of protection of this invention.

[0030] It should be noted that in the following specific embodiments, in order to clearly illustrate the structure of the present invention and facilitate explanation, the structure in the accompanying drawings is not drawn to scale and has been partially enlarged, deformed and simplified. Therefore, it should be avoided to interpret this as a limitation of the present invention.

[0031] As described in the background section, due to differences in manufacturing processes and product performance, photovoltaic silicon wafers suffer from problems such as poor purity and unevenness, making them unsuitable for direct application in semiconductor manufacturing processes.

[0032] Chinese invention patent application 201380059268.6 discloses a method for manufacturing a semiconductor epitaxial wafer, a semiconductor epitaxial wafer, and a solid-state imaging element. It points out that when monomer ions (single ions) are implanted into a semiconductor wafer before the epitaxial layer is formed, there is a problem of insufficient gettering ability. Therefore, by irradiating a semiconductor wafer having at least one bulk semiconductor wafer containing carbon and nitrogen with cluster ions, the energy of each atom or molecule collides with the semiconductor wafer is lower than that of the monomer ions. It also points out that since multiple atoms can be irradiated at once, the peak concentration of the depth direction profile of the irradiated element can be made high, and the peak position is located closer to the surface of the semiconductor wafer, thereby improving the gettering ability.

[0033] As described above, the manufacturing method is performed on a semiconductor wafer, and the carbon concentration of the semiconductor wafer is limited to 1×10⁻⁶. 15 atoms / cm 3 Above 1×10 17 atoms / cm 3 The following (ASTM F123 1981) specifies a nitrogen concentration of 5 × 10⁻⁶. 12 atoms / cm 3 Above 5×10 14 atoms / cm 3 The above manufacturing method is no longer applicable to photovoltaic silicon wafers with lower purity.

[0034] To address the problems in the prior art, this invention provides a method for manufacturing an image sensor. The technical approach of this invention is to form a first epitaxial layer on a substrate, and then use a carbon ion implantation process to form an initial first getter layer within the first epitaxial layer. The initial first getter layer actively attracts oxygen defects and / or metal ions from the substrate, and through annealing, it is further transformed into a first getter layer containing carbon-oxygen clusters, thereby enhancing the adsorption capacity of metal ions and avoiding interference from metal ions to the image sensor.

[0035] The following detailed description of a method for forming an image sensor according to the present invention, with reference to specific embodiments and accompanying drawings, will illustrate the invention in detail.

[0036] Example 1

[0037] Figure 1 This diagram shows a cross-sectional view of an image sensor according to Embodiment 1 of the present invention, as shown below. Figure 1 As shown, the image sensor includes a substrate 100, which is made of silicon. In other embodiments, the substrate 100 may also be made of germanium, silicon germanide, silicon carbide, gallium arsenide, or indium gallium ionide. Depending on the type of photosensitive area of ​​the image sensor, the substrate 100 may also be doped with different types of impurity ions. Those skilled in the art can select the type of substrate 100 based on the semiconductor device formed on the substrate 100; therefore, the type of substrate 100 should not limit the scope of protection of this invention.

[0038] The impurity ions can be oxygen ions and / or metal ions. In this embodiment, the substrate 100 is a photovoltaic silicon wafer containing oxygen ions 102 and metal ions 101, in order to reduce raw material costs and alleviate the impact of raw material shortages on production capacity.

[0039] Next, a first epitaxial layer 111 is formed on the substrate 100. The first epitaxial layer 111 covers the upper surface of the substrate 100 to isolate the substrate 100 and the subsequently formed photosensitive unit.

[0040] However, as Figure 2 As shown, carbon ions 103 are implanted using a carbon ion implantation process to form an initial first getter layer 112" within the first epitaxial layer 111. The carbon ions 103 can effectively adsorb metal ions 101, and at the same time, they can react rapidly with oxygen ions 102. Under the action of the annealing process, carbon-oxygen clusters are generated to enhance the gettering of metal ions 101, effectively improving the gettering rate of metal ions.

[0041] In this embodiment, a second epitaxial layer 111 is formed on the first epitaxial layer 111 of the first layer that has already been formed, and an initial first getter layer 112" is formed in the first epitaxial layer 111 of the second layer.

[0042] Depending on the impurity content of the photovoltaic silicon wafer, the above steps are repeated cyclically to form several stacked first epitaxial layers 111 on the substrate 100. Each first epitaxial layer 111 is implanted with carbon ions 103 through a carbon ion implantation process to form an initial first getter layer 112". By setting multiple initial first getter layers 112", oxygen ions 102 and metal ions 101 escaping from the substrate 100 are captured step by step.

[0043] The carbon ion implantation process involves implantation energies ranging from 4 keV to 80 keV and implantation doses ranging from 1E14 atom / cm². 2 ~5E15atom / cm 2 Preferably, the initial first getter layer 112" is adjacent to the top surface of the first epitaxial layer 111, thereby reducing the surface bombardment of the first epitaxial layer 111 by the carbon ion implantation process.

