CN107652900B - A gallium nitride wafer photoelectrochemical mechanical polishing liquid and polishing method - Google Patents

Abstract

The invention discloses a polishing solution for ultraviolet-assisted chemical mechanical polishing and a method thereof, and the polishing solution comprises nano abrasive particles and an oxidant; the content of the nano abrasive particles is 0.05-20 wt.%, and the content of the oxidizing agent is 0.1-10 wt.%. The polishing solution provided by the invention is mainly used for ultraviolet-assisted chemical mechanical polishing of gallium nitride wafers, high removal rate and low surface roughness can be obtained by using the polishing solution to polish the gallium nitride wafers, and meanwhile, the polishing solution has simple components, extremely low concentration of nano silicon dioxide or cerium oxide abrasive particles, convenient post-treatment of the polishing solution and small environmental pollution.

Description

A gallium nitride wafer photoelectrochemical mechanical polishing liquid and polishing method

technical field

The invention belongs to the technical field of polishing processing, in particular to a gallium nitride wafer photoelectrochemical mechanical polishing liquid and a polishing method.

technical background

The third-generation semiconductor representative materials such as gallium nitride, compared with the first and second-generation semiconductor materials, have a large band gap, high thermal conductivity, high breakdown electric field, high electron saturation rate and strong radiation resistance, and are more suitable for production High temperature, high frequency, high power, radiation resistant high power devices. When gallium nitride wafers are used as devices or LED substrates, materials are required to have high surface integrity (such as low surface roughness, no surface/subsurface damage such as scratches, microcracks, dislocations, and residual stress), and in The surface/subsurface damage of the material will occur during the grinding process of the gallium nitride wafer, and the wafer needs to be polished to remove the surface/subsurface damage of the wafer to obtain an ultra-smooth surface.

Gallium nitride crystal material has a large bond energy and almost no chemical reaction with any acid-base reagent at room temperature. It is a typical hard, brittle and difficult-to-process material. The low removal rate in the chemical mechanical polishing process leads to long processing time and high cost. question. The article published by Hideo Aida in "Mrs Proceedings", 2013, 1560(19): 2659-2666 reported that the chemical mechanical polishing removal rate of gallium nitride wafers is only 17nm/h. The post-gallium removal rate can reach 7μm/h, pointing out that in the chemical mechanical polishing process of gallium nitride wafers, the oxidation modification efficiency of the wafer is the speed-determining step of the chemical mechanical polishing removal rate of the wafer. Therefore, the material removal rate during the polishing process can be increased by increasing the oxidation modification efficiency of the wafer during the polishing process. Gallium nitride crystal, as a semiconductor material, can generate photogenerated electron-hole pairs by directly irradiating the surface of the semiconductor wafer with ultraviolet light. The cavities separated to reach the wafer surface are then used to oxidize the wafer surface, thereby increasing the material removal rate during wafer polishing.

Due to the low chemical mechanical polishing removal rate of GaN wafers, polishing fluids with a high concentration of nano-SiO2 abrasive particles are usually used. For example, in the literature published by Hideo Aida in "Journal of the Electrochemical Society", 2011, 158(12):H1206, in the experiments of their research group, when using silica sol to polish GaN, the concentration of abrasive particles is as high as 40wt%. Tsinghua University’s Pan Guoshun’s research group reported in a document published in Tribology International, 2016, 110, that in their chemical mechanical polishing experiments on gallium nitride wafers, the concentration of solid abrasive particles was also as high as 30wt%, and a removal rate of about 120nm/h was obtained. rate. However, the use of a polishing liquid with a high abrasive concentration leads to a high proportion of the cost of the polishing liquid in the polishing process, and also increases the difficulty of post-processing of the polishing liquid. D.E.Speed pointed out in Advances in Chemical Mechanical Planarization (CMP), p.27, Woodhead Publishing (2016) that polishing production units need to pretreat the polishing liquid before discharge, and manufacturers usually use coagulation and flocculation to treat the waste liquid , after sedimentation, filtration, flotation and other steps, the treatment process is complicated, and the post-treatment cost is high, which will increase the cost of the polishing manufacturer. In addition, if the treatment is not done properly, the polishing solution with high abrasive particle concentration will cause serious pollution to the environment.