[0044] The initial first getter layer 112" can have a thickness of 0.5 μm to 2 μm. A thinner initial first getter layer 112" combined with a carbon ion implantation process using a lower implantation energy, for example, an implantation dose of 1E14 atom / cm², allows for successful implantation. 2 ~5E14atom / cm 2 This avoids damage to the substrate 100 and forms several stacked initial first getter layers 112", which accelerates the adsorption rate of metal ions and oxygen ions on the substrate 100.

[0045] Then, the initial first getter layer 112 is treated with an annealing process to form the first getter layer 112. The annealing process can be performed at a temperature of 700℃ to 1150℃ for 10-360 minutes. Under the annealing process, not only is the damage caused by carbon ion implantation repaired, but the carbon ions, as gettering points, can also attract and suppress the diffusion risk of various metal ions and oxygen ions.

[0046] The carbon ions 103 can adsorb oxygen ions 102 and metal ions 101. Please refer to [reference needed]. Figure 3 Since oxygen ions escape faster than metal ions 101, carbon ions 103 adsorb oxygen ions 102 first. By setting several stacked first epitaxial layers 111, the number of oxygen ions 102 and metal ions 101 gradually decreases from bottom to top of the substrate 100, thus avoiding leakage of oxygen ions 102 and metal ions 101.

[0047] In this embodiment, as Figure 4 As shown, a low-temperature treatment is first applied, with the temperature ranging from 600℃ to 850℃ and the time from 1 hour to 8 hours. The low-temperature treatment causes carbon ions 103 and oxygen ions 102 to agglomerate and form nuclei, resulting in carbon-oxygen clusters 104.

[0048] like Figure 5 As shown, a high-temperature treatment is then applied, with the temperature ranging from 850°C to 1200°C and the time from 1 hour to 16 hours. During this high-temperature treatment, carbon-oxygen clusters 104 grow. The corresponding carbon-oxygen clusters 104 within the initial first getter layer 112" grow and connect to form the first getter layer 112. Metal ions 101 escape from the substrate 100 and are captured by the grown carbon-oxygen clusters 104, thereby preventing the precipitation of metal ions 101 and oxygen ions 102 onto the device layer of the image sensor on the first epitaxial layer 111.

[0049] Example 2

[0050] The present invention isolates metal ions 101 and / or oxygen ions 102 within the substrate 100 by forming several initial first getter layers 112" on the substrate 100.

[0051] like Figure 6 As shown, in Embodiment 2, the second epitaxial layer 113 is formed on the first epitaxial layer 111 and disposed on the substrate 100, and a plurality of photosensitive units (not shown) are formed in the second epitaxial layer 113.

[0052] Depending on the type of the photosensitive unit, the second epitaxial layer 113 can be doped with different types of impurity ions. When the type of the photosensitive unit is N-type, the second epitaxial layer 113 is doped with P-type impurity ions, wherein the P-type impurity ions are one or more of boron ions, gallium ions, or indium ions; or, when the type of the photosensitive unit is P-type, the second epitaxial layer 113 is doped with N-type impurity ions, wherein the N-type impurity ions are phosphorus ions.

[0053] The pinning layer (not shown) is doped with boron (B) ions and formed on the second epitaxial layer 113 to passivate surface defects of the second epitaxial layer 113 on the photosensitive unit.

[0054] Typically, ion sources used to form the pinning layer include boron (B) ion sources and boron difluoride (BF2) ion sources. BF2 has a low diffusion coefficient, so it hardly segregates to the surface during subsequent annealing and does not affect the isolation performance between the photosensitive unit and surrounding transistors. However, the metal components of the process equipment contain molybdenum (Mo), which ionizes during the process to generate Mo ions. Since the charge-to-mass ratio of BF2 ion clusters is close to that of Mo ions, the deflection magnetic field cannot effectively filter out Mo ions during boron difluoride (BF2) ion implantation. This leads to Mo ion implantation and the introduction of deep energy levels into the Si bandgap, resulting in increased dark current and ultimately, WP defects.

[0055] While boron (B) ion implantation can effectively avoid Mo contamination, boron ions diffuse at a high rate in Si and tend to segregate to the silicon wafer surface during subsequent annealing, forming precipitation defects. This leads to a deterioration in WP secondary properties and also narrows the isolation area between the photodiode and surrounding transistors, posing a risk of reduced FWC leakage.

[0056] In Example 2, before the pinning layer is formed, an initial second getter layer 114" is formed in the second epitaxial layer 113 using a carbon ion implantation process. The initial second getter layer 114" is adjacent to the top surface of the second epitaxial layer 113, and the pinning layer is formed below the initial second getter layer 114".

[0057] The initial second getter layer 114" can be used to adsorb Mo ions incorporated during boron difluoride (BF2) ion source implantation, and can also be used to prevent boron ion segregation during boron (B) ion source implantation, thereby improving process compatibility.

[0058] like Figure 6 As shown, since the pinning layer is formed near the surface of the second epitaxial layer 113, the implantation dose of carbon ion implantation corresponding to the initial second getter layer 114" is higher than the implantation dose of carbon ion implantation corresponding to the initial first getter layer 112". Preferably, the implantation dose of the initial second getter layer 114" is greater than or equal to 5E15 atom / cm 2 However, this will damage the surface of the second epitaxial layer 113.

[0059] like Figure 7As shown, after the initial first getter layer 112" and the initial second getter layer 114" are formed on the substrate 100, a pre-annealing treatment is first performed before the low-temperature treatment to repair the surface damage of the second epitaxial layer 113 caused by carbon ion implantation, and to promote the adsorption of oxygen ions from the external environment by the initial second getter layer 114". The temperature of the pre-annealing treatment includes 1000℃ to 1200℃, and the time includes 10min to 120min. The use of a higher temperature pre-annealing treatment can accelerate the precipitation rate of oxygen ions and metal ions in the substrate 100, thereby saving the time of the low-temperature treatment. Then, low-temperature treatment and high-temperature treatment are performed in sequence to promote the initial first getter layer 112" and the initial second getter layer 114" to fully absorb oxygen ions and form the first getter layer 112 and the second getter layer 114 respectively.

[0060] Then, a plurality of photosensitive units are formed in the second epitaxial layer 113, and a device layer (not shown) and a bonding layer (not shown) are sequentially formed on the second epitaxial layer 113, and bonded to another substrate (not shown) through the bonding layer. Then, the substrate 100, the first getter layer 112 and the first epitaxial layer 111 are removed.

[0061] The present invention avoids the influence of impurity ions in the substrate on the performance of the image sensor by forming a first epitaxial layer on a substrate, forming an initial first getter layer or a first getter layer within the first epitaxial layer, forming a device layer of a back-illuminated image sensor on the first epitaxial layer, and then removing the substrate and the first epitaxial layer, or forming a device layer of a front-illuminated image sensor on the first epitaxial layer.

[0062] Furthermore, before the formation of the photosensitive unit, the present invention forms an initial second getter layer within the second epitaxial layer. This serves two purposes: firstly, to mitigate metal contamination during the pinning layer formation process; and secondly, to repair surface damage to the second epitaxial layer caused by ion implantation through annealing. Additionally, during the formation of the photosensitive unit, metal impurities may be introduced into the second epitaxial layer. These impurities can be adsorbed by the initial second getter layer on the photosensitive unit and / or the initial first getter layer beneath the photosensitive unit, thereby reducing the dark current of the image sensor.

[0063] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and not restrictive in any way. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude a plurality. Multiple elements recited in the apparatus claims may also be implemented by a single element. The terms "first," "second," etc., are used to denote names and do not indicate any particular order.

Claims

1. A method for forming an image sensor, characterized in that, include: Step S01: Provide a substrate; Step S02: Form a first epitaxial layer on the substrate; Step S03: An initial first getter layer is formed in the first epitaxial layer using a carbon ion implantation process; Step S04: Form a second epitaxial layer on the first epitaxial layer; Step S05: Annealing the initial first getter layer to form the first getter layer; Step S06: An initial second getter layer is formed in the second epitaxial layer using a carbon ion implantation process; Step S07: A plurality of photosensitive units are formed in the second epitaxial layer.

2. The forming method as described in claim 1, characterized in that, The initial first getter layer is adjacent to the top surface of the first epitaxial layer.

3. The forming method as described in claim 1, characterized in that, The first getter layer comprises carbon-oxygen clusters.

4. The forming method as described in claim 1, characterized in that, Before step S05, steps S02 and S03 are executed several times in a loop to form a first epitaxial layer with the initial first getter layer in a corresponding manner on the substrate.

5. The forming method as described in claim 1, characterized in that, The annealing process further treats the initial second getter layer to form a second getter layer.

6. The forming method as described in claim 1, characterized in that, Each of the photosensitive units has a pinning layer adjacent to the second epitaxial layer, the pinning layer being formed under the initial second getter layer.

7. The forming method as described in claim 1, characterized in that, Also includes: Step S08: Remove the substrate, the first getter layer, and the first epitaxial layer.

8. The forming method as described in claim 1, characterized in that, The carbon ion implantation process involves implantation energies ranging from 4 keV to 80 keV and implantation doses ranging from 1 E14 atom / cm². 2 ~5E15atom / cm 2 .

9. The forming method as described in claim 1, characterized in that, The annealing process includes a low-temperature treatment followed by a high-temperature treatment.

10. The forming method as described in claim 9, characterized in that, The low-temperature treatment ranges from 600℃ to 850℃, and the high-temperature treatment ranges from 850℃ to 1200℃.

11. The forming method as described in claim 9, characterized in that, Prior to the low-temperature treatment, the annealing process also includes a pre-annealing treatment.

12. The forming method as described in claim 11, characterized in that, The temperature range for the pre-annealing treatment is 1000℃~1200℃.

13. An image sensor formed using the forming method according to any one of claims 1 to 12.

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