Therefore, in view of the low removal rate of gallium nitride wafer materials in the current chemical mechanical polishing process, the processing time is long, the composition of the polishing liquid used is complex, and the high concentration of abrasive particles is likely to cause environmental pollution. It is urgent to find a A polishing liquid with higher removal rate, simpler and more environmentally friendly components, and lower abrasive particle concentration can meet the efficient manufacturing of semiconductor devices and the increasingly stringent environmental protection requirements.

Contents of the invention

Aiming at the low removal rate of GaN chemical mechanical polishing in the prior art and the poor surface quality after polishing, the present invention provides a GaN wafer photoelectrochemical mechanical polishing solution, which includes nano-abrasive particles, oxidizing agent and water. ; The content of the nano abrasive particles is 0.05-20wt.% of the polishing liquid; the content of the oxidant is 0.1-10wt.% of the polishing liquid.

The existing polishing liquid has a high concentration of abrasive grains and relatively poor light transmittance. The polishing liquid used in the present invention achieves a fast removal rate through the synergistic effect of oxidant oxidation and mechanical removal of nano abrasive grains.

The photoelectrochemical mechanical polishing described in the present invention refers to the introduction of ultraviolet rays to directly irradiate the polished semiconductor workpiece on the basis of the existing chemical mechanical polishing, and the semiconductor workpiece is removed by mechanical polishing after being photoelectrochemically modified with the assistance of ultraviolet rays a processing method.

As a preferred technical scheme, the oxidizing agent is at least one of potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, sodium peroxide, potassium peroxide, potassium permanganate, sodium hypochlorite, potassium hypochlorite, and ammonium hypochlorite A sort of.

As a preferred technical solution, the polishing solution further includes a pH regulator.

As a preferred technical solution, the pH regulator is at least one of potassium hydroxide, sodium hydroxide, ammonia water, sodium bicarbonate, disodium hydrogen phosphate, phosphoric acid, acetic acid, hydrochloric acid, nitric acid, and sulfuric acid.

As a preferred technical solution, the average particle diameter of the nano-abrasive particles is 15-100 nm; more preferably 15-30 nm. The fine-grained abrasive particles can make the polishing liquid more transparent, and the surface quality of the polished wafer is better.

As a preferred technical solution, the nano abrasive grains are nano cerium oxide abrasive grains or nano silicon dioxide abrasive grains.

As a preferred technical solution, the polishing liquid also includes a catalyst, and the catalyst is at least one of platinum, gold, rhodium, palladium, iridium and carbon-supported catalysts thereof, and the addition of the catalyst can effectively improve the surface modification of the wafer. property rate, thereby increasing the polishing removal rate.

As a preferred technical solution, the particle size of the catalyst is 15-50nm; the content is 0.0001-0.0005wt.% of the polishing liquid.

The polishing solution provided by the present invention is used for photoelectrochemical mechanical polishing. Using the polishing solution to polish gallium nitride wafers can make the removal rate reach 201.1nm/h, the material removal rate is much higher than the existing reports, and the surface roughness can reach ~1.63nm, the removal rate and the achievable surface roughness are much better than the existing reports. At the same time, the composition of the polishing liquid is simple, and the concentration of nano-abrasive particles is extremely low, so that the light transmittance of the polishing liquid is good, and the intensity of ultraviolet light passing through the polishing liquid to the wafer surface is high, and a better modification effect is obtained; in addition, post-treatment of the polishing liquid It is convenient, and the concentration of abrasive particles in the polishing liquid is low, which can reduce the post-treatment cost of the waste liquid after polishing, and has little environmental pollution.

The present invention also provides a semiconductor photoelectrochemical mechanical polishing method, comprising the following steps:

(1) The wafer is fixed in the polishing liquid pool and rotates around the axial direction with the polishing liquid pool, and the polishing liquid in the polishing liquid pool completely immerses the wafer;

(2) The polishing pad is fixed on the polishing shaft, and is driven to generate relative motion with the wafer; the contact area between the polishing pad and the wafer is smaller than the area of the wafer;

(3) The wafer is irradiated with ultraviolet light during the polishing process.

The contact area between the polishing pad and the wafer is smaller than the surface area of the wafer, so that the remaining surface of the wafer can be directly exposed to the online radiation of the ultraviolet light source to be modified.

The ultraviolet rays emitted by the ultraviolet light source can always be irradiated online to the part of the wafer that is not shielded by the polishing pad during the polishing process, modifying the wafer in real time and then performing photoelectrochemical polishing on the wafer.

Preferably, the polishing pad has an area smaller than that of the wafer.

Preferably, the polishing pad is pasted on the polishing shaft.

Preferably, the polishing shaft can also move in a plane while rotating around the axis, thereby driving the polishing pad to selectively polish the surface of the wafer.

Preferably, the polishing pressure can be loaded through the polishing shaft during the polishing process.

As a preferred technical solution, the polishing pad is loaded with a catalyst.

As a preferred technical scheme, the catalyst is loaded on the polishing pad using the following procedures:

a) first disperse the catalyst with nanoscale particle size in deionized water by ultrasonic oscillation;

b) Submerge the polishing pad in the deionized water dispersed with the catalyst, and vibrate ultrasonically for 1-10 min, so that the catalyst is supported on the polishing pad.

As a preferred technical solution, the catalyst is dispersed in the polishing liquid.

As a preferred technical solution, the catalyst is dispersed in the polishing solution by the following procedure: the catalyst is first added to the polishing solution, and then dispersed evenly by ultrasonic vibration.

As a preferred technical solution, the polishing pad is a polishing pad containing abrasives, and the abrasives are one or both of cerium oxide or silicon oxide.

As a preferred technical solution, the material of the polishing pad is one of polyurethane polishing pad, non-woven polishing pad and flannelette polishing pad.

As a preferred technical solution, the ultraviolet light source is one or more of LED ultraviolet light source, mercury lamp ultraviolet light source, xenon lamp ultraviolet light source, deuterium lamp ultraviolet light source, wavelength<400nm

Compared with the prior art: the photoelectrochemical mechanical polishing method of the semiconductor provided by the invention has the following advantages:

1. High polishing removal efficiency. The present invention adopts the method of irradiating the surface of the wafer continuously with ultraviolet light on-line. During the polishing process, the surface of the wafer is always irradiated, which can efficiently modify the wafer, and then mechanically remove the modified layer through the polishing pad, thereby improving the polishing process. removal rate.

2. The catalyst can catalyze and promote wafer modification under the condition of ultraviolet light, and improve the material removal rate in the whole photoelectrochemical mechanical polishing process.

3. The polishing head with the polishing pad attached can rotate around the axis or move the position at will, and polish a certain position of the wafer at a fixed point with the assistance of ultraviolet light, so as to realize the fixed-point polishing removal of the wafer.

4. The polishing head attached with the polishing pad can rotate around the axial direction or move freely, which can ensure the uniformity of the movement of the polishing head on the wafer during the polishing process.

Description of drawings

3 accompanying drawings of the present invention

Figure 1 is the surface topography of the GaN wafer before polishing; in Figure 1, (a) Olympus microscope surface topography image, (b) ZYGO white light interferometer surface topography image;

Fig. 2 is the surface topography after using the GaN wafer processing of the polishing liquid in embodiment 4; Among Fig. 2, (a) Olympus microscope surface topography image, (b) ZYGO white light interferometer surface topography image Ra 1.626nm;

Fig. 3 is a schematic diagram of the Tyndall effect of a beam of laser passing through the polishing liquid and the transparency and colloid effect of the polishing liquid.

Detailed ways

(1) GaN is washed with acetone, alcohol, and deionized water in sequence and weighed. The material removal rate is converted by the change in the mass of the GaN wafer. The original morphology of GaN wafer surface was measured by Olympus microscope and ZYGO white light interferometer respectively, as shown in Fig. 1(a) and (b). The original surface was prepared by hydride vapor deposition (HVPE). Hexagonal protrusions of different sizes, the largest protrusion height reaches about 1 μm, and the surface quality is poor.

(2) Fix the GaN wafer on the workpiece fixture of the polishing machine by bonding with paraffin; the polishing pad is SUBA 800; the GaN wafer is completely immersed in the polishing solution. .

(3) The rotational speed of the GaN wafer is 60 rpm, the rotational speed of the polishing pad is 390 rpm, the polishing pressure is 6.5 psi, and the ultraviolet light intensity is 20-30 mW·cm ^-2 and the polishing time is 5 hours.

(4) Heat and melt the paraffin, take off the wafer, wash it with acetone, alcohol, and deionized water successively, then blow it dry, weigh the mass, and measure the surface roughness after polishing.

Table 1. embodiment condition and polishing effect

After the GaN wafer was polished, its surface quality improved significantly. As shown in Figure 2(a), the hexagonal protrusions disappeared, the surface became smooth, the roughness value decreased to about 1.63nm, and the removal rate reached 202.1nm/min. The surface topography measurement results after polishing are shown in Fig. 2(b). The invention has the advantages of fast polishing removal rate, obvious improvement of roughness before and after polishing, fixed-point polishing of the wafer surface by the polishing pad, simple operation, and flexible and adjustable process parameters.

Claims (9)

1. A gallium nitride wafer photoelectrochemical mechanical polishing method, characterized in that: Wafer polishing with polishing pad and polishing liquid; The contact area between the polishing pad and the wafer is smaller than the area of the wafer; The polishing liquid includes nano abrasive grains, oxidizing agent and water; the content of the nano abrasive grains is 0.18-2wt.% of the polishing liquid; the content of the oxidant is 0.1-10 wt.% of the polishing liquid; During the polishing process, an ultraviolet light source is used to irradiate the part of the wafer that is not covered by the polishing pad, so that the remaining surface of the wafer is exposed and directly accepted by the on-line irradiation of the ultraviolet light source to be modified. 2. polishing method according to claim 1, is characterized in that: described oxygenant is potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, sodium peroxide, potassium peroxide, potassium permanganate, sodium hypochlorite, At least one of potassium hypochlorite and ammonium hypochlorite. 3. The polishing method according to claim 1, characterized in that: the polishing liquid further comprises a pH regulator. 4. The polishing method according to claim 3, characterized in that: the pH regulator is potassium hydroxide, sodium hydroxide, ammonia, sodium bicarbonate, disodium hydrogen phosphate, phosphoric acid, acetic acid, hydrochloric acid, nitric acid, at least one of sulfuric acid. 5. The polishing method according to claim 1, characterized in that: the average particle size of the nano-abrasive particles is 15-100 nm. 6. The polishing method according to claim 1, characterized in that: the nano-abrasive particles are nano-cerium oxide abrasive particles or nano-silica abrasive particles. 7. The polishing method according to claim 1, wherein the polishing liquid further comprises a catalyst, and the catalyst is at least one of platinum, gold, rhodium, palladium, iridium and carbon-supported catalysts thereof. 8. The polishing method according to claim 7, characterized in that: the particle size of the catalyst is 15-50 nm, and the content of the catalyst is 0.0001-0.0005 wt.% of the polishing liquid. 9. The method according to claim 1, comprising the steps of: (1) The wafer is fixed in the polishing liquid pool and rotates around the axial direction with the polishing liquid pool, and the polishing liquid in the polishing liquid pool completely immerses the wafer; (2) Fix the polishing pad on the polishing shaft, and generate relative motion with the wafer through driving; (3) The wafer is irradiated with ultraviolet light during the polishing process.